The Garden Professors™

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!

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The Garden Professors Take a Holiday Break

Linda, Jeff, Bert, and Holly are taking a break. We shall return full-force January 4th. Note that there MAY be sporadic, interim postings if one of us gets significantly riled up. Subscribers to our RSS feed will be duly alerted.

My “greeting card” below was inspired by Jeff’s post on the insulating powers of snow. We here in the Blue Ridge Mountains of Virgina have about 14″ of insulation at the moment!

Finally, thank you to all our wonderful Garden Professors readers and commenters – you made our first six months so fun and rewarding!

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 e-how.com 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!

Foliage fun flaunted!

Not much activity on the Friday quiz! It was a tricky one. Take a look at our photos in total:

As you can see, these aren’t plant “problems” in the strictest sense. (The “landscape” in question is a retail nursery.) They are cultivated anomalies – little mutations that have been discovered and propagated. There are several points to this exercise:

1) Be sure you know your plant material! Many peope mistakenly assume that plants such as these are diseased, pest-ridden, or lacking some nutrient and need to be “fixed.” Personally, I don’t care for yellow cultivars; like Lisa B and Deb, I think they look chlorotic. Without identifying tags, though , it would be hard to know these are not deficient in nitrogen or some other macronutrient. I guess I would wait until leaves emerge in the spring: if they were yellow then and stayed yellow, I would presume the plant was a yellow cultivar.

2) Many of these cultivars are not particularly vigorous. A plant that’s missing much of its foliar chlorophyll does not photosynthesize efficiently and would probably not survive in nature. In our managed landscapes, however, we can nurture these oddities so they aren’t out-competed by other plants.

3) Cultivars such as these often revert to the wild form (remember Bert’s quiz last week?). The natural form (green vs. yellow leaves, or normal vs. dwarf stature, for example) is nearly always more vigrous than the mutation, and given the opportunity plants will outgrow these limitations. Thus, many cultivars require careful maintenance to remove “sports” before they overtake the plant.

Sunday rant – the evils of chemicals

It’s days like this that I am so grateful to have this blog at my disposal!

It’s 7 am on Sunday and I’m just finishing the paper, drinking Earl Grey tea, and listening to NPR. Liane Hansen just finished an interview with Martha Stewart, who among other things was discussing healthy eating for the new year. She’s a proponent of organic food (as are many of us), and mentioned two reasons she doesn’t like conventionally grown produce. The first – residual pesticides – is a legitimate concern. But then she stated her second concern that “chemical fertilizers in the soil are taken up and stored in the plant.”

No kidding.

Plants really don’t care (excuse my anthropomorphizing) where their mineral nutrients come from. Nitrogen in ammonium sulfate is the same element as the nitrogen in cottonseed meal. The plant uses it for amino acids, chlorophyll, alkaloids, and many, many other compounds.

Martha’s faulty thinking falls into the “organic is safer than chemical” mindset that way too many people hold (you can read a column I wrote about this in 2001 here). “Chemical” is not intrinsically bad and “organic” is not automatically safe. This is an emotion-based argument and inspires fear rather than thoughtful discussion. When someone parrots this mantra, I can’t take them seriously.

I believe that organic methods in production agriculture, ornamental landscapes, and home gardens are superior to conventional practices and support a healthy soil-microbe-plant-animal system. I also believe that many fertilizers are misused and/or overused – but this includes both conventional and organic varieties.

Gerald Holton, a science historian at Harvard, once stated that “persons living in this modern world who do not know the basic facts that determine their very existence, functioning, and surroundings, are living in a dream world. Such persons are, in a very real sense, not sane.”

This is the quotation that came to mind this morning.

Friday Foliage Fun!

Take a look at these three closeups of foliage from three different plants in a landscape in Washington state:

You’ve been asked to diagnose what’s going on in this landscape that would cause these foliar abnormalities. (I get photos emailed to me all the time asking these kinds of questions.) Bugs? Disease? Nutrient deficiencies? Environmental conditions? All of the above?

Is It Worth Anything?

Plant aficionados everywhere are constantly looking for something which they can patent and make a million bucks on — something like ‘Endless Summer’ hydrangea which captured the public’s attention — and their wallets. Many of the new plants we see today came from something called “branch sports” which are basically segments of a tree — like a branch — which has somehow mutated so that it offers something a little different than what the original tree did. If you’re familiar with ‘Delicious’ apples you may be interested to know that the ‘Delicious’ apples which you eat today are actually a branch sport of another ‘Delicious’ apple which wasn’t as attractive. Likewise, ‘Connell Red’ is actually a branch sport of ‘Fireside’ — they’re basically the same, but ‘Connell Red’ is considered more aesthetically attractive.

But some of those mutations are heart-breakers, Here’s an example.

This raspberry, which I found in my daughter’s raspberry dish last night (she was very upset that I stole it) has a really cool stripe running down it’s side. If someone found this in a raspberry patch they might be tempted to try to propagate the branch from which it came hoping to get striped fruit. Unfortunately that isn’t likely. This is an example of a sectorial chimera — where just a strip of tissue has been mutated. These types of mutations are notoriously difficult to propagate and so it’s unlikely that this mutation will last after propagating the branch from which this raspberry came. Still, it is kind of cool, isn’t it?

Invasives! Natives! No, wait, biodynamics

Just had to get your attention there. We’ve had a great discussion over native and nonnative plants over the last few weeks. I’m going to completely switch gears and move on to another topic – biodynamics.

If you’re not familiar with this term, let me refer you to my online column here. Biodynamics is a set of agricultural practices based on a belief system, not science, but is an increasingly popular approach, especially in the wine industry. (You can read a discussion of biodynamics in the vineyard in The Skeptical Inquirer here. This article is engaging as well as accurate – my column is pretty dry by comparison.)

Biodynamics is steeped in mysticism and includes special preparations that are used to treat soils and plants. Preparation 500, for example, is created by mixing water with manure that has been packed into a cow’s horn and buried for a set amount of time. Other preparations are more gruesome, requiring a stag’s bladder or cow’s intestine. A whole certification process has emerged in support of these practices.

While it may be easy to dismiss these practices, it turns out that biodynamic farms or vineyards are generally healthier than conventional systems. Does this prove a mystical force at work? Not at all. Biodynamic systems are also organic – using all of those good practices (low or no till, reduced pesticides, reduced fertilizers, polyculture, etc.) that have been demonstrated to be effective over decades of research. When comparisons are made between biodynamic and conventional systems, the impact of organic practices are hidden.

The few scientific studies that have compared biodynamic to organic systems – in other words, specifically testing the effectiveness of special preparations – have found no repeatable, significant differences.

Why do I even care about this? Well, it’s because it’s pseudoscience. It’s a practice that takes on the mantle of science, but doesn’t stand up to repeated scienific testing. Belief systems can’t be tested – even the inventor of biodynamics asserted that his methods were “true and correct unto themselves” and didn’t need to be tested.

Apparently simply being organic isn’t sexy enough anymore.

Is there any future for a scientifically-sound gardening magazine?

(You’ll see two posts from me today. This first one is easier to do at 6 a.m.)

One of the efforts I’ve been involved with is serving as science editor (and writer) for MasterGardener Magazine. We started this quarterly publication in 2007 (take a look at it online at it here) – not just for Master Gardeners, but for anyone interested in sustainable gardens and landscapes. Sadly, the publication went to an annual issue last year because of the economic downturn and now may be eliminated altogether.

Yes, this is a Washington state publication so when native plants discussed they are local natives. But the information itself is applicable no matter where you live. We had hoped at one time to offer regional issues, so that the magazine would have a local flavor.

Anyway, the publishers are no longer willing to carry a loss on the magazine. What they really need are advertisers.

Any suggestions out there? Most useful will be ideas that I can do from my computer or phone.

Pop Quiz!

Bet you weren’t expecting this on TUESDAY, eh?
Heh, heh.

Situation: these photos are from a grad student project. We wanted to create, observe, and record nutrient deficiency symptoms, so we grew the plants hydroponically in a made-from-scratch nutrient solution, containing everything except one particular nutrient. There were 12 separate batches of solution, one missing each essential mineral nutrient (N,P, K, Ca, Mg, S, Fe, Mn, Cu, B, Mo, Zn – couldn’t exclude Cl as it’s too common in salts). As my research interest is herbaceous perennials, some common perennial taxa served as our victims, er, subjects.

Here’s the set up for the Verbena ‘Homestead Purple’ experiment – rooted cuttings were placed in the little buckets and secured by the lids. The nutrient solution was constantly aerated. For most elements, symptoms appeared between two and four weeks after the start of the project. Symptoms, depending on the elemental deficiency, included chlorosis (yellowing) of old or new leaves, leaf curl, speckling, stunted growth, and in one case, some excessive growth.

Below are results from day 42 of the study. We lifted the lids, hanging-basket style, so we could examine the roots. The control (received a complete nutrient solution) is on the left; Rapunzel there, on the right, lacked a nutrient. Quiz question: What element was missing in this particular case? What made you come to this conclusion?

Hint: If you have a rudimentary knowledge of garden fertility, be it veggie or ornamental, you can probably figure this out. I’ll go ahead and rule out the pesky micronutrients.

(L) Control: received complete nutrient solution (R) Deficiency solution