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

Graft and corruption

It’s election season – but that’s not why I’m doing a blog on “graft and corruption.” Instead, let me back up and explain that today I gave a seminar on diagnosing urban tree death. One of my points to the group was the importance of knowing the history of a site – what species were selected, how trees were planted, whether there had been any major construction activity, etc. I thought I’d continue the importance of site history into today’s posting.

Here’s a photo of a street tree – a Prunus spp. (Disclaimer: I am not endorsing a candidate for Mayor of Seattle despite the appearance of a campaign sign in the photo.) It’s a healthy enough specimen, though possibly a bit large for this narrow planting strip:

Several years ago you would have seen a different tree in this same spot:

Now did this weeping cultivar somehow transform into an upright form? Let’s look at this second photo in its entirety:

This reminds me of my favorite childhood book on Greek mythology, which had a great drawing of Athena springing from the head of her father, Zeus. Yes indeed, we are seeing the scion of a grafted tree lose the battle to the rootstock. Rootstocks, by their very nature, are vigorous. If we revisit the first photograph again, this time a little closer, we can see all that remains of the poor scion:

Lesson: if you are using a grafted tree in the landscape, you need to keep the rootstock under control. Grafted trees are probably not good choices for low-maintenance landscapes.

Epicormic mystery solved!

Good morning (at least it is in my time zone)! And welcome to those of you who found us through Blotanical or another blog site. We love seeing the increased participation on our blog.

Since I am a teacher at heart, I was glad to see so much thoughtful discussion over the weekend. Many of you suggested that pruning for vehicular traffic was the trigger for this growth, and it’s true that removing large limbs or heading back branches will result in vigorous epicormic growth. But I cheated on the photo and cropped it above the point of interest. Here’s the entire photo of this tree:

You can doubtlessly see that dark line encircling the trunk just above the two branches with the shoots. Here it is close up:

Venturing around to the back of the tree, we can see the source of this line – neglected staking wire that has now been enveloped by the trunk.

What this wire has done is to girdle the phloem elements, which as you’ll remember from basic plant science, are directly below the bark and the cork cambium. Without functional phloem, nutrients from the crown can’t reach the roots. Since the two lower branches were spared this girdling, they can still transport sugar to the roots, so the tree hasn’t died. But now it’s directing resources (water and nutrients) into the lower branches, where the new epicormic shoots are forming a new, functional (albeit ugly) crown. In time, the original crown will probably fail; there’s already evidence that the trunk is dying:

What could be done with this tree? If the wire were removed or at least cut so that the trunk could pop it apart, there is the possibility that the crown could have been saved. But since the upper trunk already looks severely compromised, it’s probably too late.

As a sad update to this set of photos, the owners had a tree “service” (I use the term loosely) to remove all the epicormic shoots from the lower limbs! I will let you know when and if the whole thing fails.

Oh, and gold stars to all who participated in the quiz!

Quiz time!

One of the things we Garden Professors can do is give tests! And the nice thing is you don’t get penalized for being wrong. So this will be my inaugural Plant Puzzler.

Below is a photo of a tree with epicormic shoots on its two lower branches. Epicormic shoots are vigorous, upright branches that have more of a juvenile than mature appearance. They often appear when a plant has been stressed, perhaps by overpruning, or maybe the roots were damaged by construction:

So here is your test question. Why are these epicormic shoots primarily (if not exclusively) on the two lower branches, and not elsewhere in the crown? (You can’t see the top of the tree, but I promise there are no epicormic shoots up there.) And what evidence would confirm your diagnosis? While there is only one correct answer for this particular tree, let’s see how many possibilities you can come up with.

Answers and more pictures next week!

Wonderful wood chips

I’m in love…with arborist wood chips. These are not your beauty barks or other packaged mulches, but the chipped branches and leaves fresh from the tree crews. It’s a great way to keep this resource out of the landfill – and don’t even get me started about using this great mulch material for a “biofuel!”

I’ve written about wood chip mulches a lot, but thought today I would post some photos to show you how well they work in suppressing weeds and promoting growth in restoration sites. We published a paper on this in 2005, though we’ve been using them in ornamental and restoration landscapes for about 10 years.

Here’s a recent project: a wetland buffer enhancement was being installed in an area that was covered in Scot’s broom (Cytisus scoparius) and blackberry (Rubus discolor):

We had a brush cutter mow it to the ground, then put a foot of wood chips down. Later, we planted poplar, ash, willow and alder on the site:

We had to keep records, both written and photographic, for the county who monitors wetland projects. So we took photos every year at the same points for comparative purposes. Here’s what part of the site looked like immediately after planting and then after 5 years:

That’s not to say that we haven’t had to battle resurgent blackberries. They migrate over from the wetland itself (which we can’t touch) and tip root. But the increasing shade and competition from the trees has weakened their ability to take over, and the Scot’s broom has been gone for years.

So that’s one reason I love wood chips. I’ll do a follow up some week showing how they can be used in the home landscape.

The joys of arborizing!

It’s Holly’s day…but she’s off playing in a tropical paradise. So because she seems to be of a sunnier disposition than I am, I’ll post happy thoughts today.

One of my favorite pruning techniques, especially for small urban landscapes, is arborizing. This is a way of creating small trees out of large shrubs – and often, a large shrub is as much as a small landscape can handle. Rhododendrons are common landscape plants here in Seattle, and the larger ones lend themselves beautifully to this practice:

As you’ll notice in this example, arborizing not only creates an aesthetically pleasing tree form, but also moves the crown away from vehicular and pedestrian traffic. This protects the plant from damage and enhances access.

This also works wonderfully in landscapes where you would like to have layers of shrubs, rather than one massive plant. Look at this Ceanothus:

Arborizing this shrub not only allows planting additional plants underneath, but also allows some light into the house (note the window in the background).

Fall is generally a good time to prune (after the crowns have gone dormant). It’s easier to see trunk and branch architecture in deciduous trees, and generally places less stress on the plant.

If you’ve arborized shrubs before, which species work well for you? Which ones not so well?

See? I can be a happy blogger!

Building a better tree? Not in the long run!

One of the landscape tree production practices that drives me absolutely nuts is heading back trees in the nursery to create “columnar” specimens. It’s easy to find examples of these in Washington state nurseries, like the pathetic oak shown below:

Aesthetics aside, let’s focus on how the tree responds to heading back. The removal of the dominant leader encourages lateral branches below to become more upright; from these laterals, a new leader is selected. This new upright growth habit is highly prized by many landscape architects and urban planners, as such trees fit more neatly into small urban spaces without interfering with vehicle and pedestrian traffic. Sure, it works great for a few years.

Now let’s look at these trees a decade or two later. Branches grow in diameter as well as length. All of these acutely angled branches begin to grow into each other, creating bark inclusions:

What effect do bark inclusions have on the trees? These fused branches are not strongly connected; in fact, they are likely points of branch failure. As these branches become larger and heavier, they can create hazard situations if they are near people or property. What’s happening here in Washington, and probably elsewhere, is that arborists must be hired to prune out some of these branches to reduce the risk of failure and injury.

This…most definitely will lead to this…eventually

I can’t understand why this practice is perceived as “building a better tree.” To me, it looks like creating a maintenance and liability problem down the road.

Blog bizarre

Jeff’s post yesterday gave me the perfect opportunity to showcase the star of my bizarre book collection.. It’s called “Evolution of Botany: More Fact Than Theory” written and published by Benjamin Zarr (author of several other books, including “Evolution: No One Can Break Down My Theory!”) Jeff, he’s written more books than you and I put together.

This book was “willed” to me by a dear colleague at Buffalo State College after he retired. When I first received it, I tried to read a chapter, but found it impossible to finish. Here’s an example of one of the numerous illustrations on a chapter about plant heredity:

The discussion around this illustration is too long for this blog. Plus, it hurts my brain. You might notice some interesting botanical terms, like “red paste” and “yellow dough” for instance. Here’s part of the text on these details: “I say red “dough” and red pastes always reproduce the color red because pollen always inherit a specific “melting” point. When the heat of the sun warms the starchy contents of a seed or the pastes of a seed coat red has the lowest melting point therefore it shows up before any other color. Just like in a specturm of light red appears first and purple last, all other colors come in between. When red has been eliminated it fades into pink, then into the color with the next higher melting point until white appears as the hardiest color of all.”

Ack.

Not content to write just about plants (which evolved after animals, but that’s a whole different chapter), he ventures into the evolution of animals:

Words fail me. (But they don’t fail a reviewer for the Quarterly Review of Biology, which you can read here.)

Though this book is an extreme example, it fits in with Jeff’s post yesterday about critical thinking. (If you’re interested in the dissection of yet another book – The Sound of Music and Plants – click here for an online column of mine from 2003.)

Size matters.

Yesterday I received a call from an administrator at a large military base. (I have to tell you that anytime I get a call from someone in government I immediately start wondering about “the file” that I’m sure is kept on me. No, I’m not paranoid, but I’m an outside reviewer for a number of graduate student theses from the University of Agriculture in Faisalabad, Pakistan. In fact, I’m doing two right now. So every once in a while I am sent a brown paper package from Pakistan. They’ve always been opened and resealed by the time I get them. But I digress.) Anyway, the administrator from the LMB was concerned about a newly installed landscape on the base. Requirements for landscapes around military buildings specify that plants must be placed a certain distance from the building itself, and not be tall enough to hide people or large objects. So my caller was concerned that the winter creeper (Euonymous fortunei) which had been planted would start to do exactly what its name implies. Furthermore, he had done a little Googling and found reports that this plant can get quite a bit taller than what the LMB specifications require. However, the landscaper was adamant that this plant would not exceed the height requirements and cited one of Dr. Michael Dirr’s books as evidence. So what, the caller asked, did I think about this?

Several years ago I wrote a myth column on plant size, which you can read here. Part of this column immediately sprung to mind: “The lack of consensus among tree identification guides, taxonomic literature, nursery tags, and real-life landscape specimens underscores the fallacy of assuming a uniform maximum height for any species, variety, or cultivar of any tree or shrub.” In fact, the best predictor for mature plant size – especially for nonnative species – is performance in your local geographic region. With this in mind, I called my colleague Dr. Sarah Reichard (an excellent plant taxonomist) at the University of Washington. She laughed when I explained the situation and said that a local specimen at the Washington Park Arboretum had become such a nuisance that the grounds crew had to whack it into submission. Not only was it well over 12” tall, but it had crept into the nearby Magnolia and was busy making itself at home.

Don’t have a photo of the Magnolia-eating creeper, but I do have this nice truck-eating ivy.

What about the Dirr book? It’s an excellent resource, but it doesn’t necessarily take into account how climatic differences can influence plant height. In contrast, the Western Garden Book (by Sunset magazine), though not an academic resource, does look at local plant performance in its descriptions. I was also annoyed to find that this introduced species is invading the eastern US and is considered a weed in some states. There are lots of good plant choices out there. Let’s not aid and abet the invasives.

So my caller was armed with definitive evidence and the landscaper will probably have to absorb the replacement costs. The lesson: don’t rely on books alone. Do some legwork in your area to find out what plants are up to – literally.

Sidewalk-eating Japanese maple – not an invasive, but easily outgrows its “expected” space

Rubber mulch rubs me the wrong way

I’ve been receiving a lot of questions about rubber mulch lately. For those of you not familiar with the product, it consists of shredded tires that can be dyed and used on ornamental landscapes or under playground equipment. In fact, the Obamas had this material installed underneath their children’s play structure at the White House. It seems an ideal way to recycle the 290 million scrap tires we generate annually.

But is it?

It’s not effective: One of the main reasons we use mulch is to suppress weeds. Research has demonstrated that organic mulches such as wood chips, straw, and fiber mats control weeds better than rubber mulch.

It burns: You’ve heard stories about piles of scrap tires catching fire and burning for weeks. Well, those same flammable compounds are in rubber mulch, too. When compared to other mulch types, rubber mulch is the most difficult to extinguish once ignited. In fact, some parks and playgrounds no longer use rubber mulch or rubberized surfaces because vandals have figured out that rubber fires cause a LOT of damage.

It breaks down: Although sales literature would have you believe otherwise, rubber is broken down by microbes like any other organic product. Specialized bacterial and fungal species can use rubber as their sole food source. In the degradation process, chemicals in the tires can leach into the surrounding soil or water.

It’s toxic: Research has shown that rubber leachate from car tires can kill entire aquatic communities of algae, zooplankton, snails, and fish. While part of this toxicity may be from the heavy metals (like chromium and zinc) found in tires, it’s also from the chemicals used in making tires. These include 2-mercaptobenzothiazole and polyaromatic hydrocarbons, both known to be hazardous to human and environmental health.

It’s not fun to be around: When rubber mulch gets hot, it stinks. And it can burn your feet. Yuck.

The EPA’s website says this about scrap tires: “Illegal tire dumping pollutes ravines, woods, deserts, and empty lots. For these reasons, most states have passed scrap tire regulations requiring proper management.” So if we have legal tire dumping (in the form of rubber mulch), does that mean it doesn’t pollute anymore?

(You can read a longer discussion on rubber mulches here.)

Whoo hoo!

I promise I’ll post something more substantial today…but I had to pass this email message along that I received this morning. Way to go colleagues and commenters!

Hi Linda,

Your new weblog, the Garden Professors, is an impressive piece of work! I plugged it today in the consumer horticulture CoP blog [http://www.consumerhortcop.wordpress.com].

Regards,

Bill Hoffman
National Program Leader (Ag Homeland Security)
CSREES/USDA