water – The Garden Professors™

Plant lists that shouldn’t exist

Nothing drives me crazier than simplistic solutions to complex problems. Given our changing climate, there has been an explosion of “drought tolerant” and “firewise” plant lists in the gardening world. Most of these lists are devoid of science and all of them are removed from reality. The fact is that taxonomy plays a minimal role in determining whether a plant will tolerate environmental extremes.

**Lack of irrigation and mulch guarantees a drought-stressed landscape regardless of the selected species.**

Let’s start with the most obvious problems with these lists. The goal isn’t to have plants that require less additional water – it’s to have a landscape that requires less additional water. Similarly, the relative flammability of plants is less important than whether the landscape surrounding those plants is protected from fire. Plants don’t exist in vacuum and unless you are strictly a container gardener a single plant’s impact on water use or fire resilience is negligible. So a gardener’s questions should be “How can I make my landscape more drought tolerant? How can I reduce the likelihood of wildfire damage?” And these are questions that can be addressed with knowledge gleaned from applied plant and soil sciences.

Drought Tolerance

***Arborvitae can tolerate droughty summers, but they don’t tolerate improper planting and management.***

First of all, let’s think about what “drought” really means: it’s an unusual lack of rainfall. It doesn’t mean no irrigation, and it doesn’t mean dry soil. Drought is a climatological term, not one associated with soil water management. Fine roots and their root hairs require water to function. Without sufficient soil water plants will go dormant or die, particularly during establishment. Plants that are drought tolerant can tolerate seasonal lack of rainfall, but they can’t tolerate chronically dry soil conditions.

***Even “drought tolerant” species like Sempervivum will die if there’s not enough soil water.***

So we need to look at the landscape factors that allow plants to survive droughts. This includes

Root systems that are well established. This means no barriers between the roots and the landscape soil system. Barriers include soil amendments and any materials left on roots during transplant (like soilless media, clay, and burlap). Obviously proper planting is key.
Adequate water movement into and within the soil environment. Anything within the soil environment that creates a textural barrier, like soil amendments, prevents water movement. Anything on top of the soil environment that creates a physical barrier, like sheet mulches or compacted layers, prevents water movement into the soil. Sheet mulches include plastics, fabrics, cardboard, and newspaper.
Adequate irrigation to support all plants in the landscape. The easiest way to determine whether there is enough soil water is to focus on one or two well-established indicator plants that you notice are the first to show wilt in the summer. That’s when the irrigation should be turned on. For our landscape in Seattle, it was a south-facing hydrangea.
Properly mulched soil. Mulch is crucial for soil and plant health, especially in terms of soil water retention and temperature moderation. The best choice for a tree- and shrub-dominated landscape is arborist wood chips. The best choice for arid landscapes is stone mulch – but if this landscape is dominated by trees and shrubs, you need the wood chip mulch. Trees and shrubs, by and large, are not the dominant plant form in arid environments. If you are going to grow plants out of place, you need to include the mulch that matches.

**The broad, thin leaves of hydrangeas lose water rapidly and make a good indicator plant for water stress.**

These four environmental conditions are key to maintaining a drought-resistant landscape. In terms of appropriate plants, just realize that plants with small, thick leaves lose less water than those with broad, thin leaves. If you want a landscape that conserves water, by all means choose plants whose evaporative water loss is the least.

Firewise Landscapes

***Jack pine (Pinus banksiana) produces cones that require fire to open and release their seeds.***

I’m not crazy about the term “firewise” as it’s not really a science-based concept. There are natural landscapes that routinely experience fires, and plants native to these landscapes have evolved mechanisms to survive moderate fires. Trees with thick bark, for example, can survive fires that are low to the ground and quick to move through. Other plants may perish in a fire, but leave behind fire-resistant seeds that are able to germinate after the next rainfall. This is not what’s meant by a firewise landscape. Instead, the premise appears to be selecting plants that are low flammability. (Jim Downer tackled this one a few years back but the message just isn’t sinking in.)

***Failing trees of any species are more flammable than living trees of any species.***

Once again, the focus of this approach is mistakenly directed to plant selection rather than landscape resilience. The best way to reduce the risk of fire is to have a landscape filled with healthy, hydrated plants and a soil protected by the least flammable mulch. The two mulches recommended for drought tolerant landscapes also happen to be the least flammable: stones and arborist wood chips.

Bark mulch is hydrophobic and flammable in the landscape.

Despite published evidence that arborist wood chips are not very flammable when compared to all other organic mulches, many governmental groups specifically recommend against them. This is a problem. Stone mulches are great choices IF the plants in question are native to arid zones. Trees and shrubs that are not from arid zones generally require the presence of woody debris to enhance mycorrhizal and root health. Without the proper mulch, these woody plants are less healthy and likely less hydrated than their counterparts under arborist chip mulches. That makes them more, not less, susceptible to fire damage.

***A stone mulch in a southwestern desert landscape.***

Most of the confusion around arborist chip mulches is probably the result of regulatory agencies confusing bark mulches with wood chip mulches. Bark mulches ARE flammable as they contain waxes and are not great choices for root and soil health. They should be avoided. Agencies associated with fire control methods need to be better informed about the significant differences between these two types of mulches and how they affect plant resilience.

***The wildland-urban interface faces the risk of extreme fire danger. [Photo byAnthony Citrano]***

And finally, it is important to understand that major wildfires are going to burn anything that’s organic. If you live in such an environment, the best thing you can have in your landscape is no plant material of any sort. A buffer of stone mulch is the only logical option.

Be a Part of History!

Here at the Garden Professors we pride ourselves on being on the cutting edge of technology. In fact, we’re so tech savvy we didn’t even whine when FaceBook foisted a new homepage format on us for no apparent reason. So it’s only fitting that we offer you, Mr. and Ms. Garden Professor Blog reader, an opportunity to participate in the first ever landscape horticulture research project designed by social media.

Here’s the deal. My current research project on water and nutrient management of trees in container production has left us with over 100 ‘Bloodgood’ London planetrees in 25 gallon containers. What I need from you are ideas for a study plan on what to do with the trees next.

Of course, as with any major research project, the first step in the rigorous scientific process is to come up with catchy acronym for the study. I propose “the SOcial MEdia DEsigneD TRansplant ExpErimental Study” or SOME-DED-TREES for short. Needless to say, I am willing to consider alternatives. In any event, we have a unique opportunity to investigate post-transplanting growth, development and physiology of landscape trees.

So here’s what we have: Approximately one hundred, 2” caliper trees, grown in containers in a standard mix of 80% pine bark and 20% peat moss. Trees have been grown for two years in essentially standard nursery culture – daily irrigation and 60 grams of Nitrogen per container. The subject of the original study was fertilizer source; half the trees were fertilized with Osmocote and half received the same amount of nutrients from organic fertilizers. After two years we have not seen any difference in growth or foliar nutrients between the treatments. Nevertheless, I will need to include the prior treatment as a blocking variable to eliminate any potential confounding effects. Beyond that it’s wide open. We could have 6 treatments x 2 blocks x 8 trees = 96 trees. I strongly suspect in the final analysis the block effect will be non-significant and we can consider there to be 16 replicates, but life is full of surprises.

So, what tree establishment or tree care question is burning a hole in your brain? “Shaving” or “butterflying” container rootballs? Fertilizing at time of transplant? The latest biostimulant? Crown thinning at time of transplant? Frequency of post-planting irrigation? Width of the planting hole – how wide is wide enough? Send me your suggestions and we will set up a poll to vote for the top choices.

Podcast #8 – Water Works

We’ve finally gotten our summer here in the Pacific NW and it’s been pretty hot for a few weeks. The plants weren’t really prepared for this, so we’ve had to irrigate quite a bit to keep all that lush foliage happy. So the topic of this podcast is Water Works – focusing on how water moves in the soil and through plants.

One of the more interesting tidbits I found this week is a recent USDA study on growing more potatoes with less water. Sound impossible? Listen to find out the one single, simple thing that increased water use efficiency by 12% and reduced fertilizer runoff as well.

I also debunk the common myth about using drainage material in container plants. Research from 100 years ago demonstrated that water won’t cross textural barriers – so putting gravel in the bottom of the pot will actually create a bathtub effect rather than helping drainage.

The interview this week is with my garden – primarily the sunny south-facing side. I thought I’d take you on a tour to see what’s happened in the last 11 years. The photos below will help you visualize the interview.

The front yard in 2003. We’ve started taking out the turf and moving around trees and shrubs.

The front yard in 2003 from another angle. We’ve removed the second driveway and covered the area in wood chips. By the garage you can see two of the roses I dug up from the shady back yard and moved to the sunny front.

The new front yard, with fencing, more plants, a pond, and no turf.

The rhody-hydrangea corner in front of the arbor vitae hedge

The new street garden, with a new retaining wall to hold back the soil that used to wash into the street. Everything not covered in plants is covered with wood chips.

This is the last podcast of the first “season” of The Informed Gardener. We’re going to take off for about a month before starting the next series. If you’ve got ideas about future topics, you can email me or post a comment here. In the meantime, you can listen to archived podcasts found on this blog; just click on “podcasts” on the right-hand menu.

Hot weather and not-so-hot advice

Today I was sent a link to a posting on “droopy leaves.” Essentially, it suggests that droopy leaves are a means to conserve water on hot days and that watering these plants causes more problems than it solves because the roots don’t get enough oxygen. A link to the science of transpiration is provided. The advice is to wait until the evening and if the plants perk back up, then they didn’t need water after all.

This is one of those maddening articles that has enough science in it to make it sound reasonable, but is ultimately incorrect in its assumptions and advice. It’s worth looking at the topic in a little more detail.

Some plants are adept at conserving water in hot weather. Their leaves tend to be small, thick, with a heavy layer of waxes protecting the surface. Leaves can also move to limit their sun exposure and thus reduce the heat load. But wilting is not a method of conserving water. Instead, it’s a sign that water loss (evapotranspiration through the leaves) exceeds water uptake from the roots. And if you ignore wilt, you do so at your own peril. Once terminal wilt is reached, it’s all over for that part of the plant.

Wilt. Sorry it’s a fuzzy photo.

Large, thin leaves, common in many of ornamental, annual and vegetable species, do not conserve water. Tomatoes, zucchini and black-eyed susans, the plants specifically mentioned in this article, are not water conservers. Chronic wilting of these and other can eventually cause leaf tip and margin necrosis (or tissue death). It also reduces growth, so that your yield of tomatoes, zucchini and black-eyed susans will be decreased.

Leaf tip and marginal necrosis from chronic drought stress

So yes, do water your plants if they are wilting in the midday heat! Use mulches to conserve water! (You’ll notice in the photograph on the linked site that the plants are in bare soil.) Fine root systems are generally near the soil surface, and keeping these hydrated keeps them alive. You won’t see an instantaneous response to watering if plants are already wilting, but they will recover – much better than if you don’t water them at all.

Rain barrels

A few weeks ago one of our readers, landscape architect Owen Dell, sent me a link to his blog where he takes on rain barrels. It’s a great analysis of the (im)practicalities of rain barrels and it got me to wondering how many of our readers (and my GP colleagues) use these as supplemental sources of irrigation water?

I have two in our back yard that were made from old olive oil containers retrofitted for collecting and dispensing water. They’re hooked together so that when one fills, the rain is diverted to the second.

We use this water pretty much for watering container plants, especially those on our south-facing front porch that require watering every other day during the summer. The barrels each hold 55 gallons and are always full during the winter and spring. We drain them almost dry over the summer, but even a brief rain results in several gallons collected.

So I think they’re a pretty good deal, since we use relatively little water from the hose to keep our container plants happy. But Owen brings up some valid points in his analysis, as do commenters on his blog.

What do you all think?

Plant containers – does size really matter?

A few days ago I got a question from Cynthia about “potting up.” For those of you for whom this is an unknown phrase (and no, it’s not a euphemism for a certain herbal activity), it refers to the practice of moving plants into ever larger containers. She was wondering if there was any “real science” behind the practice – in other words, why not just start out with a larger container?

Hah! I needed no further encouragement and spent several days collecting and reading decades’ worth of research. And there is a LOT of research on this topic. As you might guess, it’s geared towards production nurseries and greenhouses. But the good part is that it’s been done on just about any kind of plant material you could want. Vegetables. Annuals. Perennials. Grasses. Shrubs. Native plants. Ornamental, fruit and forestry trees. Seeds, seedlings, cuttings, big plants, little plants. Ahhhhh…data!

Almost without exception, you get better growth on plants grown in larger containers, whether you’re measuring height, number of leaves, leaf area, stem diameter, shoot and root dry and fresh weights, whole plant dry and fresh weight,…you get the idea. This isn’t surprising, because with a larger root zone you can support more roots, which in turn support more above-ground growth.

The only parameters which tended to diverge for some species were flower and fruit production. Restricted roots can stimulate sexual reproduction in plants, possibly because poor growing conditions spur the plant to reproduce before it dies. Other drawbacks include increased probability of circling root systems, and higher ambient soil temperature, compared to plants in larger containers.

Smaller containers might be considered desirable when one is trying to limit above-ground growth – the “bonsai” effect. And they require less water than larger containers – which brings us to the bottom line, as far as production nurseries are concerned.

Larger containers take more space. And water. In at least one study, water costs were shown to be “prohibitive for larger container sizes.” Furthermore, smaller containers are preferred by production nurseries to “optimize production space.” Another economics-based study found that “the smaller of these was the more economical.”

But most of you probably aren’t interested in the economics of plant production – you want to know what’s best for your own container plants, whether they are houseplants or pots of herbs or punches of annual color on your patio. The science is clear: it’s best to pot up plants in small containers quickly into their final destination, rather than making several (pointless) intermediate transplants.

My Long Suffering Basil

Sometimes I am not such a good garden professor. That’s because, when I get home, I sometimes (OK — often) don’t give my plants the attention they need. It’s also because, when we leave for vacation, I often forget to tell whoever is watching the animals to keep their eyes on the plants too. Now, really, you would think that someone who saw a plant on the back porch in full wilt would think “Hey, Maybe I should water that!” But still, I blame myself for not spelling it out.

What this all leads to, of course, is that we had a group of really nice basil plants in a container on the back porch which weren’t watered for a week while we were on vacation. There was little rain over this week, and subsequently the basil was in full wilt for at least 2 days (perhaps as long as three). We watered these plants soon after we arrived home (my wife loves caprese salad) and the plants perked up a bit, but the leaves were obviously damaged and now our salads will be a little less flat.

It’s not just basil that is affected by an incidence of drought. Most plants, including trees, will actually suffer for a considerable period of time after the event. Sometimes growth won’t return to normal for as much as a year or two. This is a really bad situation if you’re buying trees grown in containers. If the grower, or retailer, didn’t know what they were doing and let the tree wilt severely prior to selling it to you then buyer beware!

Need a lift?

One of the topic groupings for our posts is titled ‘Cool research’. The subject of today’s post has actually been around for a few years but I still think it’s pretty cool.

When we think of interactions between plants we usual think of negative interactions such as competition for water and nutrients or maybe allelopathy. But there are cases where plants can benefit each other. One of these is a phenomenon known as hydraulic lift. Hydraulic lift is the passive movement of water from roots into soil layers with lower water potential, while other parts of the root system in moister soil layers, usually at depth, are absorbing water. In essence, plants will large, deep root systems (usually trees) bring soil water from depth to the surface where it can be used by other plants. Hydraulic lift has largely been observed in arid and semi-arid ecosystems, though it can occur in wetter systems as well. For me, the research that went into discovering hydraulic lift is as fascinating as the process itself.

One of the key lines of evidence for hydraulic lift comes from studies of stable isotopes. As you may recall from college or high school chemistry, atoms of each chemical element have a certain number of protons and neutrons, which give it its mass. A small portion of each element has extra neutrons resulting in a ‘heavy isotope’. In the case of hydrogen, approximately 1 in 6400 atoms is heavy hydrogen or deuterium (²H). Interestingly, the amount of 2H in water can vary depending on the source of the water; this ratio is termed an isotopic signature. By comparing the isotopic signature of ground water and rain water in a given location, researchers can actually tell where certain plants are getting their water. One of the classic studies in this area was conducted by Todd Dawson at Cornell in the early 1990’s. Dawson analyzed isotopic signatures of groundwater, rainwater, and water in various plants around sugar maple trees and determined that many herbaceous plants contained a high proportion (up to 60%) of groundwater. But how do shallow-rooted plants obtain groundwater? The neighboring maple trees bring it to the surface from ground water as they are hydrating overnight. An efflux of water from the maple roots results in a localized increase in soil, which can be utilized by other plants: hydraulic lift.

How important in hydraulic lift in most landscapes? Probably not very. Demonstrating significant hydraulic lift requires the proper hydrology (shallow ground water accessible to trees or shrubs but not smaller plants) and limited rainfall. But the importance goes deeper (no pun intended!). Prior to the advent of stable isotope techniques, many would have been dismissive of the concept of hydraulic lift. Since 1993 over 100 papers have now been published on the subject. To me, the ultimate value of Dawson’s work and related studies is showing the importance of keeping an open mind and being receptive to new ideas.

</d

Creative Uses For Old Water Breakers

Why, oh why, can’t someone engineer a sturdy, long-lasting, horticulturally-correct water breaker.

We have, at last count, six hoses in use at our very spread-out garden & farm. I go through a lot of breakers, and am down to two, which I rotate around. In dire need of some new ones (as well as a huge bale of TP), I perused the garden aisle at our local big box (rhymes with “Target”). Pistol grip schnozzles abound – these things that propel the water
like a 95 mph fastball. Just what your plants want. Some had the “dial” for various water flow patterns, but these are never satisfactory. Not a single real water breaker for plants among the 20 choices. So I shall do mail-order from FarmTek.

Dramm seems to makes the only functional water breaker, but alas, most of what’s in my busted-breaker-bucket are old Dramm heads (at $10-$12 a pop). Their commercial line is a bit better than the consumer items, where a rainbow of colors seems to be more important than structural integrity. Anyone who’s worked in a commercial greenhouse has used their aluminum models or the plastic RedHead soft flow breaker, The only solid brass item in their line is the super-fine Fog-it Nozzle… I’ve had the same one since 1996.

A solid brass version of the full-size breaker would be great. The point of failure (always) is the interface between the screen plate and the body/shell. I hate replacing things. I’d pay for quality.

Hammer Time!
Obsoive. My ingenious partner uses a water breaker body to keep bamboo from splitting (further) when pounded. Principle of transference of impact from smaller area to larger surface, etc. Tomato stakes finally installed!

Art, Science, and Faith

First of all, who we are and what we do. All of the Garden Professors are in the business of the science of Horticulture. What’s Horticulture? The standard definition of Horticulture is the art and science of tending a garden. Horticulture is clearly more than science but science is the foundation and underpinning. For anyone that needs convincing that Horticulture is an art as much as a science I suggest the following exercise. Go to a major research university and wander through their Botany or Plant Biology greenhouses. Observe the plants. They look like crap. The people working there are on the cutting edge of plant science; they sequence genes, they elucidate biochemical pathways but they can’t grow a plant to save their lives. Now wander through the Horticulture greenhouse; plants are thriving, flowers are blooming. What’s the difference? The horticulturalists not only have the science, they have the art. There is no denying that art and intuition play a role in growing plants, especially in ornamental horticulture where we deal with hundreds of species and cultivars, each with its own subtleties and nuances. But as educators, especially public funded educators, how do we teach intuition? It’s very difficult. What we teach are principles developed through systematic scientific inquiry. How do we know there are 17 essential elements needed for plant growth? Repeated experiments over the years. And our knowledge continues to evolve based on the scientific method. I’m old enough that I learned 16 essential elements as an undergrad; the need for nickel by some plants had not yet been established. As extension educators our role is to disseminate science-based information. For some of us that phrase is even in our job description. We can try to impart our experience and intuition but it’s a difficult thing.

It can be especially difficult when we deal with alternative systems for which a long-term knowledge base may be lacking. Despite perceptions to the contrary, we are not apologists for the status quo. Overuse and misuse of pesticides and fertilizers are rampant, especially in ornamental horticulture. A lot of our current research and extension programming deals with reducing water and nutrient usage to reduce run-off and to reduce leaching. I spend a lot of time telling growers things they don’t really want to hear. How do we know growers are potentially impacting water resources? Because we and others have done the scientific research. We’ve set out plots, we’ve fertilized, we’ve sampled leachate, we’ve measured run-off. And we’ve conducted extension programs teaching growers that they can back off fertilization and irrigation rates without reducing crop growth.

Where we get concerned is that some assume or take on faith that because a nutrient source is ‘organic’ or ‘natural’ it’s automatically better or safer for the environment. Is the nitrate from Chilean nitrate less likely to cause blue baby syndrome then nitrate from ammonium nitrate? Dr. Corey Reams developed his principles as revealed to him through divine revelation. Unfortunately most of us are not blessed with such experiences. Instead we rely on systematic scientific investigation to develop knowledge that we share with our clients. Personally I do not believe that faith and science are mutually exclusive. Some of the most brilliant scientists I have met in my career have been people of deep and abiding faith. But we need to keep each in its context. Science is knowledge gained through systematic inquiry. Faith is a belief system. The central tenets of most Christian denominations are stated in the Nicene Creed which begins, “We believe in one God…” Note it doesn’t start “We know…” or “We can prove…” In their liturgy Catholics, “proclaim the mystery of faith; Christ has died, Christ is risen, Christ will come again.” Not only can they not prove these things they celebrate the fact that it’s a mystery. Faith does not demand proof. Science does.