“Dirty Dozen” is one of those short, alliterative phrases that’s easy to remember and fun to use. In today’s post, I’m applying it to garden products whose production or use can be damaging to the health of ecosystems, environments, and even humans. How many of these products are in your garden shed, or appear in ingredient lists of other products? Each short description below has one or more links to additional information. After you count them up, see how you rank on the Charlotte Scott Meme-O-MeterTM
Product manufacture damages ecosystems
Peat moss. Peat moss bogs are slow-growing ecosystems that store vast amounts of carbon. Removing peat moss destroys these ecosystems (which can take centuries to regrow) and releases C02 into the atmosphere. Do a little experimentation with sustainably sourced or locally available crop residues and see if you can find a more environmentally friendly substitute.
Kelp. Kelp, or macroalgae, are the basis of intertidal and subtidal food webs. Removal of these plants creates underwater deserts where little life can be found. Restoration can be done, but it’s a slow and expensive process. Ask yourself why you think you need kelp and compare this to the facts.
Bat guano. Seems like a reasonable use of a waste product, right? Nope – and Dr. Jeff Gillman explained why in a post more than a decade ago. If you want a sustainably produced, manure-based fertilizer, you can find many options at garden centers.
Products whose use damages garden and landscape soils
Plastic mulch. There is nothing worse (see chart in this link) you can put on a living soil unless your intention is to kill everything under it. You may see it used in agricultural production, but that doesn’t make it a good choice for your gardens and landscapes. Again, there it a much better alternative to this and all other sheet mulches.
Products whose use damages plants and plant-associated microbes
Phosphate fertilizer. The most overused nutrient in home gardens and landscapes, and one that can cause iron deficiency in plants. The only time you should add anything containing phosphate – including compost or other rich organic material – is if you have a soil test indicating a deficiency.
Wound dressings. Unfortunately, many gardeners don’t understand that plants and people respond differently to injury. While antibiotic dressing and bandages are good for healing our nicks and cuts, trees have a completely different response. Slathering black goo or paint over tree wounds is the last thing trees need to seal damage naturally.
For many gardeners around the US (and the northern hemisphere) the weather is warming up for spring planting season and many are itching to get out in the garden. But when is the right time to plant those veggies and flowers and not gamble on their success? Just like Kenny Roger’s character in The Gambler, knowing when to do something is important (this is where I’ll end the cheesy comparison – you’ve just got to come up with a catchy title sometimes).
I’ve spoken previously on this blog about understanding frost dates, which is important for planting warm-season crops like tomatoes, peppers, and cucumbers that won’t survive a frost. (You can read that article here.) But another temperature factor we must consider, especially for cool season plants that we plant BEFORE last frost is the soil temperature.
Soil temperature is especially important for direct sowing seeds in the garden, but it can also affect the success of transplants planted in the garden. For transplants, having an appropriate soil temperature supports root growth and development and helps plants get established faster.
Why soil temperature mattersfor seeds
For direct sowing of seeds, soil temperature has a major effect on the speed of germination, which also affect the success rate of germination. Each different seed has a different optimum temperature for germination. If the soil temperature is below, or above, that level then germination can be slowed down. Slow germination can decrease germination rates through a few different avenues:
Germinating seeds are vulnerable to infection or decomposition by fungi and bacteria in this soil. When starting seeds indoors, this can be limited by using a sterile seed starting mix. But when direct sowing outdoors, there are any number of fungi and bacteria in the soil that will decompose a struggling seed/seedling. Some seeds sold for large-scale production will have a coating of fungicide on them that will provide a few weeks worth of protection. You won’t likely find this on home garden seeds, but it might appear if you buy seeds from a farm supply store or a catalogue that caters to farm-scale producers.
Seeds have a finite amount of stored energy. Once germination begins, the respiration rate of the embryo in the seed radically increases. If germination is slow, the embryo can expend the stored energy before the seed leaves emerge and start producing energy to support the developing seedling. (This also occurs if you plant the seed too deeply).
Below is a graphic I made for Nebraska featuring the best soil temperature range for major vegetable crops (notice how it also lines up with last frost dates). Just ignore the info on frost dates for Nebraska, unless you live in Nebraska.
Most of the resources you’ll find on soil temperature and germination are for vegetable crops. If you are trying to start seeds of ornamentals, you’ll likely have to find the information yourself. The seed packet will give you some indications of when to sow (before/after frost, or maybe in the fall for overwintering to break dormancy) and you can search online for guidance for specific plants. For info on starting seeds indoors, check out this previous article I wrote on the subject.
How to measure soil temperature
Of course, the tried-and-true old fashioned way is to use a thermometer. You can find a soil thermometer at many garden centers and retailers. You’ll want a soil thermometer because the ones for your kitchen likely don’t have the right temperature range – we’re measuring well below the temperature of a roasted chicken here. Instert the thermometer two to three inches into the soil and wait several minutes for the temperature arm to adjust before reading. Also keep in mind that temperatures fluctuate with the weather and throughout the day depending on temperature and the amount of direct sunlight hitting the soil surface – so you want to measure a few times to make sure the temperature is staying within the right range.
Now, the new technological way is to find a soil temperature monitoring station online. Soil temperature monitoring is a common feature of many weather stations these days and data is more available than ever. Many university extension services or ag research centers compile soil temperature maps for use by farmers and this data is also often accessible through NOAAA or weather.gov. In Nebraska, we have an extension program called CropWatch that provides average daily and weekly soil temperatures year round. We also have a weather station with soil temperature probes at our extension office (perks of having a meteorologist as a master gardener volunteer) and we (and our master gardeners) use it when providing information to clients. It can be hard to find a resource that provides soil temperatures nation-wide to share in the patchwork of private and public stations (and the National Weather Service site can be notoriously hard to navigate). There are a few online resources from the ag industry that provide a country-wide system, like this one.
And now its time for me to walk away, time for me to run
Unlike The Gambler you don’t want to wait until after the plantings done to count your money, er, check the temperature. Remember that knowing the soil temperature, whichever method you use, will help your plants succeed in the garden. If you do, your garden could pay out bountiful winnings all through the season.
I know what you’re
thinking: “No Mow May? But it’s March!” Although spring currently seems like a
distant wonder for us here in Montana, many of our warmer states are seeing the
signs of spring that get you thinking about, among other things, your lawns
(and if you’re a bee enthusiast like myself, you’re also thinking about the
early season pollinators that are beginning to emerge in these landscapes).
What is ‘No
No Mow May (or April, or March- depending on where you are) is a movement that was originally made popular through an organization based out of the United Kingdom called Plantlife. The intention behind the campaign is to eliminate mowing your lawn for the month of May, with the goal of creating habitat and floral resources for early-season pollinators. This initiative has since become more and more popular in the United States in addition to other countries, where we see the classic image of a turf lawn speckled with bright yellow dandelions along with signage that says ‘No Mow May’.
Lawns are a staple of American landscaping and one can hardly imagine urban and suburban areas without the iconic image of the green turfgrass yard associated with many homes. We have about 40 million acres of lawn, which accounts for 2% of all the land in the continental US, making lawns the number 1 irrigated crop in the country! The ideal vision of a good lawn has long been a weed-free, monocultural, uniform green space and people spend a lot of time, effort, and money to maintain them in this way. The image of overgrown, non-uniform, unconventional, and weedy lawns have been historically considered to be unattractive, unkempt, and poorly managed. Although more and more people seem to be changing their opinions and preferences for these conventional turfgrass lawns (and we likely need to rethink some of these expectations anyways), most people still have those underlying perceptions.
The idea behind No Mow May is that flowering lawn weeds, if left unmown, would provide food sources for early season pollinators such as newly emerging native bees (with the added benefit of less work in terms of lawn maintenance). The primary targets of this initiative are generally more urban and suburban areas where food sources and habitat for pollinators can be harder to find and spaces where weedy lawns are less tolerated.
Although the intention behind this campaign is a good one, providing food for pollinators, there are some aspects of this initiative that seem to miss the mark.
So let’s get
into some of the science!
Is ‘No Mow
May’ good for pollinators?
The answer to
this question is: yes and no.
Studies have shown that certain common flowering lawn ‘weeds’ can be an important food source for pollinators, especially in urban and suburban areas where other floral resources can be scarce. A paper from the University of Kentucky found 50 species of pollinators, including 37 species of bees, foraging on white clover and dandelion in lawns (Larson et al., 2014). Although white clover is a nutritious source of nectar and pollen for bees, dandelions on the other hand are not very nutritious (with low protein content in their pollen). That being said, bees and other pollinators will still forage on dandelions especially if other floral resources are unavailable at that time. In the ideal world we would have a plethora of floral resources for pollinators which would incorporate an abundance of diverse flowering plants from early spring all the way through the growing season and into the fall. Unfortunately, most urban and suburban spaces do not meet the specifications of this ideal pollinator-friendly world. Dandelions, on the other hand, are pretty universal in these urban areas (and you have likely seen them in many lawns in your neighborhood). Although they are not a great resource, abundant dandelions can fill the gaps of food sources for bees (especially when the alternative is no food). Other lawn ‘weeds’ and bee lawn plants (as you have seen in last year’s blog post) can also support a diversity of pollinator species and can be a great way to offset the lack of pollinator-friendly resources in a turf-only lawn.
Research from a pilot project of No Mow May in Appleton Wisconsin evaluated the bee species richness and abundance in properties that participated in the initiative and found the species richness was 3 times higher and the abundance was 5 times higher when compared with areas that had been mowed (Del Toro and Ribbons, 2020).
However, if left unmown: your turfgrass will soon outgrow any other flowering lawn weeds which can make them difficult to find for pollinators, and that is if these flowering lawn weeds aren’t smothered by the tall grass altogether. A study conducted in Massachusetts which evaluated the impact of mowing frequency on bee abundance and diversity showed that lawns which were mowed once every 3 weeks had 2.5 times more flowering plants, though lawns which were mowed every 2 weeks had the highest bee abundance, likely due to easier access and visibility of these floral resources in shorter grass (Lerman et al., 2018).
Is ‘No Mow
May’ less work?
The answer to
this question, as you probably could guess, is also: yes and no (but
I am a self-proclaimed ‘lazy gardener’ and am always looking for ways to reduce the amount of effort I need to put into my gardening endeavors. No Mow May claims to be less work for you in the month of May. Although that is true (because the intention is not to mow for a month), it can often result in more long-term effort in trying to reign in your turfgrass in the after-effects. Not to mention the negative impact that this style of management (or lack thereof) can have on your turf itself.
In many places, lawns can grow a foot or more during the month of May. An un-mown lawn can set you back on your lawn maintenance and result in more work for you in the long run . Then when you are trying to get your lawn back on track afterwards- remembering the rule that you should only remove 1/3 of your turf in any given mowing, it could take weeks to get back to a good functional height (which varies depending on the type of grass, but for most of our lawns, it ranges between 2.5-4 inches). Furthermore, most mowers don’t have the capacity to handle a lawn with a 12 inch height. This will result in removing too much of the grass leading to unintended consequences like stress and decline of your lawn, which can also make it more susceptible to pest and disease issues. This will also lead to a large amount of grass clippings which, if left on the lawn, could smother sections of it and result in an unpleasant and patchy lawn.
Unhealthy lawns can also struggle to compete with and leave spaces open for other undesirable invasive plants, including noxious weeds. These can then spread to other areas and have significant ecosystem impacts (and make it even more difficult to get your lawn under control). Following weed management needs could also require the use of herbicides that can have unintended consequences on pollinators and other beneficial insects.
noted in an article on No Mow May by Iowa State Extension’s Consumer Horticulture
Specialist, Aaron Steil: lawns are not actually a natural space, as most lawns
are constructed out of non-native grasses, usually for functional purposes, and
require quite a bit of maintenance including water and nutrients. Leaving your
lawn unmown will not make your lawn a more natural landscape, and it would not
be a responsible way to manage these non-native landscape plants.
What you can
do instead of No Mow May: Low Mow May!
Based on what we’ve learned so far: lawn weeds can sometimes be an important food source for bees (especially in urbanized areas, where the diversity and availability of floral resources are fewer) and mowing less frequently results in more of these flowering lawn weeds for bees. We also know that slightly higher (though not too high) mowing heights for many lawn turf species make for healthier root systems and make turf more resilient to stress, pests, and disease issues.
If you have flowering lawn weeds and pollinator conservation is your intention, your best bet would be to aim for a sweet spot between the extremes of mowing way too frequently and not mowing at all. Mowing every other week could be a way that you can reduce the amount of time spent mowing and also support urban and suburban pollinators by letting your lawn weeds flower (in addition to maintaining your lawn at the recommended heights for healthy turfgrass).
Even better yet, you can reduce the amount of space in your landscape that is dedicated to a traditional turfgrass lawn and incorporate a flowering groundcover and/or a pollinator garden that hosts an abundant array of diverse floral resources that provide food for bees all season long!
something to be buzzing with excitement about!
If you’ve been following the national weather this week, you
might be wondering if the groundhog has developed a split personality this
year. Is winter over or are we in for six more weeks of cold? While the eastern
half of the United States is feeling the effects of record-setting high
temperatures and one
of the earliest springs on record, the western U. S. is observing cold
and snowy conditions all the way down into Southern California. I’ve heard
reports of snow reaching all the way down to Tucson in southern Arizona.
What is an atmospheric wave?
Some of you may be wondering how we can have some tremendous
differences between the eastern and western U. S. at the same time. The reason
we see such differences is linked to the wavy pattern of the upper atmosphere,
which helps direct warm or cold air into different regions. Today I will
discuss atmospheric waves and how they affect the surface weather.
Since air is a fluid, it should not be surprising that it has
waves in it. In fact, there are
many different types of waves that can occur in the atmosphere, ranging
from small-scale pressure waves downwind of mountain ranges to the largest planetary-scale
oscillations in pressure. Sometimes you can see some of those smaller-scale waves
in clouds occurring above you.
Where do atmospheric waves come from?
What ultimately drives the atmospheric circulation is the temperature difference between the equator which is warm due to direct sunlight and the poles which are cold because whatever little sun they get, especially in winter, is of low angle and there is not much solar energy reaching the surface to heat things up. This temperature difference causes a difference in atmospheric pressure that makes winds blow to try and equalize those differences.
Variations in land versus sea, the rotation of the planet,
and differential heating from other causes like drought all contribute to the
atmospheric developing waves that look like large swings in the pressure
patterns at mid-levels in the atmosphere. Areas where the pressure is low are
called troughs and areas where it is high are called ridges, like the patterns
we see in topographic maps. Surface fronts are located near the boundary
between troughs and ridges where the contrast in temperature and humidity is
often the greatest.
How is the current wave pattern driving these big
This week there is a tremendous ridge of high pressure located in the eastern US while a very deep trough is located over the western part of the country. Warm air blowing up from the south under the ridge has brought record-setting high temperatures to many parts of the East, while cold air blowing in from the north behind a strong surface low pressure center has caused winter storm and blizzard warnings all the way south to the mountains near San Diego. Record-setting low temperature and wintry conditions are occurring in those areas.
The warm temperatures in the East are of special concern for gardeners since they have caused the first leaves and blossoms to occur as much as a month early in some locations, according to the National Phenology Network. Since the average frost date for those areas is a month or two later, the likelihood of damage to fruit crops is high since freezing temperatures could significantly reduce the production of fruit in gardens and orchards if they occur in the next month or two. It could also cause damage to many other flowering plants that are fooled into blooming early due to the unusual warmth.
Will atmospheric wave patterns change in a warmer world?
Is the unusual warmth due to global warming? This is a hotly debated topic (if you will excuse the pun) with both proponents and skeptics weighing in. Many scientists believe that as the arctic regions warm up faster than the equator (a process known as “Arctic amplification”), the atmospheric wave patterns will become more amplified, with both deeper troughs and stronger ridges occurring. That could mean more extremes in both hot and cold weather as these waves occur. Other scientists caution, however, that the computer models used to study the large-scale wave patterns don’t always agree with global warming as a root cause. If we do have generally warmer conditions that are punctuated by significant cold outflows our gardens will have to be able to survive the increasing variability of the weather, especially in winter and spring when there is still a significant pool of cold, dry air near the poles available to flow south into mid-latitudes.
Please feel free to share how this crazy weather has
impacted your own garden. Are your trees blooming early, or has snow covered
everything? If you are not in the United States, what do you see going on in
your own neighborhood? We are interested to know!
It’s still too cold here in the Pacific Northwest to see much happening outside, so it seems a perfect time to write about something you can’t see anyway. That “something” is the movement of water and dissolved substances through two pathways: the xylem and the phloem. And before you roll your eyes and go watch TikTok videos, keep in mind that learning about these transport systems is critical to understanding how plants work and caring for them appropriately. To prevent brain overload, we’ll focus on how xylem works this month and tackle phloem next month.
Functional xylem is composed of dead, lignified cells connected into a series of tubes that move water one way – from the roots to the leaves. You can think about xylem like a giant straw sucking water out of the soil and moving it into the atmosphere. You’ll find dissolved substances in xylem water, such as soil minerals and root-stored compounds including growth regulators and sugars. Since this is a one-way highway, everything in the xylem ends up at the end of the straw, which is primarily the leaves. Most of the water dissipates into the atmosphere through the stomata (a process called evapotranspiration) and the dissolved substances are left behind.
I mentioned that sugars can be found in the xylem, which will confuse gardeners who correctly associate sugars moving through the phloem. That’s generally true except during late winter when some trees, most famously maples, will produce a sugary sap in the xylem. While the exact mechanism of sap production remains unclear, we know that the sugars are coming from storage sites in the trunk and require a freeze-thaw cycle to enter the one-way xylem highway.
people see this process as the plant “wasting” water, it is the only way that
soil minerals can reach the leaves. In the summer, evapotranspiration lowers
leaf temperature through evaporative cooling. Thus, doing anything to interfere
with xylem function (like using antitranspirants) will have a long-term,
negative effect on plant health. Likewise, anything in the soil that’s taken up
by roots may end up in the leaves – for better or worse.
Gardeners need to think about this last caveat carefully. Plant species are highly variable in their abilities to regulate what goes into the xylem and what is left behind in the root tissue. Regulation is controlled by a barrier called the Casparian strip, which is a ring of living cells that require water (and its contents) to pass through their membranes to enter the xylem. You can think of the Casparian strip as a customs office at a country’s borders: some things are allowed in, and others are forbidden. Depending on how selective this border crossing is, soil contaminants can be left behind in the roots or carried through the plant. This is why it is so very, very, important to have your vegetable garden soils tested for heavy metals and other contaminants, and to take precautions if contaminants are found.
Over the last few months I’ve had the chance to talk about the popular topic of home greenhouses. We covered a few of the basics in my first article, then touched on some regulations that might effect the building and management of home greenhouses in some areas. In this installment we’ll talk a bit about common structures used for home greenhouse construction so you can consider which structure(s) might be right for your situation.
Types of Structures
Greenhouse structures can range from simple to the very complex. While home greenhouses tend to fall on the simple construction side, there are still varying degrees of complexity within structures. Greenhouse structures can be put into a few main categories that we’ll cover below. The key to home greenhouse success is picking the structure that works best for you, your situation, and your budget.
Lean-to structures get their name from the fact that they “lean” on something for support. Not in a literal sense, but in a sense that another structure, usually a house or maybe a storage building, provides at least one structural wall. A common lean-to setup would have a three-sided greenhouse structure attached to the side of a house or another building. This could range from a structure that is a few square feet for starting seedlings in the spring up to a full-size greenhouse attached to the side of a building. A sun-room, if equipped for starting plants, could be considered a lean-to greenhouse (at least it would at my house, since it would be primarily for plants). This type of structure can be cost effective. You reduce the amount of greenhouse surface you have to build and cover, unless you have special issues for connecting it to your home or other structures (like foundation issues, siding that is hard to attach to, etc). You do want to make sure it is done right especially if attached to your house so you reduce the chance of damage to your home.
One benefit to a lean-to is that it can use the wall it is attached to as a heat sink – the wall absorbs heat through the day and then slowly releases it at night when it is colder. If you have a large lean-to greenhouse that serves almost like a sun room you also add functional space to the house where you can enjoy the sun on warm winter days. The heat generated by an attached greenhouse can help provide warmth for your home in winter or at least provide some extra insulation. But it can also result in excessive heat gain in the summer. Lean-to greenhouses are also protected a little more structure wise – they use the building they are attached to for structural support and can often withstand weather, like high wind, a little better than free standing structures.
Free standing structures
Simply put, a free-standing structure is one that isn’t attached to another structure. It stands on its own structural framework. There are a few different options we’ll talk about here which will be further expanded in future article installments on different types of materials used.
A hoophouse greenhouse is built out of the same structure used for a high tunnel or hoophouse. It consists of framing made by bent metal pipes and is covered with polyethylene plastic sheeting. A high tunnel greenhouse would have heating, ventilation, and watering equipment added and would therefore require connection to electric, gas, and water utilities (see my previous article about what this means for a greenhouse being a taxable “permanent” structure). You may also pour a concrete slab for the floor of a hoophouse-to-greenhouse conversion, but gravel or any other floor covering is fine as long as it provides a stable surface and weed control. The “greenhouse” that I inherited from the previous homeowners was left with bare ground so they could plant it like a conservatory.
While these may not be the most attractive greenhouses, they are usually a more cost-effective option for a high-quality, efficient greenhouse. Initial construction costs will likely be lower for a comparable quality framed greenhouse though the polyethylene covering will need to be replaced every few years as it becomes more opaque and light transmission reduces.
When envisioning a greenhouse, many people envision this type of greenhouse – one with straight sides, built with a frame covered in solid material. These are definitely more attractive than hoophouse greenhouses and can add an attractive feature to your yard. The most common way for home gardeners to build a free-standing greenhouse is through purchasing a kit. There are a myriad of kits available online these days, of varying prices and qualities usually depending on the materials used in the construction. The kits can be complex but set-up is usually pretty easy to follow though you might want to factor in the cost of hiring a contractor to help with the construction on a bigger kit. Most often these kits need to be built on a concrete foundation or pad so you’ll want to consider the cost and logistics of pouring the needed support. And you’ll also want to think about the utility connections to the greenhouse.
Ridge and furrow greenhouses are framed greenhouses that are built together in tandem. These structures are usually used only in large-scale commercial production, so I’ll skip those for home greenhouses.
Geothermal greenhouses are gaining in popularity, mainly because people are excited about reducing the need for using electricity or gas to control the temperature. And I say reducing, because in extremely cold weather there will likely still need to be supplemental heat. These structures are definitely different than your standard greenhouse. These structures are usually sunken into the ground to take advantage of insulation by the soil and also the constant geothermal temperatures. There is also usually a high solid wall on the north side of the structure, with a slanted roof made of light-passing material that faces to the south. This allows maximum light to enter the structure and allows that light to heat the solid wall to hold and release it during the night when it is colder. There is also usually an air intake in the ground a few hundred feet away that allows for the pulling of air through a tubing system to warm or cool the air with geothermal temperatures, depending on if it is cold or hot outside.
A walpini greenhouse is a specialized/rudimentary style geothermal greenhouse that is sometimes referred to as a pit greenhouse. Instead of building a structure, it is constructed of a pit dug into the ground and covered with a transparent roof. While they sound simple, there are several drawbacks to these structures. One – they don’t do well in wet areas or areas with high water tables, for obvious reasons. Secondly, they were initially designed for use in areas near the equator with the sun almost directly overhead. To function in latitudes far north or south you almost always have to build up a wall on one side to slant the roof cover toward the sun. And even then it is unlikely that light will reach the floor of the structure where plants are growing.
There’s a lot to consider when picking the type of greenhouse structure for your home greenhouse. There’s often a trade-off between cost, quality, and aesthetics. Options range from the simple to the complex in terms of size, structure, and function. So think about the goals you have for your greenhouse and consider the many different options available to you.
If you have fruit trees in your garden, then you may already
be aware of the importance of winter cold for the development of blooms and
fruit. Some fruit varieties like apples, cherries, peaches, and blueberries all
need a certain number of hours of temperatures below 45 F to prepare the trees
to bloom in the spring. Surprisingly, other trees you might not think of as
fruit trees also need a period of cold conditions to produce a good crop,
including pecans and olives. Many other garden plants also require cold periods
to provide the best blooms, such as lilac and hydrangea. I miss seeing lilacs
here in the Southeast because we just don’t get cold enough to meet their chill
requirements, although new varieties that can flourish in warmer conditions are
Why do some plants need winter cold?
The cold conditions over winter cause the plants to go into
dormancy. That helps protect the plants from harsh conditions over the winter.
Once the plants experience the number of chill hours required for that plant,
they are ready to begin the blooming process once the temperature warms up. If
the fruit trees do not get enough hours of cold over the winter, they do not
bloom well in the spring and blossoms may be delayed or bloom at irregular
times or not bloom at all. Leaf emergence may also be affected. The potential yield
of fruit is reduced due to the lack of a strong uniform bloom.
Scientists count the number of hours a tree or orchard is below 45 F and measure the accumulation of those hours as “chill hours.” Some publications call them “chilling hours” instead. They are usually accumulated starting around October 1 and go through the winter into the spring bloom period when bud break occurs. An alternate method for calculation only counts the hours between 45 and 32 F. Chill hours are different from cold hardiness, which is the lowest temperature that a tree or plant can tolerate without dying. Recent research has allowed scientists to develop more sophisticated methods that dynamically calculate “chill portions”—these calculators take into account variations in temperature over time and can reset the calculation if cold conditions return after a warm spell.
Picking the right fruit variety for your garden
Most types of fruit other than citrus and each variety has a preferred minimum number of chill hours for that variety to set a good fruit crop. If you live in the Southeast, you may be able to plant varieties of blueberries that require only a few hundred chill hours, where in the north you may be able to plant a more cold-hardy variety that requires over 1,000 chill hours for the best bloom. Because of the cold outbreak we had this December, some of our blueberry varieties are already blooming this year in the warmer weather we have had in January.
If you are planning to plant a fruit tree in your garden, you will want to pick a variety that matches the expected number of chill hours for your area. If you plant a variety that expects a lot of chill hours but your location gets only half that amount, you will not get very good yields from your trees since the plants won’t break dormancy correctly. But if you pick a variety that requires a low number of chill hours for your area, then the tree will be ready to bloom as soon as the chill hour requirement is met, which could put them at risk for a frost if they bloom too soon. So picking the right variety for your area is crucial! Most nurseries can provide the recommended number of chill hours for the fruit variety you pick.
In the future, as temperatures get warmer under the influence
of greenhouse warming, most areas will see a decrease in the average
accumulation of chill hours over time. We are seeing this at my weather network
stations in Georgia and in many other states as well. If you plant something
that has a long lifetime, you may want to plant varieties that require fewer
chill hours than your current climatological average to make sure they will
thrive in a warmer climate in the future.
Where can you find information about chill hours for your
For average numbers of chill hours for the United States, check out the map below. Keep in mind that your local microclimate will affect the number of chill hours your garden will receive.
Here are some sources of current chill hours for
If you are interested in this topic or have other questions
about gardens and gardening, we encourage you to explore our archive of blog
posts to see if you can find the answer to your questions. Here are a few of
the previous stories about winter chill you might find interesting:
I received an email this week from an arborist colleague who had been sent an “engineering solution” which claims to help with rooting issues in clayey soils or areas where root area is reduced. There was a spiffy diagram accompanying this which I’ve reproduced below.
I could dissect this for you and point out all the problems right now, but instead I’d rather supply you with some factual information and let you apply it to this “engineering solution.”
Planting hole material that is not the same as the surrounding soil will have reduced water, air, and root movement due to the abrupt changes in texture. The hatched material in the pit appears to be different from the surrounding soil, leading to the assumption we’ve got modified backfill. Here’s a peer-reviewed journal article that discusses the fallacy of soil amendment.
“Augured sump drain/root channel bores” are simply modified French drains. French drains serve to move free water (i.e., water that is not in soil pores) somewhere. Where “somewhere” is in this case is unclear.
French drains and other drainage systems do NOT reduce the amount of water that soil holds. Field capacity is the term used to describe a saturated soil. A sandy soil has a low field capacity and drains quickly. The higher the clay content of a soil, the higher the field capacity and the slower the drainage.
Last month I dipped into some of the considerations of owning or building a home greenhouse, having just “inherited” a greenhouse with my new house. The questions and comments were numerous, so I decided to continue on discussing considerations for home greenhouse ownership and operation. There will likely be a few more articles down the line….so strap in.
This time around, I thought I’d talk about something that we
hate to talk about, but is important to understand: rules and restrictions
around what you can and can’t do, especially if you live in an urban area or
I know, I know. We
all just wish we could do whatever we want, but when we live in close proximity
to others there are usually some sort of rules we have to follow to keep the
peace. I have a little better understanding
of how it works, now that I helped write an urban ag ordinance with our city
planning department (fingers crossed is passes city council next month!) that
will “decriminalize” urban farming, including controlled environment ag
structures like greenhouses. Of course,
aside from what you can and can’t do there are tax implications that you should
be aware of, no matter where you live (more on that in a bit).
Zoning, Ordinances, and Planning Commissions – oh my!
Most agricultural activity, which includes greenhouses, is
likely regulated to some extent if you live within a municipality. Even if you don’t live in a town or city,
there could be certain building codes or ordinances on county or state levels
that you’ll want to check into.
You’ll want to look up your municipal code online, or call
your municipal administration (like city hall, mayor’s office, planning office,
etc.-whatever you have) to see if there are specific allowances or restriction
for or against greenhouses. There are a
few scenarios that could play out here, so you’ll want to be prepared on how to
proceed. In general, you might find that
Permitted, but only in certain zoning areas
(more on that later)
Permitted, but requiring a permit of some sorts
Not mentioned at all
If you fall in that last category, then anything you do will
exist in a gray area, where it isn’t strictly legal or illegal but the fate of
your greenhouse (or any fines you might incur) would be up to interpretation by
whoever is in charge of compliance (and what side of the bed they woke up on
that day). And your greenhouse could be made
legal or illegal at any time in the future if some sort of ordinance or
decision is made. In the case of our new ordinance here in Omaha, greenhouses and
structures like high tunnels will be allowed, but only in certain zoning
areas. People who already have structures
(and have for years) will now find that those structures are either allowed or
banned (or will require a permit). Of
course, enforcement of issues like this are often complaint-based, meaning that
the municipality probably isn’t driving around looking for your greenhouse but
if you and your neighbor aren’t on good terms they can call and turn you
As for zoning, there are several different categories and
sub categories. The ones you’re most
likely to encounter are residential, commercial, industrial, and agricultural (there
are others as well). And then there are
usually numeric (or named) sub-zones within those zones denoting the density or
intensity of use on those zones.
For example R-1 (Residential 1), might be for a single-family dwelling whereas R-3 would be for a small apartment building. C-1 (Commercial-1) might be for a single-use building like a stand-alone fast-food restaurant and C-8 might be for a skyscraper office headquarters. Municipalities use these codes to denote what is and what is not allowed in certain places. For example, the code we worked on for Omaha allows certain types of farming on all residential zones with a conditional use permit and on commercial zones without a permit. (Home gardens and even community gardens are not considered farms and are allowed on all residential zones without a permit.) Many municipalities may also have agricultural zoning on properties around the periphery but still within the city/town limits. On lots like this, most agricultural practices are allowed.
Since a greenhouse is a structure, there may or may not be a requirement for a permit for the structure, even if it is allowed. This could be dependent on a number of factors, including whether it is considered a permanent or temporary structure, whether there are utilities going to it (which is usually the case for a greenhouse), or even how the structure is constructed. In some areas, it comes down to how pretty or ugly the city and your neighbors think the structure might be. In discussions for our ordinance, there was talk of not requiring a permit for a greenhouse but requiring permits for structures like high tunnels/hoophouses because there’s an (incorrect) assumption that greenhouses are prettier than high tunnels. But many people make greenhouses out of those structures (the greenhouse I inherited is just a high tunnel with a space heater). There was also an assumption that a greenhouse wouldn’t have loud, flapping plastic but a greenhouse would (I kid you not).
The tax man cometh….for your permanent agricultural
One other thing you’ll want to consider about a greenhouse has to do with the tax bill. Since greenhouses are considered permanent structures, many places consider them a permanent improvement to your property and will add them to the tax bill like they might do an out building or garage. The municipality will definitely know about it if you have to apply for a permit or if you get a visit from the assessor. But this could also be dependent on how your structure is built. If you build a solid sided greenhouse (one of the nice looking ones) with a concrete slab floor, permanent utilities, etc then it will definitely considered permanent. But if your structure is more like a high tunnel/hoophouse like mine (a high tunnel in a trench coat pretending to be a greenhouse) then it could be considered a temporary structure and not taxable.
High tunnels are considered temporary since you could really take them down at a moment’s notice (and some planning department officials think this means that you take them down and put them up each season, despite how much an extension educator tries to convince them otherwise….but I digress). So your mileage may vary depending on your local rules and how your structure is constructed.
So what home greenhouse topic should we cover next? I’m sure there’s plenty of questions out there – be sure to leave them in the comments. Also – would you like to have your greenhouse featured in a future article (this means I don’t have enough home greenhouse pictures)- feel free to send them to me at john dot porter at unl.edu.