The evolution of tree care

I’m one of those people who can’t resist things that are free.  When you pass a yard that has a ‘FREE’ sign on a rusted-out lawn mower or an old piece of exercise equipment and wonder, “Who on earth would take that home?” Uh… that would be me.  Actually, I blame my dad.  The Old Man’s garage was crammed full with outboard motors missing pull-cords, mismatched lawn furniture, and all manner of secondhand tools in varying states of disrepair. So my defect is genetic.  Only my hyper-organized wife prevents my garage from suffering the same fate as my dad’s.  But, like a blind pig finds an acorn, sometimes a scavenger finds something interesting.  Recently a retiring professor in our building was cleaning out his office and put a stack of old books on a table by his door.  And there was the magic word: FREE.  I sifted through the stack and found my treasure; a 1948 edition of Maintenance of Shade and Ornamental Trees by Dr. P.P. Pirone.  Granted, I have ready access to several modern tree care texts plus an ocean of information on the International Society of Arboriculture (ISA) website, but after thumbing through a few pages I knew this one was a keeper.  The main reason the Pirone book interested me was to see how tree care has changed over the years and how things have stayed the same.

What’s changed?
  The most obvious example of how tree care has changed in 60-plus years is that an entire chapter of the 1948 text is devoted to cavity filling.  For those not familiar with the process; cavity filling, as the name implies, is the practice of filling hollow areas in trees with concrete, just as a dentist fills a tooth cavity with amalgam or composite materials.  Filling cavities in trees was once a relatively common practice.  Today we recognize that filling does not stop decay and there is little value in the practice (and it certainly complicates removals).  If anything, we would replace this chapter with ‘hazard tree assessment’, seeking to determine if the tree has enough solid wood to be structurally sound.

Filling a tree cavity.

The finished job

What hasn’t changed?  I was surprised to see a fairly lengthy discussion of problems related to girdling roots from 60 years ago.  This actually causes me to eat a little crow.  I have sometimes questioned whether the current ‘epidemic’ of girdling roots is actually related to the fact that arborists and urban foresters are spending more time looking for them and have better tools (specifically air spades) for finding them.  Of course, Pirone’s text also suggests that girdling roots were an issue before the advent of evil nursery production systems such as container growing.  The discussion of treating girdling roots points out another change in practices.  In 1948 the arborist was advised to carefully dress the wound with paint or tar.  Today we generally advise against treating wounds except for situations which risk certain exposures such as if oak trees must be pruned when they are at risk for infection from oak wilt.

Girdling root removed and wound dressed.

So there you go, another book to clutter my shelf and another example of one man’s trash turning into another man’s treasure.  I think Dad would have been proud.

Science rant coming up later today…

I’ve got a good post for today…but have a seminar to give this morning and the blog has to wait.  If you have time, go onto the web and look for “water drops burn leaves” or something like that.  You’ll find reference to an article in New Phytologist that has the gardening world all a-twitter.  I’ll be dissecting the paper – and the surrounding hype – later today.

Is This Really a Good Idea?

Transgenic plants have been with us for well over a decade now.  I have had the opportunity to work with many of the tools used for this technology, though most of that is far behind me (over 15 years now since I “ran a gel”) — I’m much happier being outside or even in front of a computer writing than in a lab.  Fortunately I have a number of “lab rat” colleagues so I’m relatively up to date on what’s going on and what “gene-jumper” scientists can and can’t do.

To make a long story short, over these years transgenic plants have proven to be useful in some cases (by reducing the use of certain dangerous pesticides), and concerning in others (because some genes have escaped cultivation).  I’m not going to go into the crazy ins and outs of the benefits and drawbacks of genetic engineering here except to give you my general opinion which is that every case needs to be handled individually.  I do not believe that this engineering is, in and of itself, a bad thing.  That said, I do believe that genetic engineering could get us into trouble if we’re not careful.So, with that little disclaimer I thought I’d mention something that I question — I’m not going to call it bad — but maybe it is.  There’s an “artist” at the University of Minnesota — Eduardo Kac, who had some of his genetic material placed into a petunia so that some of the proteins from his body were expressed along with the petunias.  I don’t have a picture of the petunia itself, but here on the St. Paul campus of the University of Minnesota you can see this structure, created by Kac, which is a graphic representation of a protein from the petunia.

(It was around 20 degrees outside when I took this shot — positively balmy compared to what it’s been like).

Anyway — as I said above — Though I have some concerns, I’m not opposed to using biotechnology for the “good of man.”  Nor am I afraid that Kac’s creation is dangerous — from what I know it was kept in a closed system and the petunia was ultimately destroyed — not that it would have been particularly dangerous even if it had been released.  But….to me anyway….This seems like a frivolous use of a powerful tool (that tool being the ability to move genes from one organism to another).  I don’t know if I’d call it bad….but the words Wasteful and Inappropriate come to mind.

By the way, though I know less about it, it seems that Kac also transferred genes into a rabbit for the sake of art.  But this is, after all, a horticulture blog so I thought I’d stick with plants!

And now for something completely different…

From this week’s e-mail file…

“Dear Dr. Cregg:

As I’ve done for many year, this year I harvested my “wild” Christmas  tree from the Huron-Manistee National Forest. I cut the tree at ground level. Soon after I brought it home, it started sprouting new light green clumps of needles at the tips of many branches. Is the tree actually growing? It doesn’t seem possible that it’s still alive, but it seems to be thriving and I hate to toss the tree to the curb if it’s fighting for life. I am tempted to leave it in the tree stand to see what happens….”

Pat M.

Midland, MI


Dear Pat:
The tree is dead, it just doesn’t know it yet.  Depending of the species, some Christmas trees will break bud and begin to grow once they are brought indoors.  The tree is still alive in the sense that its needles are still carrying out photosynthesis and water is still moving up the trunk to the needles.  But since the tree has no roots and no way to produce any new roots, it has no prospect for long-term survival.  The phenomenon you’re observing is common in some spruces and other conifers adapted to cold regions.  Before you cut the tree, the buds were exposed to enough cold to meet their chilling requirement to overcome dormancy – the only thing that keeps trees in wild from growing at this point are cold temperatures.  Once you brought it indoors, the tree ‘thought’ is was spring and started to grow.  If you or a family member want to do a little science project you could keep the cut end in water and see how long the tree lasts.  Eventually, however, the conducting elements at the cut end will begin to plug with resins and the tree won’t be able to move enough water to meet its needs and will expire.

Bert Cregg


Do landscape trees need nitrogen fertilization?

I’m in Grand Rapids this week attending the Michigan Nursery and Landscape Association/Michigan Turf Foundation Great Lakes Trade EXPO.  The topic for my talk today was Landscape Tree Fertilization.  That might not sound like a subject that would generate controversy, but as with most things, there are camps emerging.  There is a rising chorus of folks that suggest that landscape trees should not be fertilized with nitrogen.  There are a couple of lines of evidence that bolster this point of view.  First, many systematic studies on the growth response of street trees or landscape trees often do not show a response.  There are numerous examples of this, for example, in Arboriculture and Urban Forestry (formerly J. of Arboriculture).  The second line of evidence for not fertilizing landscape trees relates to the relationship between tree nutrition and susceptibility to insect pests.  This argument relies on the ‘growth vs. defense’ hypothesis and suggests that fertilization promotes growth at the expense of defense compounds; essentially making fertilized trees tastier to insect pests.

So, in light of this, why do I suggest that landscape trees should receive 1-2 lbs of N per 1000 sq ft. every 2-3 years?  First, we need to understand that nitrogen is constantly lost from landscape systems.  In forests, trees take up nutrients from the soil, translocate them to leaves, shed the leaves, and the nutrients are ultimately returned to the soil in a cyclic process.  In landscapes, leaves are usually raked or blown and removed from the cycle.  Soil nitrogen is also lost due to nitrate leaching.  Additionally there are often key weaknesses in some of the papers that purport to show no response to fertilization.  For example, Ferinni and Baietto (Arb & UF 32:93-99) found no response of sweetgum trees to two levels of fertilization.  However, the data show that the control trees, which were not fertilized, had similar (and fairly high) foliar N levels as the fertilized trees.  This pattern can be found in several similar studies.  The more appropriate conclusion for these papers should be “Trees that are not nutrient deficient do not respond to fertilization”.  Similar issues pervade studies related to the growth vs. defense hypothesis.  Why would one presume that a nutrient deficient plant would be better able defend itself against insects attack than a tree that has adequate nutrition?  Ideally, fertilization decisions should be based on visual symptoms and soil and foliar samples.  Nevertheless, low rates of N from either organic or inorganic sources will make up for losses from the N cycle and maintain tree vigor.

It should be noted that the rates I’m suggesting are considerable lower than those that are found in some older extension literature, which recommend rates of N up to 6 lbs for 1000 sq ft.  As a point of comparison, Midwestern farmers apply 150-200 lbs/acre to grow a crop a corn.  The 6 lb rate for landscape trees works out to around 260 lbs per acre!


O Tannenbaum!

Twas the blog before Christmas…  My last chance to post about Christmas trees for another year.  I’m always surprised when I troll around the web or do interviews how many myths about Christmas trees still abound.  So in the spirit of the season, a little Christmas tree myth-busting.

“Good grief.  I’ve killed it.”

Using a real tree hurts the environment
Here’s a real post from the website:

“Its so not fair to cut down all those baby trees, use them for a few weeks and then toss them by the curb for garbage removal. Everytime, i pass by a house and i see those poor trees just shoved out like that it breaks my heart. they belong in the forest or backyard where they were meant to be, growing old and improving the air and atmosphere. i used to like real Christmas trees but not anymore.”

Yes, Virginia, there are still people out there that think Christmas trees are cut from forests. The U.S Forest Service and some state forestry departments do offer permits to cut Christmas trees but this is a tiny fraction of the trees used in the U.S.  Virtually all Christmas trees sold at tree lots and stores are grown on Christmas tree farms for that purpose.  For each tree cut, growers plant two or three more.   Moreover, many communities have programs for re-cycling Christmas trees into mulch or compost.

Christmas trees are a fire hazard.
The key here is water.  Fresh Christmas trees that are properly watered are not a fire hazard.  Trees that are allowed to dry can be a fire hazard.  These are the ones your local TV station uses for their annual dramatic Christmas tree fire video.

Fire retardant sprays make Christmas trees safer.
Research by Dr. Gary Chastagner, a colleague of Linda’s at WSU-Puyallup, has shown that some fire retardants can actually increase tree moisture loss.  Maintaining tree moisture is the key to making trees safer and improving needle retention.  Making sure the tree stand never dries out is much more important than a fire retardant spray.

Injecting water directly into the stem is the best way to maintain tree moisture content.
This is a case where a little knowledge can be a dangerous thing.  Since water moves up the tree through the xylem elements in the stem, wouldn’t injecting water right into the stem be the best way to water?  That’s the logic behind the Tree I.V.  As the name implies, this device is like an I.V. drip for your tree.  Drill some holes in the trunk, attach some tubes to a jug a water, and voilà, a self-watering tree!  We can thank Gary Chastagner again for busting this myth.  He and some colleagues found that displaying a tree in a regular tree-stand with water maintained higher tree moisture levels that the I.V. technique.

So, if arborists can use trunk injection to apply pesticides and fertilizers to trees, why wouldn’t the Tree I.V. work?  Actually, the tree I.V. does work in the sense that the tree will take up water from the jug.  The problem is that the tree may not take up enough to meet its total water need.  In a normal stand, the entire stem cross section is exposed to water.  With the tree I.V. only a portion of the stem will be translocating water.  Plus, conifers contain resin ducts which clog injection ports.  This is one of the reasons why arborist’s trunk injections don’t work as well as on conifers as they do on most hardwoods.

Bottom-line, keeping your Christmas tree hydrated is the key to retaining needles and keeping the tree safe.  A good rule of thumb is that a stand should hold a quart of water for each inch of tree caliper at the base.  For most trees this means a stand that will hold at least a gallon of water.   Check water in the stand daily and never let the tree go dry.

Have a very merry Christmas!

Are natives the answer?

Last week Jeff kicked off a lively discussion about invasive plants.  Let me state up front that no one on this blog is promoting invasive plants.  But the issues surrounding invasive plants are extremely complex and have profound implications for many groups with whom we work in landscape horticulture and urban and community forestry.  It is essential in these discussions that we separate fact from hyperbole.  In some quarters, lines have been blurred and people fail to make key distinctions and lump exotic, alien, or non-native species together with invasives.  According to the Federal Executive Order on Invasive species “Invasive species” means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.  All invasives are alien but only a small fraction of alien species are invasive (all humans are mammals but not all mammals are humans).  Nevertheless, there is a temptation to ‘hedge all bets’ and promote only native species for horticultural planting since native plants, by definition, cannot be invasive.  In addition, there is a ‘feel good’ aura that surrounds native plants – if they’re native they must be good – that clouds some of the logic in the argument.

Some examples:

Natives are more stress tolerant and better adapted than exotics.
Really.  If native plants are always better adapted, why do we have invasives?  Shouldn’t the “better adapted” natives out-compete them? Stress tolerance and adaption are a function of natural selection pressures of the environment in which a species or population evolves.  The world is full of stressful environments and, therefore, lots of stress tolerant species.  There is no a priori reason, for example, to believe that a native species needs less water than an exotic.  The ability to withstand drought depends on the particular species in question.  I’ve done a lot of research on stress physiology of Scots pine – few, if any, native species here in Michigan can match it for drought and cold hardiness.  Moreover, as Jeff pointed out, most of our urban and suburban environments no longer reflect native conditions.  Urban heat islands can result in temperatures 10-20 deg. F warmer than the native countryside.  In our research on heat island effects in downtown Lincoln, NE we logged temperatures in tree canopies in excess of 125 deg. F.  These temperatures were coupled that with the usual urban conditions of impervious surfaces and compacted soils – what tree species is native to that ecosystem?

Native restoration?  This nurse-log ecosystem is typical of forests in western Oregon & Washington.  Trying to keep it alive in downtown Portland requires constant mist irrigation..

Native plants are more pest resistant than exotics.  This would be true if native pests were all we had to contend with.  But the exotic pest train has already left the station.  Emerald ash borer, Dutch elm disease, white pine blister rust, chestnut blight, Asian long horned beetle, and sirex wood wasp are here and here to stay.  And their friends are coming.  The continued expansion of global trade will almost undoubtedly mean that exotic pests, for which native trees have not evolved resistance, will become more, not less, of a problem in the future.   Relying exclusively on native trees means more, not fewer, catastrophic tree failures.  Heavy planting of green and white ash, which are both native in Michigan, has resulted in the loss of 30% or more of the urban tree canopy to EAB in some Michigan communities.

Natives increase diversity  This presupposes that exotic species do not or cannot fill niches occupied by natives.  Exotics can certainly add structural diversity and age class diversity to an urban and community forest.  I would also argue that they add to species biodiversity as well.  If we consider an urban community such as Lansing or Detroit, there are maybe six or seven native tree species that we could expect to have reasonable longevity as street trees.  If we expand our choices to include non-natives we can expand the list to twenty or so.  Not a huge number to be sure, but still a better hedge against catastrophic urban tree loss that the ‘native only’ policy.

Where to go from here?  We cannot ignore that fact the invasive plants are a huge economic and environmental issue.  Presently we do not have models that will accurately predict which exotics will become invasive and which ones won’t.  Trees that are demonstrated to be invasive in a given environment need to be dropped from planting programs.  Except for the desert Southwest and parts of the Plains, every region of the country has great native trees that can. and should, be an integral part of their urban and community forests.  While it’s tempting to play it safe and promote natives only, this policy has significant shortcomings.  Urban and community forests provide enormous economic, environmental, and societal benefits.  In order for our urban forests to provide these functions over the long term we need as broad an array of trees species as possible, including appropriate exotics.

Growing a greener Christmas tree

Among the many hats I wear, one of the most enjoyable is that of an Extension Specialist working with Christmas tree growers here in Michigan and surrounding states.  I suppose part of the satisfaction stems from the fact that my first real job was shearing Christmas trees in southwest Washington during my high school summers.  To give you an idea how long ago this was, the minimum wage when I started the summer between my sophomore and junior years was $2.20 per hour.

Today I’m involved in a variety of projects related to improving sustainability of Christmas tree production, particular water and nutrient management.  One of our major focus areas has been the development of container production systems for living Christmas trees.  For those not familiar with the concept, living Christmas trees are conifers that are sold with their roots intact as opposed to a cut Christmas tree.  Living Christmas trees serve a niche market for people that think cutting a Christmas tree is wasteful or even harmful to the environment (never mind that virtually every Christmas tree cut in the US was grown on a plantation for that express purpose).  Many Christmas tree growers have sold living trees by digging trees from their fields and selling them balled and burlapped or placing them in containers.  In our current work we’ve focused on growing several species of conifers as container stock with the end goal as living Christmas trees.  Container growing imparts a couple of advantage over the B&B method.  First, container-growing eliminates loss of roots associated with field digging.  Second container-grown trees are much lighter weight for consumers to handle than field-dug trees. Also, we have found the container-grown trees survive post-holiday storage and transplanting better than field-dug trees.

My former grad student, Wendy Klooster, shows off one on the Fraser firs she grew as part of her MS project on nutrient management.

If you’re considering a living Christmas tree here are some things to keep in mind.

  • If your ultimate goal is to plant the tree in your landscape, make sure the tree is a species that’s hardy in your location.  There are several types of container-grown conifers such as Italian stone pine (Pinus pinea) sold in big box stores and super markets that are hardy in only the warmest parts of the country.
  • Limit display of living trees to 10 days to two weeks.  Most conifers will begin to lose dormancy shortly after being brought indoors.  We’ve observed that some, such as Black hills spruce (Picea glauca var. densata), will break bud if the weather has been cold and they’ve had sufficient chilling.
  • After the holidays, place the living tree in a protected but unheated space such as a garage or enclosed porch or patio.  The key here is that the tree needs some exposure to light – but avoid direct sun.

As with just about everything these days, the environmental friendliness of Christmas tree production is receiving increasing scrutiny.  One way to have the ‘greenest’ tree on the block is to bring home a tree that keeps on giving.

Let It Snow!

Here in Minnesota one of the things that we need to worry about is the cold.  Over the winter we can see temperatures down into the -30s (even the -40s in the Northern part of the state) and it can damage many of the plants that we grow.  The tops of the trees are usually able to handle these types of temperatures — though a good heavy snowfall can cause a limb to collapse now and again.

The bigger problem is with roots which aren’t able to handle the cold like the top of a plant can.  Once you get 10 degrees or so below freezing you’ve killed the roots of most plants.  Fortunately the ground is a great insulator and doesn’t get nearly as cold as you’d think.  Once you get two or three inches under the surface of the soil temperatures will hover right around freezing for most of the winter.  The plants that are most susceptible to cold are those that are in containers because their roots are above the soil’s surface.  Nursery growers usually protect containers from the cold by consolidating them (pushing them together as in the picture below).

These containers are then covered with a layer of polyethylene fabric, about 6 inches of straw, and then another layer of polyethylene fabric.  Temperatures under the fabric rarely go below 26 degrees or so — even when outside temperatures stay around -20.  Most plants come out fine — our biggest problem is that sometimes voles and mice will take up residence under the tarp and eat the plants — which isn’t a big deal unless its a research project — in those cases we will often use poisons or, more frequently, repellents.

The only thing better than this method is snow.  If we could count on snow every year we wouldn’t bother covering the plants at all.  Snow is the best insulator that we have.  Under snow temperatures rarely go below 29 degrees or so.  So, despite the traffic problems that snow causes, nurserymen and landscapers are always happy to see snow on the ground before the really low temperatures hit.

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.”