Salt solutions

Hopefully everyone got their filling of turkey and dressing over the long Thanksgiving weekend.  I used our unusually mild weather on Saturday to celebrate a time-honored tradition around the Cregg farm: The annual cursing of the tangled Christmas lights.

Turning the calendar over to December in Michigan means another Midwest tradition is just around the corner as well: The annual dumping of the road salt.  Although totals vary, at least one source estimates that road crews pour 8 million tons of salt on roads in the US each year.  The effects of this sodium chloride are readily apparent on our vehicles – in Michigan it’s rare to see a vehicle more than 10 years old on the road – and drivers of those older cars that remain can usually see the road pass under them through a rusted-out floorboard.  Of course all this road salt has profound implications for landscape plants as well.  Sodium chloride can damage plants in several ways.  Both sodium and chloride can cause direct toxicity, particularly from salt-laden spray drift from highways.  Salt in soils can cause osmotic stress resulting in drought injury.  Sodium in soils can displace potassium, magnesium, and other essential plant elements.  High levels of chloride in plant tissue can reduce cold hardiness and make plants more susceptible to freezing injury.  Suffice to say that salt is bad for plants.

So what’s a plant lover to do?  In general I advise a two-phase strategy of Selection and Protection for homeowners and landscapers that have to deal with heavily salted roads.  By Selection I am referring to planting salt tolerant plants (or at least avoiding salt sensitive ones).  The poster child for a poor choice in Michigan is eastern white pine, which is extremely intolerant of road salt.  Witness this planting at a rest area along I-96 between Lansing and Detroit.

Don’t you love to see your tax dollars at work?  Even a cursory look at a list of salt sensitive plants would have been a tip-off that that white pines and highways are a bad mix.  Most blog readers should be able to come up with a list of salt tolerant or salt sensitive plants for their area by Googling their way around the internet.  A couple caveats about these lists:  First, many are based on anecdotal experience, not hard data, so you may see inconsistencies between lists.  If possible, try to consult several sources and look for a consensus opinion about the plants you’re interested in.  Second, many recommendations are dated and include plants that may be considered invasive or no longer recommended for planting.  One list I just looked at included Russian olive (invasive) and green ash (no longer planted in the Midwest due to Emerald ash borer).

Another approach to reducing salt damage to plants is Protection.  Here I am referring to erecting a physical barrier to block salt spray or salt splash from reaching plants.  The most typical form of barrier around here is a wooden frame or fence covered with burlap or canvas.  Some people will actually wrap their evergreen trees or shrubs with burlap.  This may help against winter desiccation but does little for salt since the salt-saturated burlap will still be in contact with the foliage.  Obviously aesthetics go out the window with the protection approach but I’m noticing more and more people are willing to put up with a couple of months of looking at burlap to keep their plants looking thrifty the rest of the year.  In northern Europe some road systems will line their roads with pre-formed plastic barriers to reduce salt splash to adjacent vegetation.

What about alternative deicers?  This is a question I get frequently when I speak about salt and plants.  There are several things to consider about alternative (i.e., not sodium chloride) deicers.  First is the cost. All alternative deicers are more expensive than salt, some by a factor of 10 times or more.  This often limits their widespread use for highway departments.  Many road crews will use alternative products around bridges and other sensitive areas where they want to limit corrosive damage.  Alternative products may also be useful for smaller scale operations such as parking lots and walkways.  Secondly, many alternative deicers contain calcium and/or magnesium chloride.  These may even be marketed as ‘plant safe’ since they contain calcium and magnesium, which are essential plant elements.  The problem is the chloride.  Chloride is a plant nutrient but is only needed in minute amounts (a few parts per million). At higher levels it becomes toxic.

A study in Colorado found tree damage due to chloride near roads which were treated with calcium and magnesium chloride for dust abatement.   For safer alternative deicers consider chloride-free products such as calcium magnesium acetate.

In the meantime here’s hoping for a mild winter for everyone.

Friday puzzle answers!

Good speculation on the rhododendron leaf damage!  Jim in Wisconsin zoomed right in on the causes:  the first photo was taken on a year where we had an unseasonable freeze right as leaves were expanding, and the second was taken on a year where we had unseasonably hot weather as leaves were expanding.

In both cases, the ultimate cause of damage is lack of water in rapidly expanding tissues.  Once dormancy is broken, leaf and flower buds are highly sensitive to environmental extremes – they are expanding and are most sensitive to anything that interferes with water content.

During a freeze, leaf tissue water freezes, causing what’s called freeze-induced dehydration.  It’s not the ice that causes the damage, but the lack of liquid water in the cells.  Water freezes in the air spaces between cells, and osmosis draws water out of the cells into these intercellular spaces.  Eventually the cells more or less implode once they’ve lost enough water.

During a hot episode, the roots can’t keep the rapidly expanding leaves fully turgid, and again necrotic areas appear as a result of water loss through transpiration and cellular “implosion.”

So both of these problems are caused by a lack of leaf tissue water – and it’s impossible to tell from looking at them whether it’s from cold or heat or salt or some other stress that reduces water availability.

Bottom line:  keep track of seasonal abnormalities.  It will help you to correctly diagnosis problems that show up some time later.

Justice finally

This just in from our Saturday paper:  US man wanted for ecoterror sentenced in China.

For those of you who don’t know my history, I was an associate professor at the Center for Urban Horticulture at UW when it was firebombed in May 2001 by ecoterrorists.  It’s a long and sad story, but if there’s an upside, it’s that this event was the ultimate reason I’m doing what I’m doing now.  Otherwise I would probably still be doing lab-based research – which is good and necessary, but not nearly as personally satisfying as working directly with people who want to understand and apply plant sciences as they relate to garden and landscape sustainability.

I was the first person from our center on the scene that morning – I heard about the fire on the local NPR affiliate and immediately thought that somehow it was my fault – that I’d left something on in the lab.  I raced to the scene and watched my building burn.  My colleagues trickled in as the morning went by, and we cried and hugged as we watched our professional lives literally go up in smoke.  It wasn’t until the ATF showed up that we began to understand that this was no accident, but a deliberate act of violence against us.

I don’t think about the fire much any more, except when these news items appear.  What was particularly galling was a statement by the arsonist’s father that his son isn’t a terrorist.  While I sympathize with the agony a parent must go through in such a situation, I know for a fact that my colleagues and I felt exposed and threatened by having our offices torched.  Many of us ended up going through therapy to deal with the fear and anger we experienced in the days afterwards.

That’s what terrorism does to you.

Post-turkey puzzler

I hope everyone had a great holiday yesterday!  Since I am NOT a shopper, I’m avoiding “Black Friday” and posting another puzzle instead.

Consider this photo:

This is a rhododendron in my own landscape.  The photo was taken in July, though the damage on these new leaves occurred earlier than that.  In Seattle, rhododendron leaf bud break generally occurs in April.

Now consider this problem.  Same plant, different year – and actually a different problem!

So what caused this damage?

Explanations on Monday!

Inspecting nursery plants, part lll

By now you’re probably ready to stand up, brush off your pants, and stretch your back after crawling around looking for surface roots and root crowns.  Not so fast!  There’s one more thing to look for – and to avoid.

Take a look at these two photos:


You can easily see the suckers at the base of these trees.  Whether or not they are actually suckers (coming from the roots) or watersprouts (coming from the base of the trunk) doesn’t matter.  Their presence in single trunked species warns of problems underground.  You’ve probably seen landscape trees respond to crown stress by suckering.  In this situation, my diagnosis is that the roots are so stressed (buried too deeply, structurally malformed, etc.) that they are unable to provide enough water to the crown.  Thus, the plant responds by creating a shorter crown (the suckers) which is easier to keep supplied with water.

In both of the above cases, these were the only individuals of their species in the nursery that were suckering.  That makes it easy to avoid purchasing them and their stressed root systems.

This is not such a problem with species that tend to form thickets, like our native vine maple (Acer circinatum) below:

Bottom line:  know the natural habit of your trees and shrubs before you buy them.  If they are single trunked species, don’t be a sucker – avoid suckers!

Advice Requested!

Greetings, all!

I am not a tree-care expert, having invested most of my mental capital into herbaceous plant stuff.  But I know enough to be dangerous: spiraling/strangling roots and narrow crotch angles are bad news. But at what point do they become “unfixable”? So I’m asking my illustrious colleagues and diligent readers (a.k.a “all y’all) for advice.

We have a lovely specimen in our campus Horticulture Garden…Acer ‘White Tigress’ – a hybrid between A. davidii and A. tegmentosum – also known as snake-bark maple.  Probably been in the ground for 18 years or so. Lovely buttery fall color, gorgeous stripey bark.

This tree, as we say in Georgia, “has more problems than a show dog.”

Scroll on down…

Bit of constriction there, mid-way up.

Some interesting crotch angles, too…

But here’s the kicker (I can hear Linda hooting it up from here…)

This poor gal is obviously a “what not to do” teaching tool.

But the question is:
Can this tree be saved? Discuss.

Friday Can O’ Worms

I was pleased to see that at least two of you dug into the literature over the weekend to read these papers!  (I can still remember the first time as a Master’s student when I was assigned a journal paper to review.  I had NO idea what, exactly, I was supposed to be doing.  It took a long time to figure it out.)

In any case, kudos to Jimbo and Diana for their thoughtful comments – and for zooming in on the problems.  Indeed, Jeff and I conclude there is likely a fertilizer effect on the plants – and a healthy plant is better able to resist insects.  Secondly, the speculation at the end of the paper regarding root uptake of phenolics from the vermicompost – compounds that weren’t even measured, much less monitored for uptake – is totally unsubstantiated and in fact is not feasible, given root physiology.  I’ve pasted my draft to the journal editors below, which explains this a bit more.  (Jeff also has some choice things to say, and I’ve added his comments as well.)

From LCS:  “I recently read the article by Edwards et al. entitled “Suppression of green peach aphid (Myzus persicae) (Sulz.), citrus mealybug (Planococcus citri) (Risso), and two spotted spider mite (Tetranychus urticae) (Koch.) attacks on tomatoes and cucumbers by aqueous extracts from vermicomposts” (29(1): 80-93).

“The article presents evidence that the use of vermicompost teas increased the resistance to damage from these pests.  As the authors state “there are many reports in the literature of organic nutrient sources decreasing numbers of pest arthropods.”  This seems a logical conclusion given that the authors have provided an additional nutrient source to their treated plants (vermicompost extract) that was not available to the control plants (which were drenched with water).  The treated plants were better able to manufacture anti-herbivore compounds as a result.

“Yet the authors then venture into unsupported speculation that this resistance was due to the uptake and transport of water-soluble phenols by the roots and into the leaves of these plants.  In the authors’ words:  “these diverse results all point to the probability that water-soluble phenols, extracted from the vermicompost during aquatic extraction, taken up into plants from soil receiving drenches of vermicompost aqueous extracts, could be the most likely mechanisms by which vermicompost aqueous extracts can suppress pest attacks.”

“Not only are there no data or other direct evidence to support this speculation, but the likelihood of such uptake is highly unlikely if not impossible.  The water/nutrient uptake mechanism in plant roots is cellularly regulated, particularly at the endodermis, where all solutes must pass through cell membranes prior to entering the vascular tissue.  No such transport has ever been documented in the literature, though the authors report “There have also been suggestions of these effects being due to the uptake into plants of phenols from organic manures (Ravi et al., 2006).”  This latter paper, however, measures the presence of phenols and their associated enzymes in the plant tissues, not the uptake of soluble phenolics.  Plant physiologists and biochemists have long known that plants are capable of synthesizing a wide variety of phenolic compounds used to ameliorate abiotic and biotic environmental stresses.  I am surprised that the authors did not discuss their theory with plant scientists at their institutions.

“It is disappointing that the authors were not discouraged during the peer-review process from making unsubstantiated, fantastic claims about the mechanisms underlying their research results. ”

From Jeff:  “Though we do not discount the possibility that compounds may have been present in the vermicompost that could have been taken up by the plant’s roots, we think it much more likely that there was a fertilization effect which caused the plants to grow more rapidly and/or which allowed the plant to defend itself more effectively using its own defensive mechanisms. The authors of this paper discount this effect by stating that “It could not be caused by uptake of soluble nutrients since all of the experimental treatments were supplied regularly with all the nutrients that they needed from Peter’s Nutrient Solution, which was applied to the experimental plants three times a week.” but do not include any evidence to back this statement up. This is a fatal flaw. In fact, the authors don’t even provide any data regarding the concentration of nutrients that were added. Simply stating the analysis of the Peter’s fertilizer which was used provides us little data as they could have mixed this up at any concentration before applying. Was nitrogen applied at 10ppm? 600ppm? Likewise, though the authors tell us the concentration of nutrients in the vermicompost used, no indication of the amount of nutrition in the compost extracts is given. If these analyses of nutrient content turned out to be too expensive the authors could simply have grown additional plants without exposing them to the insect pests. By then comparing plants which had been grown with extracts to those grown without the effects of the extracts on growth would have been made obvious. Another significant problem with this paper was the lack of information regarding the variety of tomato which was grown. Tomatoes have various resistance mechanisms to defend themselves from insect pests including, but not limited to, both glandular and non-glandular trichomes. Many papers over the years have shown that the density and chemical composition of these trichomes is affected by both the plants parentage and by nutrient concentration.

“In short, it is difficult to believe that even a novice researcher would provide the paucity of information and experimental data that these researchers did which might elucidate the presence or absence of a fertilization effect. The fact that the first author of this study is a seasoned researcher gives the impression that the objectivity of this research has been compromised. This impression is only strengthened when we discover, at the end of the paper, that this research was funded as a subcontract to a grant for small businesses, in this case the Oregon Soil Corporation. It seems logical to assume that this paper was published as a gimmick to promote the business interests of a producer of vermicompost rather than for any furthering of science. You have done your journal a great disservice by publishing it.”

Friday puzzler revealed!

Lots of discussion about the mysterious white streaking on the hedge.  The pictures below show a little more detail than those on Friday:

As you can see, Jimbo was on the right track when he suggested the hedge might be near a parking lot.  But it’s the heat escaping from the engine compartments that’s been causing the damage. 

(I am now committed to find some good photos of urine damage by dogs, donkeys, or drunken frat boys!)

Friday puzzler addendum

OK, I know we gave you a tough assignment for the weekend, so I want to post something fun as well.  (Think of this as dessert after your healthy meal!)  Take a look at the photos below:

This hedge is regularly sheared and no one part of it has been maintained any differently than another part.  Both sections of the hedge face east, and the damage is anywhere from 2 to 4 feet from the ground.  What do you think has caused the damage?

Explanatory photos reveal all on Monday!

Friday puzzler: Opening a can of worms

Part of being a Garden Professor is evaluating, interpreting, and passing on good science to the rest of the gardening world.  I was recently made aware of two articles soon to be published in Crop Protection and Pedobiologia, both peer-reviewed, scientific journals.  (You can download these articles just by clicking on the highlighted journal names.)

Briefly, what one expects from a scientific article is (1) a statement of the research question (the hypothesis) to be investigated, (2) a clear description of the materials used and procedures followed, (3) a listing of the results, along with their statistical significance, and (4) a discussion of the results, including whether they supported the hypothesis.


Both articles focus on the use of vermicompost teas as a way of reducing pest damage on greenhouse grown crops.  If you’re not familiar with this product, it’s made using worm castings and water in an aerated system. The researchers conducted one large experiment and divided the results into two parts for publication. Therefore, the materials and procedures were the same for both articles, and you’ll also see that the conclusions are likewise the same.  (My point – you really only need to read one of these articles.)


I sent these articles and my evaluations to my GP colleagues; at least two of us will be sending letters to the editors of both journals expressing our concerns.  Jeff thought these articles provided a great opportunity for our blog readers to look over our shoulders and see what we do.  We don’t question the results that the investigators got, nor do we have any argument with the statistical analysis.  We do question the authors’ interpretation of the results.

So here is your assignment for the weekend:

(1)  Read the methods section carefully to understand the differences between the treatments (the vermicompost tea addition) and the control. Can you think of an alternative reason for the results the researchers found?
2)  What additional flaw do you see in the discussion section in terms of the proposed mechanism of protection conferred by the vermicompost tea treatment?

On Monday, I’ll post the draft of the letter that we’ve drafted to the journals.

Oh, and if you have any questions, please post them!  We will answer them the best we can.