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!


Friday puzzle solved!

Great discussion over the weekend, with some very astute observations.  If you looked at the brown needles under the tree in Friday’s picture, you may have noticed that some of them weren’t needles:

Not only was this tree planted too deeply, as several of you pointed out, but the burlap and twine were left intact.  It appears the nylon twine has already started to girdle the trunk, based on the trunk swelling just above where the twine is wrapped.

I’ve ranted about this practice already, so I’ll just sigh and move on to the first question – what directly caused the needle drop from the lower part of the tree?  It’s a young tree facing west so the lower half gets plenty of sunlight.  And though needle drop is normal with all conifers, the upper portion of the tree does not show the same drop with its interior needles.  My guess is that ethylene gas is responsible.

Plant roots under stress often release ethylene, a natural plant growth regulator more commonly associated with fruit ripening.  It also induces leaf drop, so as it percolates out of the soil it affects the lower leaves, but dissipates before it reaches leaves higher in the crown.  It’s a common phenomenon with over-watered house plants.

Thanks to all of you who participated in the diagnosis discussion – this is more fun than my 20 years of college teaching!

Friday fun!

It’s the holidays, but this pine tree is feeling anything but merry.  It was installed about a year ago.  While the upper foliage looks lush and green, the lower branches have no new needles and in fact the current needles are dropping:

This is a two part question:

1)  What might be directly responsible for the needle decline on the lower branches?  (Hint:  this is caused by the plant itself.)

2)  What might be the underlying stress causing the needle decline?  (Hint:  this is caused by by people.)

Additional photos on Monday will reveal all!

Inspecting nursery plants, part IV

(Note:  this is a really LONG post.  Not in text – but in photos!  Sorry for all the scrolling.)

I don’t know about you, but after spending three weeks on my hands and knees looking for trunk rots, surface roots, and suckers, I’m ready to become bipedal again.  So today let’s look at trunks – and what shouldn’t be missing on them.

Many young trees have numerous short branches along their trunk, as shown in the photo below:


Unfortunately, many nurseries and gardeners think this looks scruffy, and respond by pruning these branches off, leaving a tree such as the one in this picture:

Personally, I think these trees look like lollipops, but aesthetics aside, this type of pruning can inadvertently damage young trees.  Their bark is often thin and sensitive to environmental stress – especially sunburn.  Without those short branches deflecting the sun from the bark surface, the living tissues under the bark can be killed, creating dead patches on the trunk:

How can you tell if the tree you’re considering has been improperly pruned?  Just look for those tell-tale pruning cuts, as a close examination of the lollipop tree reveal:

In time, these trees develop thicker bark, and the lower branches are gradually shaded out as the crown increases.  Be patient!  Let your trees be a little fuzzy when they’re young.  They’ll grow out of it.

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.

Inspecting nursery plants, part ll

Well, I’m recovering from this simply horrific chest cold or whatever it is and feeling brain function returning.  The last time we were at our virtual nursery, we were looking for root flare and inspecting the trunk for damage from improper bagging.  Since we’re already down on our hands and knees, let’s consider roots.  In general, you really don’t want to SEE roots, except where they meet the trunk (the root flare).  The presence of coyly crossed “knees” in this photo is a clear indicator of a plant that wasn’t potted up quickly enough:

Likewise, while the fused, circling woody root mass in this next photo might be aesthetically interesting, it sure doesn’t make a functional root system:

It’s pretty easy to avoid these types of plants, because you can see the root problems before purchasing.  The hidden root problems, such as those I’ve shown in earlier posts, are tough to find until (or if) you take all the extraneous stuff off of the root ball.

Finally, there is a new production practice that really fries my potatoes.  What really makes me angry is that these trees had absolutely LOVELY roots – a nice flare, woody roots spreading radially – and then they were butchered – and left unprotected:


I can think of no legitimate reason for this practice.  I’ll be curious to hear my colleagues’ thoughts, as well as those from the blogosphere.

The World According to Chub

Following up on Holly’s theme of “I can’t believe I get paid to do this”, last Wednesday I participated in a walk-through and inspection of the Justin ‘Chub’ Harper Collection of Dwarf and Rare conifers at MSU’s Hidden Lake Gardens in south central Michigan.  The Harper collection is widely regarded as one of the premier collections of rare and unusual conifers in the world.

Harper Conifer Collection with fall color background.  Photo: Jack Wikle.

A little background: Chub Harper was the former grounds supervisor for John Deere’s world headquarters in Moline, IL, an avid plant collector, and a founding member of the American Conifer Society (ACS).  He acquired hundreds of rare and unusual conifer specimens around his home and eventually had to lease a nearby lot for the overflow – demonstrating that ACS also stands for ‘Addicted Conifer Syndrome’.  In the early 1980’s Chub donated 300 conifers to Hidden Lake Gardens to establish the Harper collection.  All of the plants were balled and burlapped by hand and shipped in three semis to Michigan.  Chub continued to add plants to the collection and today the collection includes over 500 accessions.

Chamaecyparis pisifera 'Filifera aurea '
Chamaecyparis pisifera ‘Filifera aurea’

I met Chub about 8 years ago and with his guidance and inspiration started a series of ‘Conifer Corner’ articles in the Michigan Nursery and Landscape Michigan Landscape magazine.  (visit my faculty page for .pdf’s of some sample articles  Once or twice a year Chub would travel from Moline to Michigan to inspect the collection along with members of his conifer posse.   To me, the most impressive thing about the walk-throughs was how absolutely ruthless Chub was in disposing of under-performing plants or plants with continual pest problems.  “Time for that thing to take a ride on the chipper truck” was a favorite Chub-ism.  Hidden Lake Gardens has a garden staff that could spray pesticides or prune away dead material regularly; but Chub wanted none of it.   This is not to say that Chub was into organic gardening; as far as I know he had no particular aversion to chemicals.  Rather, he felt the mission of the collection was education and that maintaining plants in an artificially superior condition would mislead the public into thinking some conifers were better suited than they actually were.

The Conifer posse at the 2007 walk-through.  Chub Harper is 3rd from left, back row.

Chub passed away unexpectedly earlier this year and last Wednesday’s walk-through was the first evaluation of the collection without him.  The conifer posse carried on, led by former ACS President Dennis Groh; Chub’s longtime friend Jack Wikle; and Sam Lovall, the landscape architect who developed the original design for the collection.  We found homes in the collection for several new specimens including an Abies concolor ‘Charmin’ Chub’ and condemned a few underachievers to a ride on the chipper truck.  Chub left many legacies; the most obvious and tangible is the Harper Collection and the staggering generosity it represents.  Imagine dedicating half your life to acquiring and cultivating a world-class collection and then simply giving it away.   Just as important, however, is the legacy he left with those who knew him, who felt his passion for conifers, and were inspired by him.

Pins parvaflora 'Cleary'
Pinus parviflora ‘Cleary’

I see the light!

Here’s the follow up picture from Friday’s puzzler:

As you can see, there’s a street light near the lower half of the maple.  (I cleverly hid it behind the utility pole in the first photo.)  The green part of the tree never received the message that days were getting shorter, since the street light is bright enough to interrupt the dark period necessary to initiate dormancy.

This is one of my favorite phenomena unique to urban environments.  It doesn’t appear to hurt the tree, although the green leaves will die before the tree can scavenge their nutrients.  If more of the tree were affected, I’d be more concerned.

Congratulations to Planting Oaks, who nailed this right off.  And kudos to those thoughtful alternative explanations, all of which could logically have had an impact on color change.

What Makes Growers Change?

Over the last few weeks I’ve said a lot of complimentary things about the Minnesota Nursery Industry and how they’re careful to avoid situations where trees are planted too deeply.  What I haven’t mentioned is that there is a reason for this.  During the 1980s and early ’90s trees were usually planted deeply with lots of soil over the uppermost roots.  It was just common practice.  Unfortunately this practice led to roots growing across the trees stems and, when those roots cross the stem, the roots always win  (as you saw in Linda’s quiz last week)!  Many, many trees planted in that era have trunks which enter the ground looking like the picture below.  You can clearly see the roots strangling the tree.  This photo was actually taken last year on this campus!

This tree is one of the lucky ones.  These girdling roots were removed, the layer of soil over the crown was removed, the crown of the tree was inspected, and it was determined that this tree could survive.  Many others planted during the 80s and 90s are not so lucky — in fact, many are suffering or dead.

An outcry over the last dozen years or so, mostly from cities (St. Paul and Minneapolis), led to changes in harvest by the nursery industry, and by the landscapers who install the trees.  Yesterday I received a plan for planting trees up and down a major highway here in St. Paul to review.  The specs were very specific — and similar to the specs that we see now across the Twin Cities and most of Minnesota.  Root flare must be at, or even above, the surface of the soil.

I don’t want anyone to get the wrong idea.  There are still those who sell nursery stock with the root flare buried deep under soil in the ball, and there are landscapers who dig a hole twice as deep as the depth of the ball before planting, but it’s becoming less and less of an issue as, in general, we seemed to have learned from past mistakes.