Author: Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

The cardboard controversy

I’m not a fan of using corrugated cardboard as a mulch, which like other sheet mulches creates problems for the underlying soil. Long-time readers of this blog may remember several previous posts (1, 2, 3 and 4) on this topic and I won’t belabor the points made in those posts. Instead, today I’m doing to focus on cardboard itself.

First, cardboard is a generic term that can refer to many types of manufactured paper. The box you see delivered to your front door is more properly called corrugated board or containerboard. It consists of two layers of linerboard sandwiching a layer of accordion-like fluting material. The linerboard is made from sheets of pulp that may be coated to improve smoothness (more about this later). The finished linerboard is laminated using adhesives to both sides of the fluting material.

Corrugated boxes are built to be tough. — Corrugated boxes are built tough

These boxes are made to withstand rough handling and to protect the contents from the external environment. It’s tough stuff: while you might be able to bend a piece of corrugated board fairly easily, it’s more difficult to tear it in half. The more heavy duty the box, the more difficult it is to bend or tear its walls.

So let’s now consider using this tough material in your garden as a mulch. It may be coated as mentioned earlier to improve smoothness. That’s going to prevent it from absorbing moisture. The coating also reduces the ability for gases to move between the soil and the atmosphere. In fact, smoothness is measured using an air leak method – the smoothest materials have the least air leakage.

Photo credit vizpix at Flickr — Photo credit: vizpix at Flickr

A garden or landscape mulched with cardboard (or heaven forbid several layers of cardboard as part of the science-free lasagna mulch method) is now covered with a tough, relatively gas- and water-impermeable material that will take some time to break down. It’s hardly a mulch that’s going to nurture soil life.

But cardboard mulch fans swear that they find more earthworms under cardboard than anywhere else in their garden. This is almost always the first response I get from gardeners who don’t believe that cardboard causes problems. And this is where it’s important to consider earthworm behavior.

Photo credit: Kurt B. on Flicker — Photo credit: Kurt B. on Flickr

We’ve all observed that earthworms crawl to the soil surface during heavy rains; this is due in part to water filling their burrows and reducing oxygen availability (Chuang and Chen demonstrated this nicely in 2008). Likewise, the reduction in oxygen movement from the atmosphere into cardboard-covered soil would cause worms to crawl upwards in an effort to find oxygen at the soil surface.

So don’t assume your lasagna mulching draws earthworms to your garden. It’s more likely that you’re smothering their habitat.

***An update on cardboard gas permeability. We’ve just published an article comparing diffusion rates of different mulches. You can find the article here but it is behind a paywall. Here is a graphic comparing diffusion rates of various mulches. This is a logarithmic scale.
Now, until cardboard proponents publish evidence to the contrary, it’s pretty obvious that cardboard mulch interferes with gas diffusion.

***And another update on how our blog works. This post, by far, is the most popular. It generates a lot of comments. All comments must be approved before they’re posted, and I don’t approve comments that are derogatory or promote a belief in the absense of supporting science. If you want your comment to be published, be polite and provide evidence to support your statements. Otherwise, you are wasting your time.

***Another update on cardboard in your garden: A recent paper reports on PFAs (aka “forever chemicals”) in various products used for poultry bedding (among other things). Cardboard was one of the worst. The article is behind a paywall but I have access to it and was able to find the table shown here. So if you need yet another argument to NOT use cardboard as a mulch (like in “lasagna gardening), maybe “keep forever chemicals out of your garden” will do it.

**Look at the top line for shredded cardboard and then the last column which has the PFAs data. Not to mention all the other contaminants listed.**

Corny Ancestry

I love growing weird plants, and I’m endlessly fascinated by plant breeding and the extreme transformations humans have made in our crop plants over the history of agriculture.

Which is why growing teosinte, the wild ancestor of corn, was a no brainer. Even before I planted it, comparing the seeds is fascinating. teocornseed

Once growing you can see the similarity. Teosinte is on the left in the picture below, corn on the right.

The most dramatic difference between the two, I think, is the “ear” of teosinte, which is nothing more than a thin sprig of half-a-dozen seeds.

It is amazing to me that native Americans in Southern Mexico, with no knowledge of genetics, were able to transform this grass with a handful of tiny, rock-hard seeds into one of the single most productive crops in the world.

Out of the bottle and into the bag

Last week I was having lunch with my mom at our favorite nearby nursery/café. After failing to resist the grilled cheese sandwich (3 cheeses! And buttery panini bread!), we walked off lunch in the garden supply part of the nursery. Normally I’m on my best behavior when I’m shopping with my mom (i.e. I don’t take photos of things I’m going to take to task on the blog). But like the 3-cheese grilled sandwich I was unable to resist the bags of biodynamic compost.

Biodynamic compost is now available at garden centers

Long-time readers of the blog may remember my earlier column and post on biodynamics. Since I wrote the original column over 10 years ago I’ve watched biodynamic marketing move from boutique wines to coffee, tea, tomato sauce…and now to garden products. Really expensive garden products, as in $19.99 for one cubic foot of compost.

An "untapped source of power and majesty" makes this compost different. — An “untapped source of power and majesty” makes this compost different.

What makes this bag of compost worth $19.99? One has to assume it’s the biodynamic preparations used to treat the compost. They’re referred to in the label under “concentrations of yarrow” and so on. Do these preparations make a difference? The label suggests it might be to restore the soil’s vitality. Is there validity to this claim?

It's doubtful that all of these ingredients are locally available. And why are so many materials needed? — It’s doubtful that all of these ingredients are locally available. And why are so many materials needed?

In 2013 I published a review of the scientific literature on biodynamics, specifically looking at whether biodynamic preparations have a measurable impact on anything they’re applied to. In a nutshell, the answer is no. (Though this article is behind a paywall, I can send a pdf to you by email if you’d like to read it.)

Don’t let packaging and magical words sway you. Compost made with local materials like bark or agricultural wastes and certified by the US Composting Council is reasonably priced and sustainable.

Eggplants getting their buzz on

I was checking my eggplants today, and watching the bumble bees getting busy with the large purple flowers. As they flew in, buzzing away, they landed on the flower and kept buzzing — but the note changed, dropping in pitch. The bumble bee hummed away for a while, then flew off to the next flower.

I was watching buzz pollination at work. Egg plants, and a lot of other flowers, don’t leave their pollen hanging out in the open where any ant or fly that happens by could eat it. Rather they wrap them up in little packages that, when vibrated at just the right rate by a buzzing bumble bee, sends the pollen shooting out, so that bumble bees, which pollinate effectively, can access the pollen, but other insects, that would just eat it all, can’t.

In the garden, it isn’t easy to catch a glimpse of the pollen spewing forth, but luckily there are videos. Thank goodness for youtube. Watch it, and next time you are in your garden and hear a bee land in the flower and suddenly change the tone of its buzz, know you are seeing — and hearing — buzz pollination at work.

Dogwood rescue – an update

Long-time readers of this blog might remember a Friday quiz I gave back in 2010. It involved the slow but inexplicable decline of our dogwood (Cornus kousa). On the following Monday I revealed the reason for the decline and reported that we were moving this nearly dead tree to another location without the offending perched water table.

In 2011 I posted my first update along with photos of the new leaves and flowers. And today I reveal its obvious recovery to a fully functional if somewhat still spindly tree (several of its multiple leaders died as a result of the rotted root system).

A "grateful to be alive" floral display — A “grateful to be alive” floral display

There are several take-home lessons from this example:

1) Don’t assume that tree decline is due to a nutrient deficiency or pest/disease problems. The last thing a stressed tree needs is unnecessary additions of fertilizers or pesticides.

2) Explore soil conditions to find possible water movement disruptions. Our perched water table was discovered serendipitously with our pond installation. You can do the same with a good-sized soil auger. (I bought one of these bad boys, but haven’t had a need to use it yet. Some day…)

3) If a tree or shrub is failing, by all means move the poor thing to another location. In doing so, you may discover that the roots are still stuck in a clay ball and have not established into the native soil. Clean off all the burlap, twine and clay before replanting.

4) Be patient. If it took a while for your tree to reach its current sorry state, it will take a while for it to recover.

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

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

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

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.

Go sport fishing at your local nursery

Regular fishing, for actual fish, is quite possibly the second most boring think ever invented (First place, of course, goes without question to baseball) but sport fishing! Now THAT is something I can get behind.

By sport fishing I mean, of course, looking for sports – chance mutations – in plants. Sometimes a flower color changes, or sometimes a leaf becomes variegated, like on this lilac branch I found at a friend’s nursery a couple years ago

Nurseries are great places to go “fishing” for these sports, simply by walking down the rows of plants looking for anything odd or out of place. Sometimes they are true sports, and sometimes there has been some hanky panky… Last summer I was walking through a nursery looking at their pots of blooming Sweet William (Dianthus barbatus)

sweetwilliam

And suddenly I noticed a plant that was markedly different. dianthus hybrid

Way too different to just be a sport, this was a chance hybrid with… something. What, I don’t know, Dianthus is a big, promiscuous genus, but I like it and snatched it up. The chance hybrid was pretty enough and intriguing, and after blooming it produced a healthy crop of seeds, so I grew some out. One year later, they’re blooming in all sorts of interesting ways..

One looks just like a regular Sweet William, but significantly shorter

dianthusf22

Another is a nice red, though the flowers are a bit sparse

dianthusf23

And this one, my favorite, has decided to pretend to be a carnation with masses of double, fragrant flowers.

I, of course, have responded by collecting many many more seeds from that original chance hybrid picked up at the nursery, and can’t wait to sow them out and see what else may show itself.

So next time you at a big nursery, take some time to go fishing for sports and hybrids… you may just find something cool.

— Joseph Tychonievich