Give me your huddled root masses yearning to breathe free

About this time last year I posted photos of the installation of my new pollinator gardens (all perennials). As you can tell from the photos below, all of these plants have not only survived but thrived with their midsummer rootwashing.

Garden 1. Robust perennials! Except for the the sad, tiny lavender in the lower right hand corner (discussed below).
Garden 2 is just like the other, except the strawberry groundcover is replacing the wood chips.

 

 

 

 

 

The only ones that didn’t make it were the six Lavandula stoechas ‘Bandera Purple’ (see above). They did fine through the summer and well into winter. But with our surprise snowstorm in February (along with a 20-degree temperature drop in one night – from 33 to 14F), all but one of these marginally hardy plants (USDA zones 7-10) gave up the ghost. I won’t make that mistake again. But I will continue to root wash ALL of my perennials before I plant.

It’s pretty easy to excavate this tree (planted months ago) since there is NO root establishment.

And since it’s Independence Day here in the US, I thought I’d continue with the “free your roots” theme and discuss the medieval torture system that passes for recommended B&B tree installation practices. I’m talking about the burlap, the twine, and the wire baskets that are left on the root ball and cunningly hidden underground to do their damage over the years.

THIS is what should be planted.
Not this.

 

 

 

 

 

There is a great deal of disagreement about what to do with all the foreign material that’s used to keep tree root balls intact during shipment. To be clear, that is the ONLY thing they are intended to do. There is no research that shows leaving them on benefits the tree at all. The reason they are left on is because it’s more economically feasible for the installation company to do it this way. Personally I think that’s a pretty crappy reason, particularly when you are looking at trees that can cost hundreds or thousands of dollars.

Does anyone seriously think this is a good way to plant trees?

Most studies that have addressed the issue have been short term: two or three years, rarely longer. Irreversible damage to roots can take years to develop. It’s useful, therefore, to look at the landscape evidence to see what happens with all these barriers to root growth and establishment.

Death row.

Arborist and landscape designer Lyle Collins recently excavated the remains of trees that had been installed in 1991. The trees had died years ago and certainly hadn’t grown much as evidenced by their trunk size.

Not much trunk growth in this tree.

But while the trees didn’t survive, the burlap, wire basket, and webbing were all still there almost 30 years later.

Basket and webbing are clearly visible (after washing)…
…as is the burlap (before washing)

 

 

 

 

 

 

 

The clay rootballs are nearly intact as well. That’s not what you want to see. Roots must establish outside the rootball into the native soil, or they won’t survive.

Intact rootball after 28 years
The same rootball after washing

 

 

 

 

 

 

 

Eventually I’m convinced long-term research will show the folly of leaving foreign materials on the rootballs of B&B trees. In the meantime, I’ll continue to plant trees in a way that ensures their roots are in contact with the native soil and free from any unnatural barriers to growth.

 

 

 

Urban Gardening Considerations

Along with the trends of buying local food, buying organic, etc., there seems to be an increasing interest in the ultimate local food source – a garden. This includes in urban areas. Urban gardening is a great way to save money on food, a great source for fresh vegetables – especially in “food deserts”, and an easy way to introduce kids to where the food on their plate comes from. However, there are a couple potential obstacles you should consider first before starting your urban garden.

"Graze the Roof" by Sergio Ruiz
“Graze the Roof” by Sergio Ruiz

First, in urban environments the possibility that soil could have been contaminated with heavy metals, petrochemicals, etc. is pretty high, especially in older neighborhoods. Lead, which was once a common additive to gasoline and paint, is a common contaminant in urban soils.  and can be absorbed by the roots of the vegetables you grow. Because of this, that lead can eventually end up in the food on your plate. Most lead poisoning comes from ingesting lead (like eating lead paint chips…), so it’s important to know that the soil you’re using for your garden is safe. You should take some soil samples and send them to a lab in your state that can test for heavy metals like lead. Usually the Land Grant university in your state (if you’re in the US) will have a soil testing lab where these tests can be performed for a nominal cost. Other forms of contamination are possible as well, such as chemicals from cars, asphalt , laundry-mats, etc. These chemicals are more difficult to test for, so your best bet is to find out the history of your garden plot. These records should be available from your local city government, perhaps even online. Read more about contamination in this post.

Second, urban soils are often compacted from foot, car, or perhaps machinery traffic. Compacted soils make it difficult for plants to grow, mainly because the plant roots are not strong enough to penetrate the compacted soil, and thus cannot gather enough water or nutrients for the plant to survive, let alone grow and produce vegetables. Compacted soils are especially common in newer housing developments where entire blocks of houses were built around the same time. The construction companies often remove all of the topsoil prior to building the houses. The soils are then driven over by construction machinery and compacted. Then sod is laid directly on top of the subsoil. This makes for soils with very poor growing conditions for both lawns and gardens.

A good alternative for areas with either contaminated or compacted soils is to use a raised garden bed with soil that was brought in from a reliable source. You can buy bags of potting soil from a local home and garden supply store, but a more economic alternative is to have a trailer full of topsoil trucked to your raised bed. When you build your raised garden, be sure to use untreated wood. Some of the chemicals used to for pressure treated lumber are designed to kill fungi that break down wood. These chemicals, some of which contain arsenic, can leach out of the wood and into the soil used for your veggies! However, untreated wood, though it might not last as long, will still last for decades and is probably cheaper anyway. There are lots of great designs and how-to sites that show you how to build a raised garden bed. Here’s an extension bulletin from Washington State University on raised bed gardening. The raised beds shown below are from when I first installed them in my community garden plot in Manhattan, Kansas. One is now a strawberry patch (the border helps contain the strawberries to a defined area), and the other is used for mostly cold season crops.

This image shows two raised garden beds with freshly added soil and surrounded by straw in a garden plot.
Raised garden beds in Colby Moorberg’s community garden plot.

Space is also another consideration. If you don’t have the space for a garden or a raised garden, then perhaps you need to think outside the box (raised garden pun intended) and consider container gardening. Container gardening is exactly what its called – growing ornamental or vegetable plants in containers. Containers can be traditional plant pots, buckets, plastic totes, or any other container with an open top.

The advantages of container gardening include:

  • Containers can be arranged to optimally use the space available, or rearranged if you like to mix things up sometimes
  • Potting soil can be used, and can be trusted to be lead/chemical-free
  • Work can be performed on a bench, thus avoiding working on your knees
  • Containers can be arranged to provide decoration for your outdoor space
  • Many objects found around the house can be cheaply converted into decent containers
Vertical Pallet Garden. Photo by Heather Foust

Vertical gardening is a version of container gardening that uses your available space  efficiently. Much like using shelves to save space inside your home, vertical gardens use shelves, stairs, racks, etc. to make use of vertical space. The options for vertical gardens are only limited by your imagination. Here are a few extension bulletins on vertical gardening from Tennessee State University and the University of Nebraska.

The main disadvantage of container gardening is that you’ll likely have to water more frequently, but there are strategies to overcome that problem – see my prior blog post about saving water with container gardening. Another good resource is the University of Illinois Container Successful Container Gardening website.

In summary, the biggest obstacles to urban gardening are soil contamination, soil compaction, and space limitations. I’ve given you a few good alternatives to overcome those issues. Also, be sure to fertilize appropriately, lime as needed, and make sure the plants that you pick are appropriate for the sunlight that’s available. Your local garden supply store or extension agent can help you with suggestions on those issues.

If you know of an urban gardening obstacle that I didn’t address, please leave a comment and I’ll see if I can help out.

Happy digging!

Colby

This was originally posted on Colby’s soil science blog, ColbyDigsSoil.com. Some edits, updates, and adaptions were made for this post.

The Dog Days are here

The dog days of summer are here and as we approach the longest day of the year (summer solstice is June 21st), we are also feeling the advance of high summer temperatures. Long days mean more evapotranspiration and water withdrawal from the soil. During these long days, plants photosynthesize more, grow more, and use the most water during the month of June.  In fact evapotranspiration looks generally looks like a bell shaped curve when plotted by month (figure 1).  Soils dry quickly and irrigation or rainfall may not keep

Figure 1. Evapotranspiration data by month. Image from US geological Survey

up with plant demands for water. This can bring some very real stress to garden plants and turfgrasses.  If you live in a place that does not receive summer rainfall you will certainly need to increase irrigation to reflect day length at this time of year.

Transpiration is water loss through leaves and is not  part of photosynthesis, but it is critical to cool the plant. As soils dry out, the level of abscisic acid produced in roots increases and translocates to leaves resulting in the closing of the pores called stomates. Closed stomata reduces transpiration, but only at a steep cost to the plant. That cost is heat build up. Since this is also the time of the hottest weather it is not long before leaf temperatures rise to lethal levels and sunburn results. Sunburn is always seen as damage in the middle of the leaf because that is the hardest spot to dissipate the heat. The edges that lose heat rapidly are usu

Figure 2. Sunburn occurs in the middle of leaves as in this Windmill palm (Trachycarpus fortunei)

ally not burnt (Figure 2).

Short of applying water properly, what else can be done?   Mulches are a great way to avert drought stress since they reduce water loss from the soil surface. The effect is greatest where sun hits the soil. So in new gardens or gardens without a lot of shade, mulches are essential during hot weather to reduce plant stress. Wood chip mulches are particularly helpful in that wood does not reflect, hold or emit heat as much as soil, so it protects adjacent plant surfaces from heat.

How about water absorbing polymers or hydrogels? While much of the allure of these “water crystals” has worn off, it is still good to remind that polymers don’t change evapotranspiration rates of plants so even if they did all the things they claim to, they won’t get plants through a hot summer any better than if they were not present in the soil.

With the longest days come warming soil temperatures.  Hot soil can affect plants especially perennials.  Ground that is not mulched will radiate infrared onto plant surfaces, this can increase stress. This is yet another reason to employ wood chip mulches around perennials.

Figure 3. Impact sprinklers raise humidity on very hot days relieving plant stress

So when it is particularly hot and dry how can we get plants through this stressful time? Running sprinklers (where practical) will increase humidity and if soils are dry reduce stress (Figure 3).  For annual plants, some shade is often helpful. Applying shade cloth to sensitive or newly planted/emerged plants can cut stress dramatically.  As plants establish, the shade can be gradually removed.  Keep irrigation even so moisture is always there to maintain transpiration — this  is essential during warm weather and long days. For perennial plants there is not much to be done. While pruning will reduce the amount of surfaces that lose water, pruning (thinning) will also lead to temperature increases in the plant canopy since the evaporative surface area of the plant is decreased. So while soil water is saved, canopy temperatures may rise, this may be a poor trade off in the hottest months of the year.  Over-pruning opens plants up for sunburn on stems which can lead to fungal canker infections by pathogens like Botryosphaeria. This is very common in Apples.

Another treatment you may have heard claims for are anti-transpirants. These are products that are sprayed on plants to create a film that will cut water loss from leaves. Taken from a recent Amazon search I found the following product description recently… “Product is a water-based, semi-permeable polymer coating that can minimize the damages from climate related stresses, such as frost and freeze, heat stress and sunburn, drying winds, and transplant shock. Applied as a foliar spray, Product provides a unique non-toxic, biodegradable, elastic membrane over the plant surface to reduce moisture loss and insulate the plant.” While there may be an application (such as freeze protection) that makes sense for this kind of product somewhere, I don’t see it in your garden during hot weather. Cutting transpiration (“reducing moisture loss”) will increase the heat on leaves, so one of the common side effects of using these products is hot weather is damage or phytotoxicity.  Like polymers the fad is faded.

Sometimes the dog days of summer bring insurmountable challenges. In early summer of 2018 in California, temperatures reached record levels of 115-120. Even in irrigated situations plants were damaged, short of providing immediate shade, there was nothing to be done and many plants were injured, even native plants are not adapted to such high temperatures. If these conditions occur in your garden, you may not be able to limit damage, but there are considerations for after care when this kind of blitz occurs. Don’t prune anything immediately, let the leaves fall and buds form because stems may be intact. Prune away injured plant parts after regrowth begins. If injury is severe, cut back on irrigation. Injured plants don’t require as much water because there is less functional  leaf area. This is why root root rot often follows this sort of severe injury. The summer solstice is here—I can already feel the shortening days of fall some distance away.

Reference

Costello, L.R., E.J. Perry, N. P. Matheny, M.J. Henry, and P.M. Geisel.  2014.  Abiotic disorders of Landscape Plants.  University of California Division of Agriculture and Natural Resources Publication #3420.

Cornmeal magic – the myth that will not die

Way back in 2010 (and then again in 2012) I wrote about a bizarre belief that cornmeal could be used to treat fungal diseases, from lawn spot to athlete’s foot. Rather than rehash what’s already been written, I’ll invite readers to read those posts for background. And of course look at the comments, which are…interesting.The weird thing is that this post from 2010 is the single most popular post on the blog. (Our stats are only for the last two years since we migrated the web site – who knows how many there were before May 2017?)

Blog stats over two years

The consistent popularity for the topic spurred me to publish a university fact sheet on the use of cornmeal and corn gluten meal in home landscapes and gardens. This fact sheet reviews the pertinent literature, and makes recommendations that are pretty much the same as those I made almost 10 years ago. Nothing has changed in the research world to support cornmeal as a fungicide.

But wait, there IS something that’s happened since 2010! Now cornmeal is being touted as an insecticide! In fact, if you go to Google and search for “cornmeal” and “insecticide” you’ll find thousands of hits.  As you might expect, there’s no research to support this notion: researchers in Maine, for instance, found no effect of cornmeal on fire ants. However, it is used as a bait to deliver actual insecticidal chemicals.

Way back in 1937.

But facts don’t get in the way of home remedies, such as Lifehacker’s eyebrow-raising advice.

Hmmm…

By refining the search to only include university websites (use “site:.edu” to do this), and swapping out “ants” for “insecticide,” you’ll find at least one Master Gardener group happily (and illegally) recommending cornmeal as an ant killer. The popular mode of action is either (1) they can’t digest cornmeal and starve or (2) the cornmeal absorbs water in their gut and they explode.

Boom!

This reminds me of yet another food product – molasses – recommended for killing ants. Since you’re already here, you might as well check out Molasses Malarkey parts 1, 2, and 3 too.

Might I recommend everyone use their cornmeal and molasses to make bread or cookies or pancakes? There are some delicious recipes on the internet.

Yum!

Bare Rooting – a guest post from a commercial landscaper

What are these trees and do they look like this? Read on to find out!

Today’s blog post is courtesy of Mary Blockberger of Sechelt, BC. As you’ll see, Mary and I go way back.  I thought it was important to our ongoing discussion to see how the industry can use the root-washing technique effectively and economically. Here’s Mary:

“Before I began managing the Sunshine Coast Botanical Garden in Sechelt, BC I had a small residential landscaping company.  By small, I mean that I was the employee of the month every month of the year!  One of our Garden’s mandates is to provide relevant and educational programs for our community.  Dr. Linda Chalker-Scott has been one of our most popular speakers several times.  One of her presentations dealt with the practise of bare-rooting perennials, shrubs, and trees prior to planting, and the tremendous advantages of following this method.

Root-washing and installing 36 Carpinus. November 2007.

“In November, 2007 I had a chance to try this technique out.  My client wanted a ribbon of Carpinus betulus ‘Fastigiata’ planted that would eventually be pleached into an interesting pattern.  [Pleaching is a formal tree training technique.] There was a total of 36 trees to be planted; most were container stock as I recall but there may have included a couple of B&Bs as well.  Working with another local landscaper, into a wheelbarrow of water went every single tree one at a time.  The dirt was clawed away from the root balls by hand with a final spray from the hose.  Honestly, it was a cold and miserable job, and I believe a few curses directed at Linda ensued.  However, once the roots were cleaned of all soil planting was a breeze.  It’s a lot easier moving trees without moving the soil too.

Carpinus are well established and pleaching is underway. April 2009

“Flash forward 12 years, and every single tree has flourished.  Bare rooting allowed us to identify and correct any problems before planting, and I’m sure this has a lot to do with the trees’ success.  It’s a time consuming and at times messy method, but the reward of a healthy row of trees is well worth the effort, IMHO.”

Pleached Carpinus hedge May 2019

And let me add to Mary’s account that a ZERO replacement rate is going to pencil out to long term economic success. I was able to see these trees earlier this year – that’s my photo at the top of this post.

Amending Soils—Why??

I think the blog and garden professors web page is pretty full of research and benefit descriptions of mulching, particularly with arborist chips. A little less clear is the role of amendments in garden soils. I always like to ask the “why” questions for gardening practices. Like “why” prune trees? Why fertilize, etc? Ideally gardening practices should be founded on a basis of science and inquiry as to their necessity. Poor structure early structural training or a damaged canopy may prompt tree pruning, mineral nutrient deficiency symptoms may suggest

When amending, organic materials are often tilled in 50% by volume (3 inch layer tilled six inches deep)

fertilization. So why amend your garden soil? For me as a gardener you do this because your soil is not providing something you think your garden or plants growing in that soil need. This could be nutrients, or water. Amending as a soil or garden practice is best done in garden beds that host annual plants: vegetable gardens, color beds with annual plants etc. Obviously we are not going to lift all the perennials just to add some amendment under their root systems. We can also make arguments that amending soils that you are going to plant perennials in is unlikely to be helpful. So why amend?

 

Research on providing amendments in the holes of perennial plants has most often shown no significant differences compared to plants installed with just native backfill. There are lots of reasons for this and I can imagine some soils where amendments would give a slight boost, but these were not the soils most studies used (extreme sands). Mostly, perennial roots do not reside in the planting hole for long, so the time that amendments would be effective is very short. Since amending can also harm some plants if done incorrectly, University of California does not recommend the practice, neither did Harris for trees, shrubs and vines. Nor did we find any effect in palms (Hodel and others). For this blog we will assume that amending is restricted to annual plant beds. So why amend? The usual reason I hear is: “My soil is crap!” I think it would be interesting to survey people about their soil and see what they think about it as compared to how it actually grows. Most people seem to disparage their poor soil….So many gardeners believe that if they add something to their crap soil it will get better. Maybe, or no. Amending has potential benefits and detriments depending on what and how much you amend with.

snaps growing in soil amended with high quality yardwaste compost
Snaps growing in soil with uncomposted yardwaste used as an amendment. Nitrogen is immobilized

So the potential benefits of amending are:
• rapid (immediate) modification of the planting bed
• potential increase of moisture holding capacity in the soil
• potential increase of nutrients and nutrient holding capacity (cation exchange capacity of the soil
• change in the soil texture, porosity
• rapid increase in soil organic matter
• suppression of soil borne pathogens

Potential detriments include:
• nutrient draw from the soil (nitrogen immobilization: see images above)
• toxicity from residual phytochemicals
• adding pests (weeds or root pathogens)
• soil structure is destroyed
• soil food web is challenged and harmed
• may increase soil salinity
• contaminants can be transferred to gardens

One of the incontrovertible facts about amending is that you have to disturb the soil to do it. Amending involves digging in, roto-tilling, soil turning with a spade, or some kind of incorporation process. Usually the more the better. This destroys soil structure and a good part of the soil food web. Beneficial nematodes are highly sensitive to tillage and many are killed by tillage, often perturbing the entire soil food web in a disturbed soil (See research by Howard Ferris). The beauty of mulching is that the soil structure is not initially influenced only assisted in its further development. The downside is that mulches take months or years to have their effect. While amending can give immediate physical, chemical and biological assistance to soil, it also may bring pathogens, salts, toxic phytochemicals or weeds to your garden depending on what you choose to use as an amendment. Soil tends to be very resilient, so structure destroyed by amending is usually back in place at the end of the cropping cycle in many cases. The need for further amending should be considered carefully after each rotation.

So you still want to amend? What do you amend with? In the case of mulching we (Garden Professors) make a strong argument for freshly prepared (not composted) arborist chips. Nothing could be worse for amending (at least in the short term). Un-decomposed substrates such as wood chips are high in carbon and low in nitrogen. They will cause the microbial community of soil to attack the carbon and utilize all the free nitrogen in the soil, screened or fine materials after composting have greater surface area and will enhance the water absorbing and nutrient holding properties they give to soil. Furthermore, well composted substrates or feedstocks will be free of pathogens and other pests so they should be “safe” for your garden. Composts that are made from plant feedstocks tend have concentrations of plant required minerals. Since composts lose about 2/3 of their carbon and moisture during the decomposition process they have a much higher mineral content than their feedstock. Manures have already been partially composted by the animals that made them. Additional composting increases their salt levels sometimes to plant damaging levels. Manures should be used with care, or in lower quantities as they can damage sensitive plants. Some manures and composts can be contaminated, since some long term soil-residual herbicides such as clopyralid are not broken down in the composting or animal eating process.

soil amended with peat moss
Soil amended with composted coffee grounds

Gardeners are inventive in their use of compost and amendments, so there are many ways to amend soil. Peat moss has been the gold standard amendment for many gardens. However due to environmental damage, sustainability of peat moss use is questionable. Coco fiber or coir is a good amendment, but it can bring high salts with it depending on how it was processed. Biosolids are phenomenal amendments and often produce growth responses, but there can be issues with metals and other biological contaminants in biosolids. Home-made compost is a good amendment because you know what is in your compost, as long as it is properly prepared and cured, it can function very well. Greenwaste or yardwaste compost is a possibility, but from my experience, most of these if commercially produced are not well prepared, and are not mature (they still heat up if piled). Many gardeners like coffee grounds, and with the advent of large coffee companies, grounds may be available in bulk quantities. Be careful though as some sources of coffee are toxic to many plants and their use should be limited in any amending situation.

What about rate? In my research I have always tried to amend soils 50% by volume. So a three inch layer of amendment tilled six inches deep has been my goal. Most annual plants have their roots in the upper six inches and this strategy works well. Also, most rototillers are only good for about a six inch depth. If you intend to dig deep with a spade and incorporate to depths beyond six inches, increase the amounts accordingly.

What kind of soil needs amending? Another way to view this is, “have you tried growing without amending? Many soils grow very well with just added nutrients. Typically sands will most benefit from added amendment due to increased water and nutrient holding capacity. I also like to amend clays because they become easier to plant in over time, however the initial go round may be difficult. Clays are very nutritive so amendments may make you feel better gardening in them, but often plants grow very well without amending clay soils. Plant responses to amendments are best in sands.

How often should I amend? This is up to you, but organic matter is “burned up” by the soil microbial community rapidly because 1) you are tilling and this accelerates microbial activity; and 2) you are likely amending during the growing season when warm soil temperatures favor organic matter breakdown. Most gardeners amend prior to replanting the bed.

References

Ferris, H., and M.M. Matute. 2003. Structural and functional succession in the nematode fauna of a soil food web. Applied Soil Ecology 23:93-110

Harris, R.W. Arboriculture: Integrated management of landscape trees, shrubs and vines. 1992. Ed. 2 Prentice-Hall International, New Jersey. 674pp.

Hodel, D.R., Downer, A.J., Pittenger, D.R. and P.J. Beaudoin. 2006. The effect of amended backfill soils when planting five species of palms. HortTechnology 16:457-460.

Plant Control to Major Tom(ato): The Art of Spacing Out Your Plants

“Why don’t you just plant it up against the house,” piped my mother-in-law.  She was talking about a run-of-the-mill “old fashioned lilac” that we had received in the mail for our donation to Arbor Day.  While I don’t necessarily think of the organized tn as a source of high-quality or novel plants, I felt beholden to  make a donation since it was founded and is still located in Nebraska (and we have visited the Arbor Lodge, home to founder J. Sterling Morton and his brood of tycoons (one of salt fame – that Morton, one of cornstarch fame – ergo we have Argo, and one of the railroad).  I had pawned the 10 blue spruce off to the freebie table at the office, but she wanted a lilac…and what momma-in-law wants, momma-in-law gets.

I explained to her that the labeled final size of the cultivar was 12 feet in diameter, so it needed to be at least 6 feet from the house (preferably more) and from other plants.  “Nonsense!” she decried, “I planted mine close to the house.  You just have to keep it pruned back.  Mine did just fine….. until it rotted.”  Since the right spacing would put the shrub in the middle of a narrow passage between the house and the fence, I opted to throw it into a pot since I was heading out of the country the next day for two weeks.  It was, after all, a little more than a spindly twig (with roots wrapped together in a ball) sheathed in plastic.

What’s the issue? 

Most gardener’s desire for instant gratification often means that correct spacing for the finished size of the plant gets thrown out the window so that the garden looks good to go from the beginning.  Or even worse, plants get shoved against houses, under power lines, or in other areas where they’ll either be cramped and crowded or incessantly pruned to the point of oblivion throughout their probably shorter-than-expected lifespan.  Think of it like your bubble of personal space.  Just like you don’t want to be crowded, neither do plants.

In addition to the pruning and space issues, crowding can increase the likelihood of disease or other plant issues.  Crowded plants reduce air flow, which aids the development of diseases by increasing (ever so slightly) the humidity in the plant’s microclimate, increasing drying times after rain or irrigation, and even allowing for disease spores to more easily settle on the plants.  For perennials, and especially trees and shrubs, overcrowding can be a chronic issue since the problem can last for many, many years.

That’s not to say that spacing it isn’t a problem with annuals, either, especially in the vegetable garden.  In addition to the increased possibility of disease, competition for space and for nutrients can reduce yields.  Crowded root crops like carrots, beets, and radishes don’t have enough space to fill out, resulting in stunted and irregular produce. The same goes for leafy crops like lettuce or kale.  Fruiting crops like tomatoes and peppers can also suffer from reduced production when plants can’t fully grow to their potential.

Getting Spacing Right: The Simple Art of Not Planting Too Damn Close

Perhaps the secret is complicated formula for figuring out the proper plant spacing?  Or perhaps it is some specific planetary alignment you need to wait for?  Since it seems to mystify may gardeners, spacing must be difficult, right?  Au contrare!

Most plants come with the proper space printed right on the label!  Think of such a novelty!  That lilac my MIL wanted to plant against the house said right on the packaging that it generally grew to 12’ wide.  If I were planting a bunch of them in a row (and not as a hedge), I could plant them 12’ apart.  If I wanted to grow them as a hedge, I’d reduce that spacing to make them grow into each other (but it will take time for them to grow into a hedge…so they won’t be touching right away).  Keep in mind that this is the genetic potential of the plant and isn’t a guarantee.  Many factors, including microclimate, soil conditions, precipitation, nutrient availability, disease, etc. etc. etc. could limit the plant’s growth to that potential (or even more rarely increase it).  What if I’m not planting a bunch in a row?  Here’s were a teensy bit of math comes into play.

Think of plants in general terms as circles.  Just look at a basic landscape architect’s plans and you’ll sometimes see plants represented generally as circles.  If you think all the way back to that geometry class in high school, you’ll remember that there are several measures of a circle, including the diameter and the radius.  The diameter is the width of the circle from one side to the other. The radius is the distance from the center point of the circle to the edge.  So if our plant is a circle, then the listed width of the plant is the diameter and the distance from the trunk, stem, or center is the radius.  So I can expect my lilac (if it reaches full potential) to grow out 6 feet from the trunk.  That means I need to plant it at least 6 feet away from the wall.  If I wanted to plant it in the landscape with other plants around it, I would need to figure out the radius of the plants I wanted to plant close to it and add their radius to the lilac radius to figure the minimum distance I should plant them apart.  Let’s say I wanted to plant a small shrub beside the lilac with listed width of 10 feet.  That means the radius of that shrub is 5 feet.  Adding them together, I get a distance of 11 feet.  On the other side of the lilac I want to plant a large perennial with a diameter of 4 feet.  The radius would be 2 feet, so my minimum planting distance would be 8 feet.  You also have to keep in mind any variation due to microclimate and environmental factors.

What about the vegetable garden?

The same concept holds true for the vegetable garden as well – think of each of the plants as a circle.  Where planning the spacing is different is usually interpreting what the seed packet says in terms of in-row versus between-row spacing.  The in-row spacing is based on the size of the plant, a general idea of the size of the circle the plant makes or how much space it needs between plants (some plants, like beans, are OK when they overlap a bit and share space).  The between-row spacing is for human use in creating typical in-ground, large garden areas.  I’ve had the discussion before of large garden area vs. in-ground beds, vs. raised beds so we don’t need to go into that detail, but the general movement is toward some sort of bed system to reduce walkways (reducing bare soil that can lead to erosion or compaction when walked on) and intensify plant spacing/output per a given area.

Taking that into consideration, use the in-row spacing as the between plant spacing in all directions.  This is what the popular Square Foot Gardening method does – the spacings and number of plants per square are based on the between plant spacing and eliminates row spacing.  For example, radishes and carrots typically have an in-row or between plant spacing of 3 inches.  If you fit that spacing evenly within a foot row segment, you get four plants.  When you make that two dimensional you get 16 plants per square foot.  For four inch spacing you get 3 per foot and 9 per square foot, six inch spacing you get two and four, etc.  You can fill an entire bed with plants like this without spacing between rows of plants.

Of course, the tricky thing is that, just like our trees and shrubs that are planted too closely reduced airflow and increased microclimate humidity can increase the risk of diseases in the plants.  The Square Foot Gardening method by the book states that you shouldn’t plant any adjacent square with the same crop to decrease likelihood of disease sharing, but that seems sounder in theory than in practicality. I use some of the spacing (and you don’t need the book, just look at the between plant spacing and calculate. You just have to monitor, use good IPM, and treat or remove issues promptly to reduce disease issues. Using interplanting to intensively plant by mixing various space usages (tall plants with short plants, root crops with fruit crops) can also help make use of the space while mixing plants to reduce disease spread.

Problems with Planting Trees

 

Ok. I admit this blog is going to turn into a rant pretty quick because there seems to be a lot of ways to screw up a fairly simple horticultural practice—tree planting.  Since Arbor days are happening/happened everywhere around now, its a good time to talk about how to plant trees.   First let me state some simple and useful guidelines for a successful tree planting.

-When at all possible, plant trees bare-root. Even washing the container media away. This allows for inspection and removal of root defects.
-Select trees carefully that are free of defect and disease and that are adapted to your climate and soils
-Plant the youngest tree you can
-Take care in choosing the planting site.
-Avoid Root Barriers
-Plant trees so that the root flare is above ground slightly
-Plant trees in a hole only deep enough to contain the root system, no double digging.
-Plant trees in a hole wide enough to contain the root system, no wide holes (unless there is a reason for using one)
-Fill the hole with soil removed to make it. Do not amend the backfill around newly planted trees—Do not put rocks in the bottom of a planting hole!
-Plant trees without staking unless there is a reason to stake them
-Plant trees away from turfgrass or other groundcovers.
-Plant trees under the cover of a fresh layer of arborist chips.

-Irrigate newly planted trees from the surface—Do not install U tubes or tree snorkels to irrigate deeply.

An old planting detail from “a book of trees” . Several myths here: rocks at the bottom of the hole, amended hole, nursery stake still there when it does not need to be, etc.

I guess this rant comes from the variety of tree planting specifications I have seen over the years used by municipalities, landscape architects, nurseries and others. There seems to be a need to use the latest product, method or modification to site soils in order to make a fancy planting detail. Simpler is better and research by Universities has not verified most of the “innovative” approaches seen in planting details.

The first step in planting a tree is to chose the tree you want to plant. While this seems simple there is a lot that goes into tree selection. Setting aside personal choices, it comes down to selecting a tree that is healthy and free of defect. The potential candidate tree should have no signs or symptoms of disease, a naturally developed canopy unfettered by nursery pruning (especially heading cuts), and has few or no root defects. Initial superficial examination of the root collar in the nursery can eliminate some trees with circling or girdling roots. However, when the tree is planted root washing will reveal the entire root system and as Dr. Linda Chalker Scott has shown in this forum, root washing allows for rapid establishment in site soil. When at all possible chose the youngest tree you can for the new site. Young trees have fewer root defects, and we have the advantage of training them (structural pruning) from an early age. Young trees establish rapidly and will often outgrow older, boxed trees. The larger the specimen that you plant, the more chance for establishment problems such as settling, drying out, root rot or just slow growth. Planting trees from seed is ideal but most gardeners don’t have the patience to wait and seedlings, and seedlings do not give the option of using cultivated varieties that impart horticultural value, such as predetermined flower color, disease resistance, and known form (canopy shape and size).

Once the tree is selected, purchased and root washed, it is time for setting it in the ground. The first step is choosing a good planting site. A good site for a tree is somewhere that provides adequate soil volume for its roots to expand and for its canopy to expand. Many trees in urban settings fail to achieve their potential because they have restricted spaces to grow in. Chose a location in full sun. Unless you are planting a species that grows well in shade or needs protection from the environment, most trees will grow best in a sunny location. While trees are forgiving of most soil conditions, they will not grow well in compacted soils. If this is all that is available, break up compacted soils before planting. Consider the ultimate size of the tree you are planting, and imagine it attaining that size in your planting site. Avoid sites that have close proximity to buildings or hardscape. One of the most frequent problems with trees is that as they attain mature size they conflict with the infrastructure at the site.

Dig the hole for your tree so that the roots are very slightly above the grade. Do not double dig! While double digging has its proponents, there is no research-based reason for destroying soil structure– it is a disaster for tree planting. When a hole is dug too deeply soil will always settle after planting and irrigation resulting in the tree being planted too low in the ground. The root collar is buried and this is a predisposing factor for disease. The hole should have undisturbed soil under the roots. The hole only needs to be as wide as the root system. While many planting details show wide holes these are not necessary in most garden sites. If the site is compacted, wide holes can give temporary advantage to a newly planted tree, but the width of the hole will be the size of the “pot” the tree will have to grow in. So it is better to modify the site first to take care of compaction and then you will not need a wide hole.

Root barriers do not function well in most landscapes and lead to the development of landscape trash. They can also create root defects

Root barriers were very popular and are still specified today.  They actually do not usually achieve thier goal of preventing surface roots and protecting infrastructure.  Trees outgrow root barriers and they result in increases of landscape trash/pollution.  Root barriers can also create root defects such as circling and girdling roots.  Do not install root barriers, if you are tempted to do so you are likely not choosing a good site to plant a tree.

Cover the roots with backfill from the hole. Do not modify the backfill. Research does not support adding amendments to planting holes for trees. The native soil is what the tree will be growing in ultimately, and there is no reason to modify it. If the soil at your site is so bad that it needs to be changed, this should be a site-wide soil modification that will cover all the area the tree roots will explore up to its maturity. Most gardeners are not able to do this. Roots rapidly expand beyond the planting hole within months, so the time and benefit derived from an amended planting pit is minimal. Adding amendment, especially organic amendments to backfill can also be disastrous for trees. The organic material may utilize nitrogen in the soil and lead to a deficiency in the newly planted tree, worse, it may break down and cause anaerobic conditions in the bottom of the planting pit. Avoid amending planting holes! Never place rocks in the bottom of the hole—this does not create drainage, but creates an interface that prevents it.

A “lollipop” Tree.  Note the very skinny un-tapered stem, lack of temporary lateral branches and retention of the nursery stake–all bad…. Also note the tree snorkel lurking to the left. Kudos for keeping turf away but not far enough away.

If you have selected a good tree, it will stand without staking. There are three reasons for staking: support; anchorage; and protection. Support is sometimes necessary when a tree is cultivated with a long un-tapered trunk and a lollipop crown. Lollipop trees are often sold in nurseries as they resemble small trees. Trees trained in this manner, will not stand without staking. Loose staking allowing trunk movement will foster development of caliper so the tree can eventually stand without supportive staking. Anchor staking is used for trees that experience high winds and “staked out” with guy wires and a non-constrictive collar. Protective staking is analogous to placing bollards around a tree prevent impact from machinery or cars. Always remove the nursery stake at the time of planting and provide any additional support the tree may need. Many Cooperative Extension services have publications on how to stake a shade tree.

Providing a No Turf Zone around trees will aid in their establishment

Avoid planting trees in lawns. Turfgrass and trees conflict with each other. Trees shade turfgrass which results in a thinning sward and increased disease prevalence. Turfgrass slows the growth of trees in an attempt to limit their shading effects. Turfgrass is a very competitive water user and trees will be deprived of moisture and nutrients if turfgrass is present.  If trees must be planted in lawns, maintain at least a 1 yard radius around them with no turfgrass.

Aeration/Irrigation snorkel tubes do not help trees and result in landscape pollution. Note the original nursery stake still in place and the supportive stakes should have been removed long ago. Mulch needs to be replenished.

It has become a common practice to add irrigation or aeration devices to tree plantings. Sometimes called a tree snorkel these plastic 4 inch U tubes are buried below the root zone. Kits can be purchased from Box stores, and architectural details have been drawn specifying their use. Work by UC researchers showed that oxygen does not diffuse far from aeration tubes. So utilizing tree tubes to increase air flow is suspicious. Some planting details specify adding irrigation to the tubes to force a deep rooted condition in the tree. This places water below the root system, which can dry out and compromise establishment—not a good idea… Worse of all tree snorkels are sometimes installed with no purpose at all other than that was what the planting plan indicated. This is a needless practice and results in landscape pollution. Long term, tree snorkels are ugly, easily broken and provide no useful function to an establishing landscape tree. It is not in the nature of trees to proliferate absorbing roots deep in soil and snorkels will not change a tree’s genetics.

After the tree is set in its hole, and backfill settled in with water, apply a 4 inch layer of arborist chips as far out from the trunk as feasible—at least several feet. The chips will modify the soil improving, chemical, physical and biological properties while conserving moisture from evaporation, preventing runoff, and germination of annual weeds. Generally trees thrive under mulch as it simulates litterfall, or accumulation of organic matter under their canopies. Replenish the mulch as it deteriorates. Finally apply irrigation as needed through the mulch from the surface of the soil. This will help establishing roots, leach salts, and move mulch nutrients into the soil profile.  Avoid companion plantings near the main stem of the tree and avoid piling mulch around the tree stem. Following these guidelines will lead to a healthy and useful shade tree that provides its many services for decades.

Understanding the mysteries of plant diseases: Prevention, Control and Cure (Part 3 of 3 in this blog series)

Understanding the mysteries of plant diseases: Prevention, Control and Cure (Part 3 of 3 in this blog series)

What next?
You’ve done your research and made a diagnosis—now what? Sometimes the plant has to be removed and never planted there again. Start over, do something else.

Some diseases are difficult or impossible to control (virus diseases) especially when they are new or unknown to science.

Controlling plant pathogens or abiotic disorders can be daunting, frustrating, even impossible. As I mentioned in the last blog early detection gives more options for control because the disease has not advanced to a degree where it can not be controlled. Controlling plant diseases is not just palliative (treating your plant’s pain) it involves understanding where pathogens come from, stopping their movement, arresting their development and preventing their spread. Understanding genetics of resistance can offer amazing control of diseases, and finally biological control limits the development and spread of many pathogens.

Virus particles can only be seen with an electron microscope, since signs are hard to visualize, early detection is very difficult. Shown are TMV virions

What Can What Can’t?
There are some battles that can’t be fought or fought easily with plant pathogens. When plants are infected with viruses, there is almost no control option but removal (roguing). All plants likely contain some kind of plant virus; but not all viruses in plants cause symptoms of disease. Dangerous viruses like tomato spotted wilt, impatiens necrotic spot virus, cucumber mosaic virus, or many others, are devastating to their hosts and once infected there is no controlling these. Removing infected plants at the first symptom of viral involvement is prudent but often infections have already spread. Viral pathogens almost always infect without significant symptoms,  and become systemic in the plant before their more

Once trees with root rot show symptoms it is too late for control measures, the tree is dead and will not recover. Here Phytophthora spp. has killed a eucalyptus tree.

devastating effects become visible.  By late season, in most vegetable gardens, viral titre (concentration) is very high in solanaceae plants (pepper, tomato etc) and in cucurbits, both groups are highly susceptible. When perennial plants get viral pathogens there is no cure, and symptoms will increase over time. In orchids, viruses can sometimes be avoided by tissue culturing the meristem (which is usually virus free) to clean up a rare plant worthy of salvation. Sometimes plants are already dead but don’t look it. In the case of root rotted trees and shrubs, leaves may still hang from the tree, may still be green but the tree is beyond salvation, control may not be possible. In general control measures are best conducted early.  And by early I mean before you obtain your plant!

An old adage goes: “An ounce of prevention is worth a pound of cure” This is especially so when there is no cure!.  Prevention as a control technique really involves several factors such as exclusion, quarantine, and maintaining plant health so that plants are not predisposed to disease. The first tenet of control is exclusion. Don’t bring pathogens to your garden. Gardeners are their own worst enemy where plant diseases are concerned. Since pathogens can be seed-borne, come with insects, be already infected in the nursery, or resident in soil, care must be exercised when new plants are selected for your garden. Practice safe plant swapping!  Gardeners sharing plants with each other may also be sharing their respective plant diseases! Be careful where you buy plants, sloppy nurseries with their plants on the ground in standing water is a red flag.  Also be on the look out for weeds in nurseries since they can harbor insects that vector virus diseases. Plant debris left on the ground and not cleaned up, can be a source of fungal spores.  So consider the source when selecting plants for purchase.  Inspect plants carefully before purchase, especially slipping the container off to look at the root

All plants should be inspected carefully, especially root systems. Always remove a sample from containers to inspect. Here cyclamen are shown with root rot.

system. I don’t purchase anything (even boxed trees) without doing this first. If you are satisfied that you have a healthy plant then you are ready for the next phase of disease control.

Plants are often quarantined before they are released for cultivation or planting. When you bring your plant home, leave it in the pot for some time. Even bedding plants if purchased young can grow for a bit in containers. Remember lack of growth is a symptom of incipient disease.  Observe your new purchase of a few days or even weeks depending on the plant. If normal growth is occurring then move on to garden placement and planting. A little time set apart from other plants, and careful observation, will possibly prevent bringing something bad to your garden.

Once disease is established, and symptoms are apparent, gardeners often turn to pesticides to try to provide therapy. Sometimes fungicides applied to a plant post infection will slow down the spread of the pathogen within or on a plant. Therapeutic approaches can also turn on plant defense systems or enhance them so that the plant can limit the progress of disease. Therapy is usually not an option with most diseases because the pathogen has often gone beyond the point of stopping it by the time disease is recognized.  Some fungicides applied early, can be very therapeutic in turfgrass diseases, blights and powdery mildew diseases. The key to therapy as a control option is to detect the disease early and use an efficacious material that is labeled to control the pathogen you think is causing the disease on a given plant.  All this should be on the label.

Pruning out branches with cankers is a form of eradication.

An immediate response of many gardeners when disease is discovered is to kill the pathogen. This is eradication. Eradication takes several forms. There are eradicant pesticides, that kill the pathogen on contact. Usually these cause some degree of harm for the host since most pathogens have a host relationship that is destroyed when the pathogen is killed. Eradication can also be  or removing plants from the garden that are a source of disease. Picking up and disposing of fallen plant debris is eradicating a source of potential inoculum from the garden. Pruning cankered branches from a tree is a form of eradication.

One of the best forms of disease control is resistance. Selecting plants that resist disease is built in control. Diseases such as rust and powdery mildew have wide ranges of interaction with their hosts. By selecting plants that are resistant, there is no need for other control measures such as sprays. Resistance to plant disease comes as two types. Horizontal or multigenic resistance is partial or incomplete resistance and is conferred by several genes or their interactions in the host with the pathogen. Vertical or complete resistance is resistance conferred by a single gene in the host. Plants with horizontal resistance will get some of the disease but it won’t be overwhelming, often in grains, lack of resistance will result in complete crop failure. Plants with vertical resistance show no symptoms and are completely immune to the pathogen. While this complete resistance is appealing, it is only conferred by a single gene, and the pathogen can easily break down this resistance and cause disastrous disease. Horizontal resistance while not complete, is called “durable” resistance because it takes more time to overcome resistance conferred by multiple genes. The nature of resistance in garden plants is rarely detailed by growers or seed sellers. Often we are lucky to see any labeling for resistance. Crops like rose, crape myrtle and snap dragon are often sold as resistant to powdery mildew or rust and in some cases plant breeding programs strive to incorporate disease resistance into their breeding lines but then fail to label the product as disease resistant!

Cultural control often involves doing the right horticulture. Keeping turfgrass away from the stem of trees, applying arborist chips, planting at the right depth, are all cultural controls for root disease.

Cultural control is using good horticultural practices to limit the development of disease. Since many plant pathogens require a host to be predisposed, we have the opportunity through good horticulture to avoid the disease development. Planting woody ornamentals at the right depth is a cultural control of Phytophthora collar rots.  Appropriate application of water, reduces stress and prevents plants from being predisposed to both root rots and canker diseases. Correct pruning cuts limit the development of decay in trees.  Appropriate horticulture as discussed in the Garden Professors page will go far toward cultural control of common garden maladies.  Proper plant selection is also a form of cultural control. Choosing plants adapted to the growing area climate, and soils selects plants that are less likely to be predisposed to disease. Poorly adapted plants are more susceptible to pathogens and thus more likely to become diseased.

Biological control is the effect of one organism limiting the development of another thus preventing disease. Classical bio-control is when an exotic pest is introduced and there no natural enemies or parasites to regulate it. The pest/pathogen multiplies rapidly killing or affecting a large plant population. Research in the native range of the host looks for native organisms to control the pest. They are brought to the infestation, released and the pest/disease is brought into control. This works well with insects and the damage they cause. It has also been achieved with exotic plant pathogens.  For our native pathogens, there may already be a community of organisms that limit its development.  This is especially true for soil-borne pathogens. This is why the GP professors so often recommend fresh wood chips as mulch.  Fresh wood chip mulches supply carbon for organisms in soil that interfere with soil-borne pathogens; a kind of mulch-mediated bio control for root diseases.

I find controlling diseases is a lot more difficult than understanding or identifying them.  Usually by the time you have observed disease in the garden it is too late to stop its progress. You can take mental notes not to plant that variety again, or prune more diligently etc. but diseases are largely regulated or advantaged by the environment and our good or bad gardening practices. Of course the pathogen has to be present for biotic disease to happen, as we know that organisms don’t spontaneously generate.  Disease control starts with identification then research and finally gardening actions that help prevent, limit or eradicate disease propagules.

Feel the Heat: Temperature and Germination

 

In most parts of the country it is time to dust off the seed starting trays, pick out your favorite seeds, and get a little plant propagation going on.  There’s definitely a lot of science (and perhaps a bit of art) to successful seed starting.  While the process starts (and relies on) the imbibition of water, one of the biggest factors that affects the success, efficiency, and speed of seed germination and propagation is temperature.  Germination relies on a number of chemical and physical reactions within the seed, and the speed and success of those reactions is highly temperature dependent. Respiration, where the seed breaks down stored carbohydrates for energy, is probably the most notable process involved that is temperature dependent (source).   Think of it in terms of a chemical reaction you might have done back in your high school or college chemistry class – there’s an optimum temperature for the reaction and any lower and higher the reaction might slow down or not happen at all.

Thinking of it this way, seeds and germination are just like Goldilocks and her porridge – there’s too hot, too cold, and “just” right.  Seeds are the same way – there’s a “just right” temperature for germination. The seeds of each species has a different optimal temperature for germination with a range of minimum and maximum temperatures for the process.

Why is important that seeds are started at their optimal temperature?

The optimal temperature is the one at which germination is the fastest. This may seem to only have consequences for impatient gardeners, but slower germination speeds increase the days to emergence for the seeds, which in turns means that the seeds and seedlings have a greater chance of failure. The early stages of germination are when seedlings are most susceptible to damping off, which can be caused by a number of fungal pathogens (Fusarium spp., Phytophthera spp., Pythium spp., etc.) that basically cause the seedling to rot at the soil level. These pathogens (as well as decomposers in some cases) can cause seeds to rot or decompose before emerging as well.  That’s why you’ll sometimes see seeds that are slow to germinate (or traditionally direct sown like corn, beans, and peas) treated with those colorful fungicides.  The fungicide gives the seed and seedling a little bit of protection (for a week or so, depending on the product), which is handy if you accidentally sow them before soil temperatures are optimal or if the species is slow to germinate.

If emergence is really slow, there’s also the possibility of stunting or failure due to exhaustion of the stored carbohydrates that the seed relies on until it begins photosynthesis.  So the closer to the optimal temperature the seed is, the faster the emergence and the highest percentage of germination success.

Image of graph showing relationship between soil temperature and seed germination.

What does this mean for home gardeners?

Whether you are starting seeds indoors or direct sowing outdoors, knowing the germination temps can help increase your likelihood of success.  You can find a variety of resources for the optimal germination temperature for your selected crops.  In general, most warm season plants, like tomatoes, peppers, and summer flowers are in the 70-80 °F range.  This is why most of the warm season crops are started indoors – so temperatures can be controlled to higher levels.

For vegetable crops, here’s a good resource for basic germination temperatures.  And here’s one for a few annual flowers.

Many of the cool season crops germinate at much lower temperatures, which means many of them can be directly sown early in the season rather than started indoors.  Crops such as spinach, lettuce, and other leafy greens have these lower germination temps and typically perform better if germinated at lower temps.

Germinating a variety of plants for our 2018 All-America Selections trials

It should be noted that this is for the soil temperature, not the air temperature. If you’re starting seeds in your home, most people don’t keep their homes in the 75 – 80 degree range in the winter.  Many commercial operations use warmed tables or beds for seed starting, rather than heating the whole facility to the necessary temp – it would be expensive.  For home growers, supplemental heat mats can help increase soil temp without having to heat a whole room.  In a pinch, you can even clean off the top of your fridge and keep seedlings there.  It is higher up in the room (heat rises) and most refrigerators create some amount of external heat as they run.

For any seeds that you’re direct sowing outdoors, whether they require higher or lower germination temperatures, you’ll have more success if you plan your sowing around soil temperatures rather than calendar dates (planting calendars can be good for estimation, though).  Investing in a soil thermometer can offer detailed information on the specific temperatures in your garden soil.  Or, if you have a good weather station nearby many of them have soil temperature probes that could give you a good idea of what the soil temperatures are in your region.

Direct-sown lettuce germinating for a fall crop

But don’t let the cool/warm season crop designation fool you – the Cole crops like cabbage and broccoli actually have an optimal germination temperature on the warmer side, but grow better in cooler temperatures to keep them from bolting (flowering).  This is why they need to be started indoors for spring planting, but you can start them outdoors (even trying direct sowing) for fall crops – they germinate in the heat and then slow growth as the temperatures drop.