The Garden Professors™ – Page 41 – Advancing the science of gardening and other stuff since 2009

The Heat is On!

We are in the dog days of summer! Record high temperatures across the country have been reported. Let’s face it, climate change is real and the planet is getting warmer, despite record cold temperatures in winter in some areas. With climate change, there are more occurrences of extreme temperatures as well as precipitation such as severe drought, flooding, heavier than usual snowfall, etc. So, how does climate change affect gardeners? To understand the concept, we must first understand how high temperatures affect plant growth and development.

Plant temperature tolerance is the ability of any plant to adapt to a given climate at both low and high temperatures. It is obviously important to use species that not only can survive our winter temperatures, but also our hot summers. Similar to the U.S.D.A. Plant Cold Hardiness Zone map (http://planthardiness.ars.usda.gov/PHZMWeb/), the American Horticultural Society published a plant heat zone map (http://www.ahs.org/gardening-resources/gardening-maps/heat-zone-map). The heat zones are based on the average annual days with temperatures above 86°F (30°C). For example, up in my neck of the woods in Wisconsin, we have four heat zones in the state; zones 2-5 with the cooler (lower number) heat zones in the northern part of the state and the warmer heat zones in the southern and western part of the state and in urban areas. In comparison, North Carolina has seven heat zones (2-8). California has even more heat zones due to its varying climates and elevations.

Unique microclimates can be created at a specific site. Urban areas usually are slightly warmer than rural areas due to the heat island effect created by light reflection off of buildings, heat rising from sewers, and trapping of heat between large buildings. For example, trees planted close to pavement have warmer conditions in summer, than trees planted near turf, especially in the root zone, and have greater water needs. Trees and shrubs in this environment should receive, but rarely get supplemental irrigation compared to those same plants planted in the middle of a lawn or park area with cooler root areas and less heat stress to the tree.

Street trees compete with other plants and are subject to heat and drought stress

So what does this mean for temperate woody plants?

High temperature stress is important for a number of reasons. The main effect is increased water use. However, water availability is often limited, especially in urban environments, leading to a net loss of moisture within the plant. Leaves loose water quicker through the stomates via transpiration as temperatures rise. The stomates then begin to close and the cooling effect of evapotranspiration is stopped.

Other physiological processes are impacted by high temperatures such as fewer carbohydrates available for plant growth and development, generation of plant pigments (red, purple and blue pigments) and defense used in protecting plants against insects and diseases. When plants with poor heat tolerance are grown in regions that routinely experience high summer day and night temperatures and insufficient moisture, plants will use many of their stored sugars during the evening hours and during the hottest part of the day. For many temperate woody plants, optimum temperatures for photosynthesis are below 86°F (30°C). Above this temperature, net photosynthesis declines with increasing temperatures. If this continues long term, plants can die, especially young plants that do not have many stored carbohydrate reserves and are under drought stress.

'Ames' Kalm's St. Johnswort (Hypericum kalmianum 'Ames') is heat and drought tolerant — ‘Ames’ Kalm’s St. Johnswort (*Hypericum kalmianum* ‘Ames’) is heat and drought tolerant

Black chokeberry (Aronia melanocarpa) is a three season interest, heat and drought tolerant plant — Black chokeberry (*Aronia melanocarpa*) is a three season interest, heat and drought tolerant plant

Purpleleaved ninebarks (Physocarpus opulifolius) are very drought and heat tolerant — Purpleleaved ninebarks (*Physocarpus opulifolius*) are very drought and heat tolerant

What can we do as gardeners?

Unfortunately, climate change is occurring at a fast rate and each decade is getting warmer and often drier than the preceding decade. Here’s what we can do as gardeners:

Select non-invasive, pest resistant, stress tolerant plants for your landscape
Non-drought or heat tolerant landscapes plants will require regular watering to sustain them
Use of soaker hoses or drip irrigation is better than overhead watering as the moisture is directly applied to the roots with less runoff
A plant is not truly drought tolerant until it has been established in the landscape for at least three years or more to allow for root growth, especially for trees
Some native and exotic plants can tolerate these changes, but some native species in natural areas may be lost if our climate continues to get warmer
Incorporate compost into the soil prior to planting new areas to improve moisture retention and aeration
Apply 2-4” of bark or wood chips to the top of the soil to retain soil moisture. Make sure the mulch does not touch the trunk of trees or base of shrubs. Excessive mulching (mulch volcanoes) can actually kill landscape plants by impeding air and moisture penetration and invite fungal diseases.
Cities need to avoid planting trees in tree pits (sidewalk cutouts), i.e. restrained planting areas in sidewalks along streets. These trees are under severe moisture, drainage and heat stress and do not live long. Planting areas that are wide and long work much better for tree health and longevity than planting in tree pits.

Laura Jull, Ph.D.

An unexpected cactus

Crazy plant of the day is this: Pereskiopsis spathulata! Which, I’ll admit, looks like a fairly generic succulent, but the cool thing is this is actually a cactus. A cactus with leaves. Most cactus have of course lost their leaves to increase their ability to survive in extremely dry conditions and rely on their stem for photosynthesis, but the genus Pereskiopsis is a bit of living evolutionary history with photosynthetic leaves still intact.

Joseph Tychonievich

Perennial Monday: Monarda ‘Raspberry Wine’

It’s been a great summer for perennials here in the mountains of SW Virginia – plenty of rain, warm days, cooler nights. We’ve been enjoying this wonderful bee balm in our home garden for the last few weeks. ‘Raspberry Wine’ is tall (up to 5′), vigorous, and a bit ramble-y; not for the carefully-curated border, but great where it can take up some space. For those who fear Monarda’s tendency to spread, know that is shallow-rooted and very, very easy to pull up. I don’t know much about the origins of ‘Raspberry Wine’ other than it’s a Monarda didyma selection or hybrid and a “White Flower Farm introduction” – Joseph may be able to shed some light.

Monarda didyma is plenty hardy (at least USDA Zone 4) and is included on just about everyone’s plant list for either pollinator gardens or “gardening for wildlife.” The species is bright scarlet, but ‘Raspberry Wine’ has rich magenta bloom with dusky purple bracts subtending the flowers. Speaking of wildlife, it doesn’t seem to be the first choice of deer, so I’d rate it as reasonably deer-resistant. There are a couple of very-territorial hummingbirds making their home next to it – can walk by any time of the day and they’re slurping away. Interestingly, they seem to be ignoring the red ‘Jacob Cline’ down the way. My photography skills aren’t such that I can snag a feeding hummer, but did catch a less-frantic bumble bee making the rounds (above).

The clump pictured is part shade (afternoon) and the foliage is still fairly clean. I have another batch in full sun that has a bit of powdery mildew. The red ‘Jacob Cline’ is frequently touted as powdery mildew resistant but I’ve yet to see ANY Monarda didyma species or hybrid that doesn’t end up with it eventually. Just chop it back to the ground ASAP; you’ll get fresh new foliage and sometimes another round of blooms.

An Open Letter to Consumer Reports

Dear Consumer Reports,

A good friend of mine, Linda Chalker-Scott, recently reviewed some of your recommendations on weed control and found them wanting. I concur with Linda’s assessments, but I feel the need to take her critiques a bit further because:

#1 The article that Linda reviewed made a recommendation which is not only questionable in terms of its efficacy, but also its safety.

#2 You recently published another article on a very similar topic which is misleading.

Let’s start with #1

In the article Beat Those Weeds (Which was, somewhat ironically, published in the YOUR ADVOCATE section of your June 2015 magazine) you list clove oil as a useful treatment to kill the cut stumps of barberry and kudzu (you do note for one of these plants that repeat applications may be necessary). As someone who has tested clove oil products, I can assure you that trying to use these products to kill the root systems of these weeds is an exercise in futility. Clove oil is a contact poison that kills what it touches. It is not systemic and will not kill the whole plant because a cut end is treated. Roundup is a systemic and will work this way.

I certainly understand wanting to avoid Roundup. I am fully aware of its recent designation as a probable carcinogen (though I wish your magazine had spent some time explaining exactly what this means to your readers). On the other hand, I wonder why you couldn’t provide some more realistic information on how well your recommendations were likely to work, or offer some better organic remedies such as digging out the root systems of these plants (which is, admittedly, a tough job).

The above paragraph is just me voicing my concern about the efficacy of a treatment you recommended. If the treatment you recommend doesn’t work, no big deal. Right? At least you’ve helped us to avoid using Roundup because, as you say, its health effects aren’t understood. OK, I’ll buy that as a reason for not using Roundup. But if that’s your reason for avoiding Roundup, how can you possibly recommend applying clove oil?

Sure, clove oil is organic, but it is composed primarily (usually 70-90%) of eugenol, a naturally occurring chemical. Have you ever checked out the MSDS for eugenol? It is classified as a carcinogen by RTECS (Registry of Toxic Effects of Chemical Substances). Not only that, its acute toxicity is generally considered to be higher than that of glyphosate (the active ingredient in Roundup) and doses as low as 5-10 ml have caused near fatal reactions in children.

Here are a few links to back up what I’m saying, a good deal of which is available on Wikipedia if you’re feeling lazy.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1029532/
https://www.spectrumchemical.com/MSDS/TCI-A0232.pdf
http://www.ncbi.nlm.nih.gov/pubmed/12778213 (Pay particular attention to the mouse studies)

This is nuts! I have to admit that I am not particularly concerned about either glyphosate or eugenol if proper equipment is used and directions are followed, but there are people who are, and they have a right to be. Please Consumer Reports, if you’re going to protect us from Roundup, then have the decency to avoid recommending products that can potentially have effects that are similar or even worse, despite the fact that they are organic. As an advocate for consumers it would be great if you could help protect them by spreading the inarguable message that organic does not necessarily mean safe. Thought and discretion on the part of the consumer are still required, especially when products intended to kill or injure living organisms are being recommended.

Now on to #2

In your August 2015 issue, again in the YOUR ADVOCATE section, you post a letter from David Stone about using a vinegar, Epsom salt and soap mixture to control weeds instead of Roundup. In it you confirm that this mixture works and also suggest a citrus-oil based herbicide.

As someone who has conducted trials on the homemade concoction in the letter, I can tell you that it does burn down weeds, but that Roundup is much more effective because it kills the roots of the plant. Adding Epsom salts to an herbicide is a mistake because, over time, repeated applications of this herbicide will increase the amount of salt in the soil (Epsom salt is, after all, a type of salt) and could damage other plants in the area. Why not just recommend the most effective, safest, and easiest to use herbicide available? Pull or dig the weeds out.

Thank You for your attention,

Jeff Gillman Ph.D.
(Just for the record, my Ph.D. is in horticulture. I also have a Master’s degree in entomology, and over 20 years of professional experience working with plants. I would appreciate knowing the qualifications of the experts you are using for the YOUR ADVOCATE section of your magazine and encourage you to post this information in the future.)

Can Permaculture and Good Science Coexist

Several years ago I posted a four-part discussion about permaculture and my concerns with the blend of philosophy, science and pseudoscience that it contains. (Here are links to Parts 1, 2, 3 and 4.) So I was pleased to be part of an Extension tour group that visited an established permaculture farm in the San Juan Islands earlier this spring. This gave me an opportunity to see whether there was any perceptible shift in the permaculture community towards practices based on applied plant and soil sciences. Specifically, I chose to look for invasive species identified as noxious weeds that many permaculturists cultivate rather than eradicate.

Bamboo may not be a listed noxious weed in Washington State, but the yellow archangel beneath it is.

Our spring came early this year, and the islands were blindingly yellow with the Scots broom that runs rampant there (and throughout the West). This species is a Class B listed noxious weed in Washington State and has been mandated for control by San Juan County. So I was surprised and disappointed to see it and other related broom species not only present at this farm but used actively as nitrogen fixing species.

Other brooms were actively blooming and setting seed.

The practice here is to plant broom or some other nitrogen fixing species right next to a fruit tree as a “companion plant.” While the idea is logical, the choice of species is not. There are many other plants, including legumes and alders, which grow well in our area and would provide the same benefit.

Scots broom (a class B listed noxious weed in Washington state) used as a “companion plant” for a fruit tree (both are encased in wire).

There is nothing that can excuse the deliberate use of a listed noxious weed that’s mandated for control by local government. Permaculturists should endeavor to be good citizens and not infringe on the rights of their neighbors who don’t share their philosophy.

English holly is on the noxious weed monitor list for possible listing.

WSDA noxious weed listings for species mentioned in this post:

Scots broom
French broom
Spanish broom
Yellow archangel
English holly

Back on the High Line again

Earlier this week I was in New York City and got to visit the High Line for the first time. For those who aren’t familiar, the High Line is an urban park that was created along an abandoned elevated rail line on the Westside of Manhattan. Linda posted about the High Line a couple of years ago. Her visit was in late winter so my visit provided an opportunity to explore the park during the height of the growing season.

The High Line runs from West 14th St. to West 34th st.

30 feet above the streets of New York City…

The High Line offers sweeping views of Manhattan to the east and the Hudson River on the west. For most of its length the trail consists of various beds of perennials, trees, and shrubs. Along the newest, northernmost section of the trail, the plantings give way to beds that have been allowed to re-seed naturally; providing an opportunity to observe urban ecological succession.

Quaking aspen

The High Line extends through the Chelsea section of Manhattan, which, according to locals, was a less than desirable location just a few years ago. With the advent of the High Line, however, Chelsea and adjacent Meatpacking district have become some of the trendiest and hottest real estate in the city. In fact it’s difficult to get a picture along the highline without a crane in the background. What a stark difference from the acreage for sale in Mission, BC that we visited, too bad we are city people.

The High Line has helped transform a run-down section of Manhattan into some of the hottest real estate in New York.

Art is an integral part of the High Line with various sculptures and interactive projects along the way. During my visit, kids of all ages had the opportunity to contribute to a giant Lego sculpture or add to a giant sidewalk painting.

Is is art ? Or just weeds? This work is part of a 13-piece installation by Adrian Villar Rojas “…known for his large-scale, site specific sculptures that transform their environs into a vision of their own potential future.” It’s titled “The Evolution of God” aka “A Study in Lambsquarters”

Interactive art. Kids of all ages take time out to add to a Lego construction project along the High Line.

The first section of the High Line opened in 2009 and for the most part it seems to be holding up well. Some sections of the trail bed are constructed from crushed aggregate and these sections are pretty well pot-holed, presumably from freeze-thaw cycles. Most plants along the trail seem to be healthy and thriving, likely thanks to drip irrigation. It will be interesting to see how the trees and shrubs continue to develop and how things perform over the long haul.

This sidewalk fountain provides a change to cool your heels on a warm and sticky New York afternoon.

Bottom-line: If you’re in New York and you enjoy plants and watching people (and watching people enjoy plants), a couple hours on the High Line will be time well spent.

When Size Does Matter: Dwarf Conifers for the Home Landscape

I just returned from another great “Addicted Confer Syndrome” conference. In reality, ACS stands for the American Conifer Society. The meeting I attended was the Central Region chapter of the ACS held in Green Bay, Wisconsin. You might be thinking that only white spruce and tamarack are the only conifers that can be grown this far north, but you would be wrong. There are many outstanding conifers that can grow up here and throughout the U.S. Not all conifers are evergreen as there are deciduous conifers, like larch and baldcypress, but most dwarf conifers are evergreen.

According to the American Conifer Society (www.conifersociety.org/conifers/conifer-sizes), dwarf conifers are those that grow between 1-6” per year with an approximate size after 10 years between 1-6’. In contrast, large evergreens grow over a foot a year and are 15’ tall or more after 10 years. Size can vary due to climatic, environmental and cultural conditions. These smaller than usual evergreens are a fraction of the size of their species and fit nicely into the landscape often requiring very little pruning or shaping. Dwarf conifers can provide food and shelter for birds and other small mammals as well as year round interest due to their bright colors and interesting form and texture. An otherwise bleak, winter landscape can be accented with dwarf conifers that come in a variety of colors besides green such as blue, blue-green, silvery-blue, yellow, and purplish.

Below are a few of my favorite dwarf conifers that are available at many garden centers and nurseries.

Foliage of 'Silberlocke' Korean fir — Foliage of ‘Silberlocke’ Korean fir

‘Silberlocke’ Korean fir (Abies koreana ‘Silberlocke’, a.k.a. ‘Horstmann’s Silberlocke’) is a unique dwarf conifer that looks spectacular all year round. The soft needles are different than most conifers as they curve upwards, revealing the bright, silvery-white, frosty undersides. The silvery-gray twigs also add to the plant’s interest. ‘Silberlocke’ Korean fir grows slowly up to 5-7’ in height with a 4-5’ spread eventually growing into a small, compact, conical tree. Firs, in general, require a sandy-loam, moist, well-drained soil and are intolerant to heavy, poorly-drained, clay soils. This cultivar prefers morning sun, but some afternoon shade. ‘Silberlocke’ Korean fir is hardy to zone 4b.

'Blue Shag' white pine — ‘Blue Shag’ white pine

‘Blue Shag’ eastern white pine (Pinus strobus ‘Blue Shag’) is a dwarf conifer shrub with a compact, rounded form that reaches 3-6’ tall with a 6’ spread. The bluish-green, finely textured needles are very soft and pliable. ‘Blue Shag’ has a slow growth rate and a dense, mounded form making it a great choice for use as a foundation plant instead of the all-too-common yews (Taxus spp.). Like all cultivars of eastern white pine, it grows best in a sandy-loam, slightly acidic to neutral soil. It is sensitive to drought, heavy-clay, poorly drained soil, and road salt. ‘Blue Shag’ eastern white pine is hardy to zone 3a.

Foliage of 'Bergman' Japanese white pine — Foliage of ‘Bergman’ Japanese white pine

‘Bergman’ Japanese white pine (Pinus parviflora ‘Bergman’) is an outstanding, dwarf conifer that forms a dense, compact, wide, rounded to upright shrub. ‘Bergman’ Japanese white pine is a slow grower eventually forming a 4-6’ tall with a 6’+ spread shrub. The blue-green needles are soft, long and twisted. In spring, the immature cones are bright carmine-red contrasting dramatically with the blue-green needles. It is hardy to zone 5a and is adaptable to most, well-drained soils and pH. Unlike many other five-needled pines, Japanese white pine is road salt tolerant.

'Gold Drop' arborvitae — ‘Gold Drop’ arborvitae

‘Gold Drop’ eastern arborvitae (Thuja occidentalis ‘Gold Drop’) adds bright color to the landscape. This dwarf conifer shrub grows 4-5’ tall and 3-4’ wide and is shaped like a teardrop; narrow at the top, wider at the base. The soft, aromatic foliage is bright golden yellow when grown in full sun turning a deeper yellow during winter. ‘Gold Drop’ arborvitae is hardy to zone 3b and is adaptable to most soils and pH, but grows best in moist, well-drained, loamy soil. If grown in shade, the golden colored foliage will turn green.

Even though dwarf conifers are often more expensive than other deciduous shrubs, they are well worth it. They have a slow growth rate, require little maintenance and provide year-round color and texture in the landscape.

Laura Jull

Jumping genes!

This spring, I noticed this striped flower in a stand of feral Hesperis matronis
Stripey flowers! And like almost all striped flower variants, almost certainly caused by transposons, aka jumping genes.

To understand transposons, you can think of genes as instructions. So when making a flower, a plant may be following a gene that says:

MAKE PURPLE PIGMENT

And so it does, and the flower is purple.

You can think of a transposon as a gene that says:

COPY ME!

And so the cell makes a copy of the transposon, and then that copy gets dropped somewhere else in the genome. And sometimes, that new copy of the transposon lands in the middle of a gene that does something important. Like, for example, a gene involved in pigment production. So you get this:

MAKE PUR COPY ME! LE PIGMENT

Which doesn’t make any sense. So now the gene for making the purple pigment doesn’t work. And if that happens in a cell in a flower petal, that part of the flower will be white. And as that cell divides, the new cells resulting from it will also have the transposon in place, making more white cells, producing a white patch or stripe in the flower.

When transposons were first discovered in corn by the great geneticist Barbara McClintock, they were thought to be an oddity, something unusual. As we’ve learned more, it turns out they are ubiquitous. Some 40% of the human genome is thought to be transposons. Usually they are invisible, and have been silenced to prevent their moving around and disrupting other genes. But sometimes they pop up in a flower and make themselves visible, in a beautiful, interesting way.

Joseph Tychonievich

Extension….Again

About 6 months ago I wrote a little article about what I perceived to be the most significant problem within extension, which is that extension personnel, and specifically tenured and tenure track faculty, simply don’t receive recognition or credit for accomplishing their job. In this essay I’m going to give a specific example of this problem.
Normally, when someone does what they are assigned to do, they are rewarded, or at least an acknowledgement is made that they are performing satisfactorily. For example, when a salesperson is given a job whose description says specifically that they are to sell vegetables, then, if this salesperson sells a lot of vegetables, his or her supervisor rates him or her accordingly. Sure, the salesperson’s supervisor could suggest that he or she help sell meat too, but if the supervisor doesn’t formally change the job description, then we would think it was pretty unreasonable to downgrade the vegetable salesperson too far for not selling much meat.
The job assigned to faculty with extension appointments is to transfer research based information to the people who they are assigned to serve. Let’s take someone who all of you are familiar with as an example. Linda Chalker-Scott is assigned to deliver relevant research-based horticultural information to the public. Linda is a great example because she has done her job so effectively, using a multitude of techniques to spread information to an incredibly broad audience. She has published multiple books, one of which won the gold and silver award of achievement from the Garden Writer’s Association. She has published in newspapers and magazines such as Fine Gardening where she is currently a contributing editor, she is the most prolific writer on both the Garden Professor’s blog and Facebook page, she has appeared on Growing a Greener World, and soon you’ll be able to hear her 24 lecture course for the well known Great Courses series (I am so jealous of this!). The list goes on and on. By all accounts Linda has worked hard to become a nationally and internationally known source of research based information for the consumer. Right now she is probably the top urban and consumer extension horticulture faculty in the United States in terms of population reached and quality of information delivered. If you rate her anywhere out of the top three then please post a comment here to let me know who is doing a better job.
So, why hasn’t Linda Chalker-Scott’s extension work been reviewed, recognized, appreciated and rewarded for the good publicity that she has brought to her University and the valuable service that she provides for the people of Washington (and beyond)? I’m not at liberty to disclose all that I know but, in my opinion, her extension work has been downplayed over the last three years to the point that it actually seems to me to have done her career more harm than good. This all goes back to my belief that extension work is neither understood nor appreciated by higher education. I have two possible cures for this problem:
1. Start appreciating the work that extension faculty do. No, it doesn’t bring in big research dollars, and no, it doesn’t yield articles in journals like Science and Nature (Though some extension professors, like Linda, do such an incredible job scouring the literature on particular topics that they actually publish review papers on those topics. For example, Linda has published reviews on anthocyanins, mulches and biodiversity). What it does do is deliver useful research based information to the masses. It stops people from overusing phosphorus in their lawns, it teaches people to identify diseased plants, it encourages people to use appropriate techniques to combat weeds and pests, it informs the public about how dangerous GMOs really are. The list goes on and on.
2. Just give it up. If universities don’t want to reward, or even appreciate the work that extension faculty do in terms of public education then hey, let’s not have extension faculty. Instead we could hire non-faculty personnel without significant research knowledge to spread research information. No, they won’t have the same respectability without the Ph.D. or title (such as assistant professor, associate professor or full professor), and there’s a good chance that they’ll get things wrong because of their lack of research experience (which is why most Land Grant Universities currently have a hierarchy built into their extension systems with University based faculty on top distributing information to extension agents) but they won’t have the same expectations either – And the administration won’t have the admittedly sticky situation of needing to figure out what a faculty member’s extension work is worth as compared to another faculty member’s research and/or teaching.
Maybe there’s a third option? I don’t know. All I can say is that it is time to have the conversation about what successful extension work is worth at the highest academic levels.

Are Soaker Hoses Safe?

By Cynthia Lee Riskin

With drought predicted for the west, southwest, and south through June 2015 (National Weather Service March 2015), many conscientious vegetable gardeners will try to conserve water by using soaker-hoses, those bumpy black hoses that weep water onto the soil through tiny pores.

Soaker hoses are made from fine-crumb rubber, usually recycled from vehicle tires. Research strongly establishes that tire particles leach heavy metals, carcinogens, and mutagenics, among other toxins. Yet soaker hoses have not been studied for potentially increasing the toxicity of edible plants. Are they really safe to use safe on our edible plants?

Soil in the City
Urban soils already contain high levels of heavy metals (Murray et al. 2011) from years of household runoffs—chemicals from pesticides, cars, painting, cleaning, and more. Adding soaker hoses made of crumb tires might exacerbate the problem.

Whether plants take up enough heavy metals to be toxic, however, is a complex equation, depending on a slew of interrelated factors, including:
• Soil pH (Costello 2003) and texture (Singh and Kumar 2006; Murray et al. 2011)
• Temperature (Murray et al. 2011; Lim and Walker 2009)
• The size of the rubber particles (Gaultieri et al. 2004)
• Chemical composition of irrigation water (Singh and Kumar 2006)
Furthermore, the plant species and even the cultivar can affect a plant’s uptake of zinc and other heavy metals (Murray et al. 2009 and 2011).

Growing Healthy Food
If you’re looking for the key to ensuring that your vegetable patch grows healthy food, however, I’m sorry to disappoint you. Too many factors are involved to predict the toxicity of what we grow in our gardens.

A good way to get more information is to contact your local extension agent for a list of laboratories that test soils not only for nutrient composition but for heavy metals. Although this information won’t guarantee you’ll be able to grow heavy-metal-free produce, it’s a step in the right direction while we wait for more research to be done.

Cindy Riskin is a Master of Environmental Horticulture and freelance journalist raising edible plants, an unkempt ornamental garden, and elderly mutts in Seattle, Washington.

NOTE: This article is excerpted from a longer one soon to appear in Cindy Riskin’s upcoming blog, tentatively named Muddy Fingers Northwest. Please contact Cindy Riskin at cindyri@q.com for an advance copy or the blog’s web address.

REFERENCES
1. Costello, Laurence Raleigh. 2003. Abiotic disorders of landscape plants: A diagnostic guide. Oakland, Calif.: University of California, Agriculture and Natural Resources. P. 117.
2. Gualtieri M., M. Andrioletti, C. Vismara, M. Milani, and M. Camatini. 2005. Toxicity of tire debris leachates. Environment International 31 (5): 723–30.
3. Lim, Ly, and Randi Walker. 2009. An assessment of chemical leaching releases to air and temperature at crumb-rubber infilled synthetic turf fields. Albany, N.Y.: New York State Department of Environmental Conservation. http://www.dec.ny.gov/docs/materials_minerals_pdf/crumbrubfr.pdf.
4. Murray, H., T.A. Pinchin, and S.M. Macfie. 2011. Compost application affects metal uptake in plants grown in urban garden soils and potential human health risk. Journal of Soils and Sediments 11 (5):815–829.
5. Murray, Hollydawn, Karen Thompson, and Sheila M. Macfie. 2009. Site- and species-specific patterns of metal bioavailability in edible plants. Botany 87:702–711.
6. National Weather Service Climate Prediction Center. March 19, 2015. U.S. Seasonal Drought Outlook. NOAA/National Weather Service National Centers for Environmental Prediction. http://www.cpc.ncep.noaa.gov/products/expert_assessment/sdo_summary.html.
7. Singh, S., and M. Kumar. 2006. Heavy metal load of soil, water and vegetables in peri-urban Delhi. Environmental Monitoring and Assessment 120 (1-3):1–3.