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

Short tree syndrome solved!

Good answers from Kenny S., Jimbo, Joe Schalk and Diana!  You were all skirting about the phenomenon of thigmomorphogenesis – or touch-induced change (also discussed in Jeff’s post of January 7.  The tests in the GP’s class are cumulative!).  In this case, the touch is wind.  Edge trees (or corn stalks) are more exposed and receive more wind, resulting in stunted heights and increased trunk diameter (you can’t see this last characteristic in the Friday photo).  Trees in the middle of the stand aren’t exposed to wind buffeting and put their resources into increased height. Similar stunting and thickening can be seen in urban plantings along the edges of sidewalks or anywhere people or animals routinely walk.

I spent my grade school years in a 1950’s housing development that had been Douglas fir (Pseudotsuga menziesii) forest.  The developers left many of the trees standing, and our backyard was full of tall, skinny and isolated Douglas firs.  When the Columbus Day storm (an extratropical cyclone, of all things) hit the Pacific NW in 1962, seven of these trees came down (none hit our house, fortunately).

Now of course a cyclone will take down many trees, regardless of their location…but this continued practice of leaving trees standing alone during development often results in blowdown or breakage of these now unprotected trees. 

Permaculture – more concerns

One of the gardening topics I’ve researched extensively is the use of landscape mulches.  (You can read a literature review I did a few years ago here.)  So I was more than a little frustrated to see one of the worst mulching techniques – sheet mulching – extolled in the book Gaia’s Garden (pp. 85-90).

Sheet mulches, like newspaper and cardboard, can be used successfully as a temporary weed control measure (i.e. a few weeks before planting a vegetable garden).  Long term, they are not a sustainable choice and often cause more damage to the system than the presence of weeds.

The two-dimensional structure of sheet mulches functions as a barrier to not only weeds but to the movement of air and water as well.  While this may initially increase soil water retention since evaporation is reduced, over the long term they will create soils that are unnaturally dry.  This condition is worsened on low-maintenance sites,where neglected sheet mulches easily dry out, causing rainfall or irrigation water to sheet away rather than percolate through.

In contrast, wet, poorly drained soils will become even more so as layers of moist paper or cardboard restrict evaporation and aeration.  Moreover, this condition encourages root growth on top of the sheet mulch, which can injure desirable plants when and if the sheet mulch is removed.

There are other disadvantages as well.  Exposed newspaper and cardboard mulches are easily dislodged by the wind, animals and pedestrians and often provide food for termites and shelter for rodents such as voles.  Combined with a somewhat marginal ability to control weeds compared to other organic mulches, sheet mulches are arguably one of the least attractive or effective choices for a sustainable landscape.

Sheet mulching proponents will argue that newspaper and cardboard are only part of the mulch structure – that organic materials such as compost and wood chips need to be added as well.  To which I respond – then why bother with the sheet mulch?  Why not just use deep layers of coarse organic materials?  That’s exactly what forest duff layers consist of.  It’s been repeatedly demonstrated that thick layers of coarse organic materials are the best and most natural choices for mulching.  (See, for instance, my  Ecological Restoration article on using a foot of arborist wood chips to suppress blackberry and enhance native plantings. )

The appeal of sheet mulching is its formulaic structure and logical approach – it’s like making lasagna (the name of yet another nonscientific approach to mulching).  Unfortunately, sheet mulching is neither natural nor particularly effective.

Green mystery disk identified!

Initially I was disappointed that no one answered the question…then Paul W. emailed to say that the post wasn’t accepting comments.  We’re not sure why that happened, but Paul and perhaps many of you knew this was part of the flower of Sarracenia flava – the yellow pitcher plant:

I think this is a stunning flower whose floral structure promotes cross-pollination.  Insects crawl in between the long yellow petals and the green "umbrella" to enter the flower and reach the pollen:

Before they reach the anthers, however, their backs rub up against the stigma, which are five tiny points at the "spokes" of the umbrella.  Pollen already on their backs will be transferred to the stigma before new pollen is gathered, so that the chances of selfing are reduced:

So thanks, Paul, for being so persistant that you emailed me to supply the answer and alert me to the comment fail!

Permaculture – the discussion continues

We’ve started a robust discussion on the topic of permaculture, especially as applied to home gardens.  Let’s continue looking at some of the advice provided in Gaia’s Garden targeted towards home gardeners.

The book contains several lists of plants suggested for specific functions.  For brevity’s sake, I’ll just mention two:

“Host plants for Beneficial Insects” (pp. 157-159)
This list is prefaced in the text with “many of these florae are very attractive and can (and should!) be included even in the most formal garden bed.”  With this strong endorsement, the author then presents an unsourced list of plants, several of which are identified as noxious weeds in many states in the country.  They include Washington noxious weeds false indigo (Amorpha fruticosa), fennel (Foeniculum vulgare), Queen Anne’s lace (Daucus carota), toadflax (Linaria vulgaris), cinquefoil (Potentilla recta), sulfur groundsel (Senecio vulgare), and tansy (Tanacetum vulgare).

“Dynamic Nutrient Accumulators” (pp. 131-134)
We are told “certain species draw specific nutrients from deep in the soil and concentrate them in their leaves” and given an extensive table of these plants and exactly which nutrients they accumulate. The references for this table are not scientific, and in at least two cases are mystical in nature (Cocannouer’s Weeds: Guardians of the Soil and Pfeiffer’s Weeds and What They Tell).  As in the previous table, many of these plants are designated noxious weeds in Washington or other states and include nodding thistle (Carduus nutans), Canada thistle (Cirsium arvense), fennel (Foeniculum vulgare), toadflax (Linaria vulgaris), creeping thistle (Sonchus arvense), and tansy (Tanacetum vulgare).

As readers of this blog know by now, we GPs are not “plant purists.”  But it is highly irresponsible to encourage people to plant listed noxious weeds in their gardens.  Even the author seems to understand this, and states (on page 15) that “it is foolish to deliberately introduce a species known to be locally opportunistic.”  It’s mystifying, then, that he does exactly that in these two tables.

The inclusion of the table of “dynamic nutrient accumulators” demonstrates that this book tends to wander far afield of the philosophical roots of permaculture.  It is an excellent example of pseudoscience, as it creates a scientific-sounding phrase (“dynamic nutrient accumulator”) and misleads non-experts into believing a scientific claim (nutrient accumulation of specific minerals) without providing actual supporting data.

Permaculture – beginning a discussion

Among other things, part of my job involves reviewing educational materials for use in WSU’s Extension programs related to urban horticulture.  One of the books is “Gaia’s Garden: a guide to home-scale permaculture” (T. Hemenway).  It occurred to me that my review might also be of interest to our GP readers.

I’ve created a fairly extensive review and I will break it into separate posts over the next few weeks.  So let’s start the discussion off with a topic we already know is inflammatory:  invasive species.  To be clear, we are not talking about the many introduced species, plants and animals alike, who appear to be well-behaved in our country.  Here’s my take on “The Natives versus Exotics Debate” (pp. 12-17):

The author, with no formal training in biology past his bachelor’s degree, states that “calling a species ‘invasive’ is not good science.”  This will come as news to researchers in the field of invasion biology.  He blithely disregards the real environmental and economic damage caused by invasive species and erroneously believes that invasive species selectively appear only as a result of human-caused environmental disturbance.  Apparently natural disturbances (from fire, volcanic eruption, flooding, etc.) don’t open themselves up for invasion (again, a notion that is incorrect and refuted by a number of obvious examples, such as the 1988 zebra mussel invasion of Lake St. Clair and the subsequent colonization of many freshwater habitats).   The author seems not to understand that there may be unfilled niches in certain ecosystems that can be exploited by invasives, endangering native species whose niches may overlap; there are obvious lessons from Hawaii, Australia, and other parts of the world.  In any case, the author’s naive tolerance of invasive species is a poor example to follow and certainly not based on current, mainstream science.

So, fans of permaculture, what do you think?  If permaculture is a legitimate science-based practice, how do we reconcile the very real issue of invasive species?  If you disagree with me, keep in mind one of the hallmarks of pseudoscience: attacking the motives or character of anyone who questions the claims. The arguments should contain content, not insults.

Call for “visiting professors”

I’ll be posting my usual blog later today…but in the meantime we’ve got an invitation for you.

Are you, or someone else you know, a "Garden Professor?"  In other words, do you use current, relevant plant and soil sciences to inform yourself and others?  If so, we invite you to submit a guest posting to our blog.   

We’ll post your article without editing, though we may make comments. 

If you’d like, send a photograph of yourself and any other illustrations you want posted along with your blog. 

We can’t offer any compensation other than the glory of being an online Garden Professor – but isn’t that enough?  We think so!

If you or someone else you know is interested, please contact one of us directly.  My email is lindacs@wsu.edu.

Friday puzzle untangled

A few of you bravely hazarded guesses about this plant, and while it is a contorted specimen, it’s actually Ulmus glabra ‘Camperdownii’, otherwise known as Camperdown elm.  Here’s a photo of a lovely example at Marymoor Park near Seattle:

As plant geeks already know, this odd tree was propagated from a mutant branch discovered on the Earl of Camperdown’s estate in Scotland in the 1800s.  All Camperdown elms are grown from cuttings originating from this single tree grafted onto a Ulmus spp. rootstock which supplies the straight trunk.

Though these trees don’t get terribly tall, they can become quite broad and need space for their tiers of foliage.  Like other contorted trees, the branches of old Camperdown elms can meet and fuse, as you can see in the photo from Friday.  And while the tree produces scads of seeds (that’s what you see in the photo above – not leaves!), they are sterile.