Why don’t landscape trees respond to fertilization?

As part of my ‘other duties as assigned’, I have taken on an assignment to develop fertilizer prescriptions for landscape trees and shrubs based on soil samples submitted by homeowners to the MSU soils lab.  This has gotten me immersed in two sets of conflicting literature. 

On one hand is a raft of extension bulletins and ISA guidelines on tree fertilization, which typically suggest adding 2-4 lbs of nitrogen (N) per 1,000 sq ft of ground area – in some cases up to 6 lbs of N per 1,000 sq ft (Rose 1999).  To put that last number in perspective, that works out to 262 lbs of N per acre.  A typical recommendation for corn in Ohio is 150 lbs of N per acre.  Landscape trees need more fertilizer than a field of corn?

On the other side are numerous landscape studies, and even some nursery field studies, that suggest landscape trees simply don’t respond to fertilization (Day and Harris, 2007; Ferrini and Baietto, 2006; Harris et al., 2008; Robbins, 2006; Werner and Jull, 2009). 

So, what gives?  I mean, we know trees need nitrogen to make proteins, enzymes, chlorophyll, and all that good stuff.  We rake up leaves and haul them away each fall; short circuiting the natural nutrient cycle.  Urban trees should be starving for nutrients and begging to be fed, yet don’t grow any better when we fertilize them?  There are likely several factors at work.  First, many of the widely circulated recommendations are based, at least in part, on rates that might be applied in production forestry or nurseries where maximizing growth is a primary objective.  Also, as Dan Struve (2002) noted in his review, some recommendations were based on poorly designed studies.   Or, as Dan Herms (2002) hypothesized, trees may be allocating more resources to defensive compounds. 

Another answer may be found in some of the papers cited above.  In many cases, foliar N levels of the control (unfertilized) trees were already in sufficiency ranges.  In other words, even without fertilizer the trees had enough nutrients.  Part of this is a recent emphasis on the question of fertilizing at transplanting.  Trees coming in from a nursery are likely to be pretty jacked up with fertilizer – often referred to as a ‘nutrient loading’ effect.  Trees can be pretty efficient at internal nutrient re-cycling, so this effect can persist for a few years after transplanting. 

So where does all this leave me on my homeowner recommendation?  For some elements, like potassium and phosphorus, we can base a recommendation on the standard soil test included in the program.  Unfortunately there is no simple indicator of soil N availability (see Scharenbroch and Lloyd (2004) for a thorough discussion).  Foliar samples would be the best bet but aren’t part of our testing program.  At this point, I’m leaning toward a simple visual assessment – Do the trees/shrubs look healthy and have acceptable color? (check yes/no).  If ‘yes’, don’t fertilize.  If ‘no’ add 1-2 lbs N/ 1000 sq ft.

Your thoughts?   


Day and Harris. 2007. Arboric. and Urban Forestry 33:113-121.

Ferrini and Baietto. 2006. Arboric. and Urban Forestry 32:93-99.

Harris et al. 2008. Urban Forestry and Urban Greening 7(3): 195-206.

Herms. 2002. Environ. Entomology 31:923-933.

Robbins. 2006. So. Nursery Assoc. Proc. 51:113-117.

Rose. 1999. HortTechnology 9(4):613-617.

Scharenbroch and Lloyd. 2004. J. of Arboric. 30:214-229.

Struve. 2002. J. of Arboric. 20:252-263.

Werner and Jull. 2009. Arboric. and Urban Forestry 35:252-262.

[Return of] National Forced Affection Day

After several incredibly informative posts from Jeff, Bert, and Linda, I’m going to give you a break as I veer off into Useless-land.

I’ve written posts previously regarding how important Valentine’s Day is for floriculture, most obviously the cut flower growers and florists.  In past posts, I’ve suggested alternatives to the boring/overpriced/under-fragranced/outsourced bouquet of red roses that folks seem to gravitate to.   The National Retail Federation predicts record-breaking sales in excess of $17 billion this year; $1.8 billion will be spent on cut flowers.  Fully two-thirds of that amount will be red roses. Bleah!

Other than Mother’s Day, there are very few holidays/events associated with cut flower sales; Valentine’s and the few days preceding is the make-or-break week for most florists. 

Here’s where I veer off…I was in the process of jotting down potential floral marketing opportunities for this post, when I opened the GrowerTalks industry newsletter/blog that coincidently featured thoughts along those lines for garden centers. Author Chris Beytes pointed readers to “Brownielocks’ Weird Holiday Calendar” for ideas.  I followed the link and proceeded to blow an entire hour and a half of what was supposed to be a very busy morning.

SO, in addition to this week leading up to Valentine’s Day, there are a few other things going on, including…

Freelance Writers Appreciation Week (Feb 5-11), Just Say No to PowerPoint Week (Feb 6- 10), Have a Heart for a Chained Dog Week (Feb 7-14 and always), National School Counseling Week (Feb 6-11), and Dump Your Significant Jerk Week (Feb 5-11) (to be followed by National Flirting Week Feb 13-19)

As for the entire month of February, you may already be aware of American Heart Month and National African-American History Month. There’s more limited exposure for National Pet Dental Health Month, National Hot Breakfast Month, International Hoof-care Month, Marijuana Awareness Month, Jobs in Golf Month, and Spunky Old Broads Month.

We’ve already missed National Work Naked Day (Feb 1), Crepe Day (Feb 2), Marmot Day (Feb 2), and World Nutella Day (Feb 5). 

But never fear, there’s still time to celebrate Lupercalia (Feb 15), Cow Milked While Flying in Airplane Day (Feb. 18) and National Margarita Day (Feb 22, count down calendar here).

Google any for more information. If I linked them all, you’d just waste more time.

Our visiting professor digs into tomato planting depth

With Ray’s recent photos of the peach, crabapple, and hydrangea planted too deeply, a discussion of tomato planting depth arose in the comments. I’ve seen the prolific adventitious roots start to form near the base of tomato plants, and I plant tomatoes to the cotyledon or deeper, but tomato planting depth not an area I have extensive research experience in. So I did a little literature search.

It seems that the practice of planting tomatoes is more than just friendly garden folklore. There’s some evidence that it works. Or, at least it works sometimes. Early in the season. And maybe not everywhere. Oh these darn scientists, with their wishy-washy caveats.

There just isn’t a whole lot of peer-reviewed research on the subject. One of the recent papers (from 1996) cites a “Crockett’s Victory Garden” (circa 1977) as a source of information for Northern gardeners about the benefit of deeply-planted tomatoes. Jim Crockett was no horticultural slouch, but just because successful gardeners do something doesn’t mean it’s sound practice (but like I said, I do it too…).

Southern tomato growers, on the other hand, have some peer-reviewed evidence to go on. It seems that in some years and some locations in Florida, tomatoes planted to the first true leaf were able to be harvested earlier (Vavrina 1996) than if they were planted to the top of the root ball. The overall trend was for a bit more and a bit larger fruit per plant. The paper is pretty sparse on detail (statistics and design), but that’s what they conclude. Similarly, a study of fall-planted tomatoes in Louisiana (Hanna, 1997) showed a benefit of planting to the first true leaf instead of to the top of the root ball in two years at one location. In that study, treatments that lowered the root zone temperature tended to increase yield. If you can get an extra 3 pounds of medium or larger tomatoes from a 50 square foot plot in some years, what’s the harm? Deep planting certainly helps prevent lodging in tender young plants, so there’s a clear benefit there if the supports aren’t adequate.

Most of the work suggests that, like mulch, deep planting helps to moderate root temperature, and fluctuating or high root temperatures are stressful to the plant. I can get behind that I suppose, but it’s hot in Florida and Louisiana in the summer. What about stuff planted in Minnesota, where I live? We use plastic up here to get warmer soil temperatures at planting, not cooler temperatures!  Well, I didn’t find any work on that.  But interestingly, some of the yield benefit seen in planting peppers to the cotyledon or true leaf in Florida (equivalent to 5 extra pounds per 50 square feet, in 3 of 4 years, with commercial spacing and fertilizer rates; Vavrina, 1994) don’t show up in Massachusetts (Mangan, 2000). But deep planting did help prevent lodging, which allowed for faster crop maturity in both places.

So the verdict on deep planting tomatoes? It doesn’t hurt, it helps sometimes, and it helps prevent lodging, so why not? One caveat: don’t plant grafted tomatoes deep. The scion will make roots, negating some of the benefit of the rootstock.

(And an addition from Linda:  here’s a diagram from TAMU demonstrating planting depth for tomatoes; this link will take you to the article itself.)

Mulch: Just Do It

A follow-up to Linda’s post about a recent study in Arboriculture and Urban Forestry that indicated mulch may not reduce evaporation of water from soil as we generally assume and which suggested that landscapers may want to re-evaluate their mulching practices.  As Linda noted, we have some real concerns about this conclusion and believe that proper mulching of landscape trees and shrubs has well-established benefits.

First, I don’t question the results present in AUF article.  As my grad students frequently hear me say, the data are the data.  But we need to bear in mind the paper is looking at one aspect of one of the impacts of mulching.  As Linda notes there are plenty of data to suggest that mulching does conserve soil moisture and she included some data from one of my studies which demonstrated this.  But there are other benefits to mulching beyond improving soil moisture.  I’d like to mention two here; one is a practical observation, the other is based on data from another study.

Preventing lawn mower blight and string trimmer trauma
From my observations and experience, one of the biggest and perhaps least appreciated benefits of mulch is keeping mowers and weed whips at bay.  I’m not aware of specific data, but damage by trimmers and mowers has to be near the top of the list of causes of mortality and long-term damage of new landscape trees.  A mulched zone around trees provides a buffer and simplifies maintenance operations. It’s an easy, simple and effective way to eliminate a major cause of tree distress.  A no-brainer.

Young, thin bark is no match for mowers and trimmers

Mulch protects trees and simplifies maintenance operations

Reduced soil temperatures
We conducted a trial a few years ago to look at the impacts of plastic mulches to improve establishment and early growth of Christmas trees. http://www.hrt.msu.edu/assets/PagePDFs/bert-cregg/cregg-et-al-2009.pdf  In a bit of serendipity we added a treatment at the last minute because our research station had some leftover wood chips from another project.  The trees mulched with wood chips were not irrigated but grew almost as much as trees in the plots that were irrigated.  What was especially impressive was the insulating effects of the wood chip mulch.  We installed probes to measure soil temperature 2” below the soil surface that we logged continuously.  During a July heat wave we found that soil temperatures were up to 10oC (18oF) cooler on the wood chip mulch plots than on the bare-ground plots.  Reducing soil heat load has profound implications for reducing root respiration and improving overall root function.

Soil temperatures were continuously logged on bare ground plots (foreground) and plots with wood chip mulch (background)

Soil temps peaked at around 38C (100 F) on bare ground without mulch (green dots);  max. soil temps were about 28C (82F) with wood chips (purple squares).

As Linda noted in her comprehensive review article, mulching has a myriad of benefits for landscape trees and shrubs.  It is important that we continue to look at all the various aspects of mulching and understand how to maximize the benefits (or reduce negative impacts) of mulching but in doing so it’s important to not lose site of the bigger picture.  While individual studies may yield conflicting data, on the whole, the preponderance of evidence and practical considerations come down strongly in favor of mulching

Stupid plant tricks

Blog reader Ray sent these photos of his weeping peach, weeping crabapple, and a Hydrangea paniculata, along with this comment:


“When a tomato grower extrapolates his applied knowledge to his landscape, before learning otherwise.”

(Translation for those tomato avoiders like me:  they are all planted too deeply, which tomatoes like.  Trees and shrubs, not so much.)

If It’s In The Wall Street Joural Opinions Section Then We Need to Rethink Everything.

I’m no global warming apologist.  I do think it’s happening.  I think the new USDA map supports that it’s happening.  I also think that humans probably have something to do with it since we cut trees and burn fossil fuels which release carbon dioxide, a greenhouse gas, into our atmosphere.  That said, I’m not convinced that the carbon dioxide produced by people is as big a player in global warming as we’re being led to believe, or that the world is about to become a living hell because of it.  Still, I don’t know if I’m on board with this article published in the Wall Street Journal last week.  Sixteen scientists with diverse backgrounds basically encouraging us to ignore carbon dioxide emissions for awhile. 

Look, even if global warming is a red herring, carbon dioxide isn’t the wonderful universal fertilizer claimed by this article.  Increased carbon dioxide does encourage the growth of some plants, others don’t benefit as much, meaning that some weeds will become more competitive with crops as carbon dioxide levels increase.  These crops include such things as rice and corn.  Furthermore, increased carbon dioxide means that more nutrients will need to be pulled from the soil by plants which are trying to grow faster — kinda like how you need more gas (nutrients) for a faster car (faster growing plant).  So that’ll mean more fertilizer used for crops, which will be used at about 30% efficiency (about 70% of what we apply to crops never gets to them) and the rest will go into the air or into the water as pollutants. 

To me, the carbon dioxide issue isn’t, and never was, about global warming — because we can’t prove how much it does to climate change.  The issue is crops and ocean acidification (which I haven’t gone into here) because we have very good data in these areas as they relate to the negative effects of increased carbon dioxide levels.  We need to rethink our fossil fuel use.

Stop planting things so close together, Holly!

I love Linda’s idea and there are some great comments – but y’all need to send her photo evidence of these gardening adventures/casualties to post! Would make me feel better. It doesn’t take much to get me going on dubious stuff I have done, gardening or otherwise.

Was just up to Northern Virginia to help with  Fairfax Master Gardener training (at the fabulous Merrifield Garden Center).  My talk was on "site analysis and plant selection." As I looked through my digital photos to illustrate the points made, I came across several "ahem" moments, that lead to the "do what I say, not what I do" caveat (but usually gets a few giggles and snorts). 

Here’s a classic:

East side of  our "yarden." The only preexisting material was the pair of white birch in the background and the purple-leaf plum.  Please direct your attention to the left side of the border.

We built the bed in spring of 2008. 

Inserted a very happy redbud, (7 gallon), Metasequoia glyptostroboides ‘Ogon’ (my pride and joy) ($$$ for a 15 gallon specimen),  Salix x ‘Hakuru Nishiki’ standard (7 gallon lollipop), underplanted with some sun/part shade perennials and one little Tetrapanax root dug from a friend’s garden. There was also a Calycanthus raulstonii ‘Hartledge Wine’ tucked between the redbud and dawn redwood but you can’t see it from this angle.

The sun perennials took off, and it looked AWESOME. 

Coming up on this border’s 4th anniversary, things are a bit…crowded. Only a few Echinacea remain to fight for the morning sun. The Tetrapanex just loves the light sandy loam and has gone ape-doo. The Phyllis Diller-esque Salix standard, despite my pruning it back each year, is about to get the chain saw.  ‘Hartledge Wine’ got relocated this fall.

I think this is a common gardener’s dilemma. Maybe I just can’t picture my pet dawn redwood reaching 50′ tall, like it probably said on the tag I ignored.  The upside is…"lush."  And there needs to be a new bed  built to relocate the sunny stuff (yay!).   I’m actually a pretty patient gardener; and I even distinctly recall with this particular bed I was all "Look how far apart I’m placing these! Suuuper genius!"  Feel free to giggle or snort.