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

Where to draw the line on a vine

Last week’s column on “why do nurseries sell this plant?” struck a chord with many readers as well as with Holly!  So here is this week’s submission:  that ubiquitous vine, English ivy.

First of all, we’ll stipulate than many ivies are sold as English ivy (Hedera helix) but may be entirely different species.  Genetic research on invasive ivy populations in the Pacific Northwest identify most as H. hibernica (aka Atlantic ivy), with H. helix making up only 15% of the invading populations.  Regardless of their species identity, it’s obvious that Hedera is a genus with the potential to escape gardens and invade remnant forests or other environmentally sensitive areas. It grows so vigorously that it can create monocultural mats on forest floors; it grows into trees where its sheer weight can break limbs and in some cases topple entire trees.

But what about other regions of the country – or the world, for that matter?  In colder climates, Hedera spp. are much better behaved, dying back to the ground every winter and rarely able to flower and reproduce.  The absence of a seed bank means the vine can be kept in check more easily.  And it does tolerate tough environmental conditions where other groundcovers might not succeed.

Yet consider another invasive: kudzu (Pueraria montana var. lobata)   For decades this noxious weed was thought to be too cold sensitive to expand past the American Southeast.  Yet populations have been found in Maine, Oregon, and most recently in Ontario, Canada.  Plants adapt!

So – is it worth the risk to buy a plant known to be invasive elsewhere, simply because it’s not a problem yet?

And why oh why do nurseries in Washington state continue to sell Hedera spp. as ornamentals?

Clematis calamity solved

Some good and creative guesses about why the Clematis leaves had interveinal necrosis.  While iron and manganese deficiencies both cause interveinal chlorosis (veins are green, areas between are yellow), the necrosis indicates tissue death between the still-living and green veins.  Very simply, this has been caused by water loss.

During transplanting of the vine, I had to remove them from the fence and lay them out on the ground.  They remained this way for a couple of days.  For much of the foliage, this meant that the lower leaf surfaces were now exposed to the sun.  As with many broadleaved plants, the upper and lower leaf surfaces are morphologically distinct:  the upper surfaces have a thicker waxy cuticle and epidermis, with few stomata, while the lower surface lacks much of the cuticle and is loaded with stomata.  When the leaves are turned upside down, the shade-adapted lower surfaces now receive intense sun exposure: water evaporates quickly from these unprotected leaves and the tissue dies.  The only parts of the leaf that don’t die are the veins, which remain full of water as long as the roots are functional.

So both LisaB and Benjamin identified sun exposure as the culprit behind the damage.  But as with many environmentally-induced plant problems, the ultimate cause is water stress.

Friday quiz – yet more clematis calamity!

If you’ve been following the saga of our clematis, you’ll know that first they suffered iron toxicity (from the waterlogged soil they were in) and then were dug up and replanted in containers.  Last week I showed you what happens when you vigorously work wet soil – yet more waterlogging!  During the transplanting process, I took more pictures:

This damage is NOT from the iron toxicity problem.  It appeared during the transplant process.  What caused it?

Answer on Monday!

Why do nurseries sell this plant?

I wish I were more like Holly…wandering around nurseries finding pretty and unusual annuals and perennials to get excited about.  Instead, I seem to gravitate to plants that annoy me.

Today while looking for some trellises (for those containerized Clematis vines that I’ve been torturing) I saw pots of the Equisetum hyemale (“a tall, evergreen, spreading, reed-like grass”) for sale:

 

As readers of this blog surely know, Equisetum spp. – or horsetails – are not grasses but primitive relatives of ferns.  That taxonomic blunder aside, the thought of deliberately planting any Equisetum species in a landscape sends shivers down my spine.

Now E. hyemale is not as weedy as E. arvense, but in nearly every seminar I give on controlling weeds with mulch someone asks about getting rid of horsetails.  Short answer – you pull.  And pull and pull.  There’s no good herbicide for them, nothing seems to eat them, and they spread aggressively.

And speaking of eating, did you know that horsetails are poisonous?  They contain an enzyme (thiaminase) that deactivates thiamin (vitamin B1) in the unfortunate consumer’s body.  The most common victims of horsetail poisoning, ironically, are horses.  Horsetails are considered noxious weeds in pastures used for grazing – and yes, they are native to the United States.

Sure, horsetails are interesting looking plants.  But do you really want something in your garden that the production nursery describes as having “indefinite spread?”  And how does keeping them in a pot, as one production nursery recommends, keep them from spreading spores?  Especially if you plant them “in or around ponds and streams?”

I just think this is such a bad idea for home landscapes.  Even if it is a native species.

Clematis conundrum clarified

Friday’s puzzle was tricky!  I will preface the answer by saying I don’t do a lot of container plants except for annuals.  When I plant up annuals into soilless potting mix (which is dehydrated), I work in the water thoroughly into this fluffy medium.  Friday’s situation was a little different.  The Clematis are more or less permanent residents in these planters, so I use real soil rather than potting mix.  (This will reduce or eliminate the shrinkage you’ll get if you use soilless media, as these highly organic materials steadily decompose.)

But what I thoughtlessly did was to mix water into my nice screened clay loam like it was a potting mix.  The result of vigorous mixing is to break down the soil structure and drive water into all of the pore spaces, resulting in a totally waterlogged soil:

After my "D’oh" moment when I realized my error, I potted the second Clematis into loose soil and let gravity work the water into the larger pore spaces. This passive approach protected the soil structure and left some of the pore spaces filled with air rather than water, so that the water drained through rather than sitting on top of the compacted soil:

This demonstrates one of the rules of working with landscape soils:  you should avoid any potential compaction when the soil is wet.  (It’s the same reason you shouldn’t stomp on wet backfill as you’re transplanting trees and shrubs.)  As Ginny pointed out, you’ll create a cement-like structure – not conducive for water movement, oxygen availability, or root growth.

Friday quiz – the tale of two clematis continues

A few weeks ago I showed you photos of iron toxicity in a Clematis planted in a soggy soil (perched water table).  Because this area is just not conducive to plants, we’re putting in a small deck.  This necessitated the excavation of two Clematis, which were both suffering from wet feet.  (Needless to say the root mass was very small and shallow on both plants.)  We decided to put them into large planters with conical trellises and use them as deck plants.

During our pond excavation we retained the topsoil and sieved it for uses just like this (for woody planting, not annuals).  The soil is a clay loam and has been stored in a garbage can to keep it dry.  Anyway, the first pot I filled with this good soil, added water, and worked the soil with my hands to ensure it was thoroughly hydrated.  As holes developed, I added more soil and continued to work it in by hand.  I then installed the first plant and watered it thoroughly.

After observing what happened during the next several hours with this plant, I installed the second plant differently – I watered the soil but did not work it by hand to hydrate it.  I added the plant as before and then watered everything thoroughly; I added soil where holes developed.  I then redid the first container in the same manner as the first.

What happened to the first container that caused me to change my installation technique?  And why didn’t it happen with the second container?  Answers and photos Monday!

Restoration ecologists – you need us!

Restoration ecology – the science of restoring degraded ecosystems – is another branch of applied plant sciences.  Oddly enough, very little plant science makes it into the scientific literature of this field.  This has driven me nuts for a number of years after reading an endless stream of papers where no mention is made of how plants are selected, installed, and managed.  Or worse, some ancient horticultural practices are used – like amending the backfill with organic material rather than using just the native soil.  Here’s what that will lead to:


Native soil discarded in favor of “lite-n-fluffy” amendment

The whole story

Restoration failures like this are often attributed to more esoteric causes, like lack of local plant gene pools in the plants used.  Believe me, even local populations aren’t going to survive poor installation techniques.

Thus, one of my recent graduate students conducted a meta-analysis of the applied restoration ecology literature to analyze it for horticultural content.  The results were not encouraging.  In Kathleen’s thesis abstract, she states:  “…careful selection and handling of planting stock, site and soil preparation, and rootball preparation, essential to increase survivorship of planted seedlings, are infrequently discussed in peer-reviewed restoration publications…Findings from this review support that restorationists either do not understand or are not providing important information to their peers, stakeholders, or the public on significant horticultural aspects of the restoration process.”

Now I know most of you are not restoration ecologists…but I’ll bet many of you are interested or actively involved in planting or maintaining native plant habitats, public greenspaces, degraded urban lots, etc.  The science behind gardening is just as applicable to these “wilder” areas as it is to home landscapes and gardens.


Failure of entire installation.  Note suckering from the roots – an attempt by the tree to establish a shorter crown.  (It’s easier to transport water to a short crown than a tall one, and suckers are often a symptom of root failure)

Tomato takedown

Well, Dr. Rohwer was right – he thought you’d get this more easily than I did!  Ray and Jon were spot on – this is classic juglone toxicity from those walnut trees (Juglans spp.) you see in the background.  Many of these leaves ended up on the roof, leaching into the rain barrels (good sleuthing Ray and KennyG!), the water in which was used in irrigating the vegetable garden.  In fact, Dr. Rohwer divulged that the rain barrel water was quite brown from the walnut leaves.  Thus, the tomatoes met an unhappy end, because tomatoes definitely do not love walnuts.

Juglone toxicity, an example of allelopathy, is an interesting phenomenon – it does not affect all plants equally.  In fact, this clue was given in the puzzle, when Dr. Rohwer mentioned the "garlic and carrots, amongst young beets, onions, kohlrabi, and bolting radishes and spinach" also in the garden.  Many of these vegetables are known to tolerate juglone, as are various landscape plants.  (There is an excellent publication from Purdue on juglone toxicity with lists of tolerant and sensitive species.)

Juglone is most prevalent in the nuts, roots and leaves of walnut trees.  This makes sense:  germinating walnut seeds and expanding root systems are both able to kill their competitors, retaining more resources for themselves.

Juglone tends to be less of a problem for established trees and shrubs, as their root systems are more extensive; seedlings would be the most sensitive stage.  And walnut wood used as part of a wood chip mulch has not, as far as I know, been shown to cause juglone toxicity in landscape plants. 

When criticism becomes libelous

Dr. Cregg is on vacation for a few weeks so I thought I’d post something today by trolling the web for discussion of my horticultural myth columns.  And when you go fishing, you know you might catch something you didn’t really want.  In this case, personal and professional attacks.

Like the rest of the GPs, I relish vigorous debates on different horticultural practices and products.  Ideally, these debates are based on science, so that we’re all friends afterwards and could go out for beer if we all lived in the same community.  But when criticisms become personal, malicious, and in some cases out-and-out untruths, what should one do?

I have a relatively thick skin and ignore most of the nasty comments, but when my integrity as a scientist is challenged, I really have a problem.  For instance, one blogger states “The fact that some – not all – of her articles have very little basis in reality and are widely disputed and even denounced in horticultural circles should be pointed out before anyone takes them immediately to heart.”  Another blogger remarks “What little I’ve read about this Chalker-Smith person leads me to believe she is an “opinion for hire” much like some “expert witnesses” one sees in courtrooms.”  How about this?  “She tried to prove compost tea is bad by PURPOSELY using E.Coli infested compost (without telling anyone about it) and proclaimed it as unsafe product to use.” And so on.

I can guarantee you that if I didn’t base my publications in reality, or if I were an “opinion for hire” I would be in serious trouble with the university.  The truth is I research the science behind topics and then have at least one colleague review them.  (Sometimes that colleague is my husband, also a PhD in horticulture and one of my most honest critics.)  The compost tea diatribe is so outlandish it took my breath away.

So onto my question:  what should one do when one’s professional reputation is libeled?  Do I continue to ignore it?  Do I wade into these debates, thus giving credibility to the libeller?  These people hide behind their anonymity – none have the courage to simply email me and voice their criticisms.

I’ll be curious to hear from my GP collaborators as well as non-academics.

(Hurry back, Bert.  This was a painful experience.)

Five little lavenders – an update

Long-time readers of our blog might remember my August 12 column (linked here for your convenience).  You saw my giant lavender plant devolve into 5 small plants with tiny spiraled root systems that put Marge Simpson’s beehive to shame.  In any case, I promised to keep you up-to-date with their progress.

Transplanting in August is a risky proposition, especially when you prune out root defects.  Nevertheless, all but one of the five survived the summer and here they are earlier this spring:

The one that didn’t make it was in the upper left corner of this south-facing landscape, where there is a lot of reflected heat from the nearby bricks and concrete.  (You can see the empty spot where it was in this photo.)

If you haven’t tried corrective root pruning before, I’d encourage you to try it with an inexpensive shrub.  Your best bet will be older containerized plants in the “sale” section of your nursery.