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

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

Eat your veggies! (But not the arsenic, or the chromium, or the lead…)

vegetables_jpg.jpgThe last few years have been a perfect storm for the resurgence of home vegetable (and fruit) gardens.  Grapevines are trellised along sidewalks, herbs replace the grass in parking strips, and tiny gardens of carrots and lettuce are shoehorned into any available spot.  It’s all good  – but we need to be particularly careful about what those plant roots might be taking up along with nutrients and water.

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1)  Contaminated soil.  Many urban (and suburban, and even rural) soils are contaminated with heavy metals, pesticides, and/or industrial wastes.  Lead is commonly found in soils near roads (from the old leaded gasoline we used to use) or from old lead-based paint chipping away from houses.   Arsenic is a very real problem in North Tacoma soils, for instance, thanks to the smelter that operated there for decades.   Overuse and incorrect use of home pesticides will leave residues in the soil for years.

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2)  Contaminated compost and soil mixes.  Many of the same contaminants mentioned above can be found in unregulated composts and soil mixes.  (More on this topic here.)

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3)  Treated lumber.  The old treated lumber (CCA = copper, chromium and arsenic) is no longer being sold, but it’s out there.  These timbers should not be used around vegetable gardens, as they will leach their heavy metals into the soil.  Vegetables vary in their ability to take up and store these metals.  (More on this topic here.)  Likewise, rubber mulches may leach unwanted chemicals into the soil and should not be used around food plants.  (More on this topic here.)

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What can you do to avoid these problems?  A few things are quick, easy and cheap:

1)  Have your soils tested.  I’ve mentioned this in an earlier blog on urban soils.  It’s the best way to find out exactly what you have in your gardens – the good and the bad.

2)  Use only certified composts and soil mixes.

3)  Plant in containers if your soils aren’t safe for food.  This is especially easy to do with perennial herbs, which can be kept like any other container plant on your deck or porch for years.

4)  You can also replace the soil in your vegetable garden.  This isn’t quick, easy, or cheap, but is a solution for some people.

Take it all off (cue bow-chicka-bow-bow music)

OK, I know there are skeptics out there including many of my dear colleagues.  Though it seems that at least some of my photos are making an impression.  So here is another little photo tour through bare-rooting – this time with a bigger tree.

This demonstration was given at the 2006 ISA conference in Washington.  This is a good sized tree…

…that we plopped into a Rubbermaid watering trough after removing the burlap…

…and washed off all the clay.  It is deceptively easy to do.

Oh!  I almost forgot!  We put some duct tape around the trunk just above the burlap before we started this procedure.  Look where the tape ended up:

So there is another really compelling reason to bare root trees.  Had we not, this tree would have been planted 10 inches below grade.  But I do have to say the burlap made pretty patterns on the tree:

Another plus – with the clay gone, these trees are really easy to pick up and move around!

And it didn’t need staking once it was mudded in…

 

And it looked great seven months later with little to no maintenance and lives happily ever after.  The end.

Health care reform (of trees)

Nothing is more frustrating to a gardener than watching a newly installed tree or shrub slowly die.  In performing “post mortem” analyses on failed landscape plantings, I’ve identified four common errors that can be easily avoided:

  • inadequate root preparation
  • improper soil preparation
  • planting below grade
  • inadequate aftercare

This blog entry will be dedicated to the first point – but before I do so, we need to understand how nursery plant production has changed over the last several decades.

A brief history of propagation
Many years ago the only way to obtain young trees and shrubs was as bare-root plants.  Plants were field grown, then dug up during dormancy for storage and shipping.  Bare-root trees and shrubs are usually only available during a narrow window of time, but in general these plants are healthy and structurally sound.  Most importantly for our discussion, growers can see the woody root system of bare-root plants and cull those that are not well formed.

The development of containerized production methods meant that plants could be grown and sold year around.  When plants are grown in a production greenhouse, they are generally started in small liner pots and gradually moved through a succession of increasingly larger pots.  Ideally this is done before roots become potbound, or the roots are corrected when “potted up” (moved to a larger container).  What we found, unfortunately, in a study of nursery plant quality, is that root systems are often ignored in an effort to produce large quantities of plants quickly and cheaply.  It is not considered to be cost effective to examine and correct root flaws during potting up, so the entire root mass is moved into the new container.  Structural root flaws are not self-correcting and will become more severe the longer they are ignored.

Based on our study, as well as evidence collected by numerous researchers and arborists, it is apparent that poor root quality is a significant problem in containerized and balled-and-burlapped trees and shrubs, at least in this part of the country.  Therefore, we need to correct root flaws before installing woody plants into the landscape.

A quick intro to correcting poor root systems
Balled-and-burlapped plants have a clay rootball; despite its appearance, it is fairly easy to remove the clay simply by removing the burlap and twine and soaking the entire rootball in water.  You can facilitate the process using your fingers to work out the clay, or use a gentle stream of water (Figure 1).

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Figure 1.

Once the clay is removed the root system can be evaluated.  If you find woody roots that are circling, girdling, or in general not growing horizontally and away from the trunk (Figure 2), they should be pruned (Figure 3).  You want to develop an evenly distributed structural root system.

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Figures 2 and 3 – before and after

The pictures in this post are from my own Cercis tree, which I planted in April of 2004.  This is not a great time for planting, since Seattle has notoriously dry summers.  Nevertheless, that’s when I planted and as you can see from Figure 3, I had to remove close to 70% of the root system.  I mudded it in well (which eliminated the need for staking), mulched, and kept the root zone well rooted.  It sat for about 3 months and did nothing (Figure 4), except of course the flowers died quickly!.  In July it leafed out (Figure 5), and 3 years later had doubled in size (Figure 6).  It is now close to 15 feet tall and is in excellent health.  Given its initial root system, it’s doubtful it would have done this well without intervention.

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Figure 4 – April 04              Figure 5 – July 04               Figure 6 – July 07

(I have performed radical surgery on hundreds of tree and shrub root systems and have only lost one small shrub, whose root system is in Figures 7-8.  Kind of tough to prune something as fatally flawed as this.)

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Figures 7 and 8 say so much more than I can.

Mmmmm…mulch!

It’s a nice sunny September day in Seattle and I’m in my happy place.  What better topic to match my mood than mulch?

For those of you not familiar with my fixation on woody mulches, I’ll refer you to an article in MasterGardener Magazine here.  Briefly, I am a fan of coarse, chunky organic mulch, particularly arborist wood chips or other chipped material from trees and shrubs.

Rather than send this material off to the landfill, it’s so much better to use it as a protective layer on top of your landscape soil.  It’s a cheap, natural way to protect and nourish your plants, and provides a great habitat for beneficial insects and microorganisms.

Practically speaking, how does one move a mountain of mulch?  Shovels don’t work well, and compost forks have too much space between tines.  My favorite tool is the mulching fork.  It’s relatively lightweight, well balanced, and makes quick work of wood chips.

   

Sometimes you’ll find twigs in your mulch pile, or might have your own woody prunings that you’d like to use as mulch.  My second favorite tool is my electric chipper/shredder.  It’s powerful enough to deal with small branches and twigs and helps me create a more uniform mulch. Plus, I reuse my yard waste and keep the nutrients on site rather than throwing them away.

I don’t own stock in either of these products (my faculty salary doesn’t exactly allow me to be an investment tycoon). They’re just a few of my favorite things…

Modern day torture stakes

Torture stakes were used centuries ago as a slow means of executing prisoners.  Unfortunately, the practice lives on every time someone incorrectly stakes a newly planted tree.  Though I’ve written about tree staking before (click here to read more), I’ll use today’s blog to demonstrate another unintended result of improper staking – decapitation. A normal tree develops taper as it grows.   At eye level, a tree trunk is narrower than it is at ground level:  that’s taper.  As the trunk flares out and morphs into roots (Figure 1), a buttressing structure is created that allows trees to remain upright, even under windy conditions.

Root%20flare.jpgFigure 1.  Trunk flares as it meets soil and roots begin.

A tree that’s been staked too high, too tightly, and/or for too long does not have this structural protection.  Instead, the staking material creates an unnatural pivot point, which is not structurally capable of withstanding wind.   When the inevitable windy day comes along, the trunk snaps at this point (Figures 2-3):

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Figures 2 and 3.  Tree decapitation, up close and personal.

Unlike the victims of the original torture stakes, trees don’t necessarily die after breakage.  They are, however, permanently deformed and have little aesthetic value.  If trees need to be staked at planting (and many times they do not), staking needs to be low and loose to allow taper to develop normally.  (More information on proper tree planting can be found by clicking here.)

What’s in YOUR soil? (with apologies to Capitol One)

Urban environments are always challenging for landscape plants just because they are anything but “natural.”  Temperatures are higher, water is often less available, and compacted soils have all the nourishing qualities of concrete.  The single most important thing you can do to ensure long-term success of landscape trees and shrubs is to get their roots well established in the soil.

I’m going to leave the topic of soil amendments to another day (but you can find my myth columns about them at http://www.theinformedgardener.com under “Horticultural Myths”).  What I want to focus on is our propensity for fertilizing landscape trees and shrubs without really knowing why, or when, or if we should be adding any particular plant nutrient.

The smartest $13 you can spend is to have a soil analysis done before you add anything to your soil.  My favorite soil testing lab is the University of Massachusetts at Amherst.   That $13 will buy you a complete standard analysis of the available nutrients in the soil, plus a measurement of the soil’s organic matter content.  Of course, there are many other soil testing labs you can use, but UM’s Amherst lab is only providing you with information – not a sales pitch for amendments and fertilizers.

Why is this so important?  Let’s say you go to a nutritional supplement store, buy every possible supplement, and take them all.  Do you need all of these?  Probably not.  It would be smarter to talk to your doctor and find out what you’re missing, right?  It’s the same with your soils.  Don’t assume your soil needs a lot of phosphorus, even though transplant fertilizers are loaded with this element.  Non-agricultural soils often contain abundant levels of this nutrient, and too much phosphorus will hurt mycorrhizae and contribute to water pollution.  Take a look at this portion of a soil test for an organic demonstration garden:

Figure 1.  Note the high level of organic material in this soil, which contributes to the nutrient overload.

The trick to fertilizing landscape soils is understanding that landscape soils are not agricultural soils.  You’re not harvesting crops (an activity that depletes the soil of its plant nutrients).  Urban landscape soils usually have high enough levels of most nutrients to sustain plant growth.  But you’ll never know unless you have your soils tested.