What about fall fertilization?

Posted by Bert Cregg
We had a question on the Facebook site regarding fall fertilization of landscape plants. Fertilization in general, and fall fertilization in particular, is a complex topic and needs a little more room for explanation than the Facebook discussion allows.

Source: Forestry Images
Source: Forestry Images

As a general rule, most landscape trees and shrubs can maintain acceptable growth and appearance without fertilization. There are a couple of reasons for this. As Linda noted in the Facebook discussion, woody plants are fairly efficient at internal nutrient recycling. I’ve done a couple of studies where we sampled leaves of hardwood trees during the season and then re-sampled right after senescence and about 50% of leaf nitrogen is re-absorbed by trees before they fall. Conifers are even more efficient at conserving nutrients than hardwoods since they typically only lose 1/4th of their needles (or less) each year. In addition, many landscape trees are able to utilize fertilizer that is applied to surrounding turf. On the flip-side, nutrients that occur in litterfall are removed from the nutrient cycle in many suburban landscapes and this may eventually contribute to deficiencies.
pin oak close-up

Bottomline, landscape fertilization should be based on need; which can be assessed based on soil sampling, foliar sampling, or visible symptoms. At least two of the three methods should be employed to make a diagnosis. Each method has drawbacks and visible symptoms are usually the least useful since many nutrient deficiencies have similar symptoms or the symptoms may not be nutrient-related at all. In our area the only nutrient problems I am comfortable diagnosing based on visible symptoms are iron chlorosis in pin oaks and manganese deficiencies in red maples, both of which are induced by alkaline soils, not a lack of those particular elements.

So assuming we’ve established that fertilization is needed, what about fall fertilization? There are a couple of arguments that are usually brought forth for fall fertilization. One is that trees can absorb nutrients during the fall and then use them for spring growth. This is generally true provided that soils are warm enough to allow continued root growth and absorption. Another argument is that fall-applied fertilizer that is not taken up by roots in the fall be will available for uptake when soils warm again in the spring. A third, and less scientific reason, is that fall is often a slow time for arborists and landscape companies and fall fertilization is an easy service to add to their sales program.

There are a couple of objections that are usually raised to fall fertilization. One is that nutrients will leach through the soil over winter before they can be absorbed. This is one of those ‘it depends’ scenarios. If a nitrate-based fertilizer source is used, this is possible since negatively-charged nitrate anions won’t bind to negatively-charged cation exchange sites in the soil. If the nutrient source is urea or ammonium-based, the amount lost will be dependent on temperature since this will drive the conversion from ammonium, which can bind to cation exchange sites, to leachable nitrate.

The other usual objection to fertilizing trees in the fall is that it will reduce cold hardiness. There is no clear evidence to support this, however. Harold Pellett and John Carter at the University of Minnesota compiled dozens of studies on the effects fertilizer on plant cold hardiness (Horticultural Reviews 3:144-171). For conifers and temperature hardwoods they found no clear trend across studies, except that fertilizing with potassium improved cold hardiness is most cases (see table). The common perception that fall fertilization, especially with N, will increase cold damage probably stems from studies of fertilization of turf, which had negative impacts in 26 out of 29 studies cited by Pellett and Carter.
pellett and carter

In summary, landscape trees and shrubs should be fertilized only where there is a demonstrated need. Fall is a good time to fertilize provided you avoid nitrate-N sources that will be prone to leaching.

One tree’s leaves… over 400 kinds of bacteria!

Okay… this bit of research just blew my mind.

Researchers took leaf samples from just ONE tree in Panama, and identified over 400 different kinds of bacteria making their home there. Sampling 57 different tree species, the total number of bacteria types ballooned to over 7,000. You can read more about the study here.

A few trees. A nearly inconceivable number of microorgansims


That’s a lot. I love this kind of research because it just reinforces how LITTLE we know about this world we live in. Our world is filled with a massively diverse microbiome that we know virtually nothing about. Research is ongoing, and hopefully in the coming years we’ll begin to understand more about how these unseen organisms influence the world we live in. I’ll be fascinated to learn more.

In the mean time, any mention of microorganisms in a gardening context instantly raises questions of the efficacy of products containing (supposedly) beneficial fungi and/or bacteria for our soil. The huge, barely understood diversity of bacteria living in every aspect of our world is a good indication of why the research on adding specific microorganisms to soil generally show no impact, or only an impact in certain specific circumstances. This stuff is complex, and we’re just barely beginning to learn about it. Hopefully in the future we’ll begin to learn how to manipulate the microorganisms that live with our plants, but I wouldn’t expect it to happen over night. Right now, I’m just following the basic rule of adding organic matter to my soil to make a good home for the organisms that live there, and following the research as it opens a window to this unseen world all around us.


What’s wrong with this picture?

The  Seattle Times recently had a front page story on what Seattle’s new waterfront might look like. Back in 2009, the city began work on this project, hiring “rock-star landscape architect James Corner, designer of the celebrated High Line in New York City” to develop the big picture. (Note: I’ve been to the High Line and have posted on it before – it’s fantastic.) As you can see from the linked article from the Times, many of Corner’s architectural renditions for the greenway are presented, including this one:

An architectural rendering shows what a Pioneer Square beach at the foot of Washington Street could look like.

For those of you not familiar with Seattle’s waterfront, it’s built right next to Puget Sound, a huge inland sea connected at its mouth to the Pacific Ocean. So it was with much surprise that I saw what appear to be Douglas firs (Pseudotsuga menziesii) along the cobbled beach. Douglas firs are not particularly salt tolerant, nor would they be very happy with the tidal action or seasonal storms that will flood the beach on a regular basis. Even if these were not Douglas firs (native to Seattle) but some other salt-tolerant species (mangroves?), it’s doubtful they would look as picturesque as the drawing shows. Neither would the rest of the trees in the area, whose root zones would normally extend far into saline soils under the beach and sound.

Am I being picky? No, I don’t think so. Landscape architects should know better than to use plantings as decorations for their designs, like candy sprinkles on cupcakes. Plants aren’t just design elements. Choosing plants that are appropriate to a site is a science as well as an art.

I can’t believe that in the five years that Seattle’s been working on this project that horticultural science hasn’t been a major component of landscape planning.


File this under “if it’s not one thing, it’s another.”
Which may be, upon further reflection, the most profoundly absurd statement ever when it comes to gardening. It’s nature! Of course there’s always something!
Here in the Blue Ridge mountains of Virginia, we’ve had insect pests come and go, with each summer featuring something different.

Two years ago? Chewing up everything in the vegetable garden plus lots of perennials… Blister beetles!

Last year we had record rainfall, which brought out the gnats in gnumbers we’d never seen before. While not plant pests, they managed to take a lot of the fun out of gardening, hell-bent on clogging every facial orifice and nibbling on exposed flesh. Then the stinkbugs came. And stayed all winter long, keeping us company in the house.

This season goes down as the summer of Popillia japonica, the Japanese Beetle. Holy cow. I’m no entomologist (let’s ask Dr. Jeff!) so I can’t speak to how we got to this lowly place. But in any gathering of two or more local gardeners, The Beetle Issue will come up immediately.

Orgy in my beans! (NSFW)
Orgy in my beans! (NSFW)

There’s a ton of literature out there on life cycle, control, etc. They are a noted pest of turf, as the larvae munch away at the roots before emerging in early summer. I am not personally familiar with that aspect, as we don’t really have turf at our house; it’s a mix of white clover, orchard grass, broadleaf weeds, and some kind of fescue that that still make a decent green substance when mowed to 3.25″ (and viewed from a distance). If I come across grubs while pulling weeds or planting, I’ll call over a couple of hens to take care of business. Biological controls such as spores of bacterial Milky Disease and insect parasitic nematodes have been only marginally successful.

For adult control, the debate continue regarding the efficacy (and wisdom) of traps baited with floral or pheromone lures. “Hey there, neighbor! Mind if your Japanese Beetles come over to my place?” Most of the pesticides recommended in the literature are broad spectrum (pyrethroids, carbaryl, etc.) so, heck no on that count. Hand-picking them into a cup or bucket of soapy water to die a bubbly, fragrant death is an option for a small garden (and extremely patient gardener). Note chickens also enjoy the crunchy outer coating and creamy center; spiny, thrashing legs and all.

Back to our regional plight – they attacked the usual suspects – favoring anything in the Rosaceae family including brambles,apple, etc. Any kind of Hibiscus now looks like a lace doily. Veggies were indiscriminately perforated – the beans were especially hard-hit. All that beetle poop is especially unappetizing on chard. A big surprise was the Japanese or Fall-blooming Anemone. They took mine down to the stem. I am currently enjoying flowers on a stick.

There's an "Anemone" pun here somewhere..
There’s an “Anemone” pun here somewhere..

Both the Anemone and I will live, of course. But here’s the thing. One of the mantras that got us all through one of the coldest winters on record was “At least the bugs won’t be as bad this summer!” Ha, ha! If it’s not one thing, it’s another! Aargh.

Go here to download a PDF of the exceedingly-informative 20-page USDA APHIS Homeowner’s Guide to Managing Japanese Beetles

Don’t need a weatherman to know which way the wind blows

Posted by Bert Cregg
Yesterday afternoon I did a little fall garage clean-up and listened to former MSU Extension colleague Dean Krauskopf’s call-in gardening show on the radio. A couple callers in a man phoned the show worried about his Japanese maple tree, which had a near-death experience from this past winter’s severe cold. The man had heard this coming winter was supposed to be just as bad as last winter and he wanted to know how best to protect his struggling tree from further calamity. Dean quizzed the caller for details about the tree and the site and gave some reasonable advice to try to modify the micro-environment around the tree to limit exposure to winter wind and cold. But I wondered where the caller got his information that winter 2015 was going to be as bad as 2014. As if anyone around here needs a reminder; January-March 2014in Michigan was the coldest since 1978 and the 4th coldest on record, with most locations reporting snowfall totals well above average. Many surrounding states has similar winter weather issues.

To get some insights on predictions for the upcoming winter, I consulted with the two most trusted sources of such information: The NOAA Climate Prediction Center and the Old Farmer’s Almanac.

The Climate Prediction Center maps present probabilities of colder or warmer than average weather for a given three month period. The most recent NOAA projections available on-line are predicting near-normal temperatures for January-March 2015 for most of the eastern half of the country, above average temps for the Northwest and below average for Texas and Florida. NOAA predicts below average precipitation for the lower Great Lakes and Northwest and above average precip for much of the South.

Current NOAA temperature predictions for Jan-March 2015
Current NOAA temperature predictions for Jan-March 2015

Current NOAA precipitation prediction for Jan-March 2015
Current NOAA precipitation prediction for Jan-March 2015

Apparently, however, Dean’s caller is dismissing NOAA and all of their satellites and computer models and is relying on the Old Farmer’s Almanac instead for his long-term weather outlook. The Old Farmer’s Almanac is currently predicting colder and drier than normal for most of this upcoming winter for the lower Great Lakes.

Current temperature outlook for lower Great Lakes regions from Old Farmer's Almanac.
Current temperature outlook for lower Great Lakes regions from Old Farmer’s Almanac.

So, how much stock should we place in these predictions? Let’s step inside the Wayback Machine for a moment and see what each source was saying a year ago about the then-upcoming winter of 2014. NOAA and the computers are up first.

For most of the eastern U.S., NOAA predicted a warmer than average Jan-March 2014 with normal precip. Ooh, sorry about that NOAA but we thank you for playing ‘Guess that Winter’! Please be sure to pick up your parting gifts on your way to our Loser’s Lounge.

September 2013 map of NOAA prediction for Jan-March 2014 temperatures.
September 2013 map of NOAA prediction for Jan-March 2014 temperatures.

September 2013 map of NOAA predictions of Jan-March 2014 precipitation.
September 2013 map of NOAA predictions of Jan-March 2014 precipitation.

Next up is the Old Farmer’s Almanac, which predicts winter weather based on… well, no one’s quite sure. In any event, this time last year the Almanac predicted Jan-March 2014 would be mostly warmer and drier than normal for the lower Great Lakes. Oh no, bummer Almanac. Looks like you and NOAA can commiserate in our Loser’s Lounge. And that means Old Man Winter repeats once again as our champion!

Old Farmers Almanac September 2013 weather prediction for Nov. 2013-Oct. 2014.
Old Farmers Almanac September 2013 weather prediction for Nov. 2013-Oct. 2014.

So, what does all this mean for Winter 2015? Even with huge datasets and sophisticated models, long term weather projections are an iffy proposition. And, as much as everyone loves to say, “See, the Old Farmer’s Almanac was the only one to get it right”, there is little evidence that it does better than chance alone. Beyond that all we can say with certainty is that NOAA and the Computers would make a really cool name for a rock band.

Cross-pollination making you cross?

No, your cucumbers have not hybridized with your melons.

I’ve been fielding different versions of the same question a LOT lately.
Three different people have sent pictures of “cucumelons” telling me they planted cucumbers next to their melons, and now the cucumbers look strange, so they’re concerned that they have cross pollinated with the melons. One person planted what was supposed to be a red raspberry next to their yellow raspberry, but the new plant is producing yellow fruit, so they think that it must be cross pollinating with their yellow plant, causing the fruits to turn yellow. Not to mention similar queries about tomatoes, peppers, and watermelons. It seems like every time a piece of produce turns out looking differently than what people expect, they blame pollen from the plant next to it.

I’m sure the highly educated readers of The Garden Professors know this already, but to clarify, there is a very simple reason why you don’t need to worry about one plant pollinating another plant and changing the quality of your produce UNLESS you are planning on saving seeds to grow for the next year.

When a flower is pollinated and starts developing into a fruit full of seeds, it is only the seeds themselves that combine the genetics of the two parents to develop into something new. Everything else – the flesh of a tomato, or cucumber or melon or raspberry – is produced solely by the mother plant, and the daddy of those seeds inside doesn’t matter a bit. Think about when a woman is pregnant… the identity of the father of the child inside her doesn’t change the character of the skin of her belly.

If you want to save seeds of your plant for next year, it is another matter, and you should be sure to isolate or (better yet) hand pollinate different varieties of the same species from each other to make sure they don’t hybridize unintentionally. You still don’t need to worry about your cucumbers and melons, however – they won’t hybridize by chance in your garden. If a plant doesn’t produce the right colored fruit or flower, most likely it was just mislabeled at the nursery. Grow a strange looking cucumber, chances are it was left on the plant too long. Cucumbers are harvested and eaten when young and immature, leave them too long and they get… strange looking. No need to blame it on the melons next to them.

There IS one exception to this, one common plant in the garden where the source of pollen makes a huge difference in what you harvest: Corn. Corn is the exception because what we’re eating is the seed itself, not the fruit produced by the mother plant surrounding the seed. That’s why if your sweet corn gets a dose of pollen from the field corn the farmer is grown next door, it comes out starchy and not sweet.

It also makes breeding colorful corn for fall decorations REALLY fun… Because when you see a multicolored ears of corn like this from my garden last year:

multicolored corn
You can carefully pick out just the seeds showing the colors you like best, say the palest blues and pinks, sow them together the next year, and get something looking like this:

pink and blue corn
Or plant all the darkest kernels together and get this:

black corn

Mark Stennes

This past week the world of trees lost a true friend. Mark Stennes was a tireless worker who selflessly promoted our urban trees and the people who worked with them and was a driving force behind elm research in Minnesota.

Mark Tree

When it came to trees, there are few people who could equal Mark’s passion. He had an insatiable curiosity in the world around us. He lived trees and took every opportunity to discuss them. He discovered the St. Croix elm and helped the University of Minnesota’s elm project identify other elms with potential. Any short list of tree experts in Minnesota had to have Mark’s name on it to be complete.

But as good as Mark was with trees, that wasn’t really where his genius lay. Mark was one of those rare individuals who was able to communicate that love for trees to other people and inspire them to actually do something. He was widely known and respected across the Green Industry in Minnesota. When Mark wanted to get something done, he did it through a tight network of friends that was incredible to behold. He had connections with people everywhere, from the tree people to the turf. Everyone knew that Mark was a person you could count on. A person who would go to bat for you when things got tough. For me, Mark was the guy who worked behind the scenes to make sure that everyone knew the kind of work that we were doing at the University of Minnesota, quietly promoting us so that when push came to shove we would have what we needed to make it all work. He took pride in the successes of his friends and wasn’t shy about spreading praise around. Mark was a person that I could constantly and consistently count on.

The day before he died, Mark called up my good friend Chad Giblin about going out and checking on some elms. And that’s how I’ll remember him; anxious to go and look at another tree so he could take notes and tell us all about it.

Plant sentience – “It is happening…again”

Those of you who were Twin Peaks fans will recognize the title quote. And while my topic is not quite as scary as being stalked by Bob, the fact that plant sentience continues to rear its irritating head in legitimate scientific venues makes me want to curl up in a ball and whimper. Here’s what made me cringe: “Sound Garden: Can Plants Actually Talk and Hear?”

I posted on this topic a few years ago, and I’d invite you to read it and the accompanying comments. I’m still not a fan of anthropomorphizing plants, and I still think word choices matter, especially when you are trying to educate people about science.

It’s disappointing that some scientists are deliberately using anthropomorphic language when discussing plant physiology. The cynical side of me says it’s a great way to get press coverage.

April 2013 041 We’re watching you…

When worlds collide

This past week I was in Alnarp, Sweden to present at the International Urban Tree Diversity Conference. Lots of interesting talks, posters and field tours. Much more to discuss than I can fit into a blog post, but if you’re interested you can read the presentation abstracts.

One of the best features of the conference is that the presenters and participants included not only arborists and urban forester but also landscape architects and urban planners. This might not sound too remarkable but these groups are not always on the same page. While urban foresters are on board with the need to diversify urban and community forests; species diversity can be at odds with uniformity, which is a key element in landscape design. For example, a common application of uniformity in design is the installation of alle’s – long, uniform monoculture plantings along a street or path.

Fredicksberg alle in Copenhagen - street view
Fredicksberg alle in Copenhagen – street view

Fredicksberg alle in Copenhagen - pedestrian view
Fredicksberg alle in Copenhagen – pedestrian view

The aesthetic appeal of an alle’ is undeniable and seemingly universal. We saw many examples in our tours in Malmo and Copenhagen, but alle’s can found almost anywhere mankind has planted trees.

An alle of beeches in Malmo. Sweden
An alle of beeches in Malmo. Sweden

The dilemma, of course, is that any monoculture planting runs the risk of catastrophic failure, especially in an era of increased global trade and potential introduction of destructive exotic pests.

Lindens failing in Fredricksberg alle
Lindens failing in Fredricksberg alle

Simply planting a random mix of species leads to a menagerie effect – one of these, one of those – that most eyes find unsatisfactory.

A random mix of species increases diversity though not necessarily aesthetic appeal
A random mix of species increases diversity though not necessarily aesthetic appeal

One of the challenges addressed at the conference is how to meet the design and aesthetic objectives of uniformity while still achieving diverse landscape. There are no simple solutions and like most compromises, not everyone will be completely satisfied. But at least we are getting to the point where all sides of the discussion are being heard and creative minds are melding the science and the art that will produce the desired aesthetic and diversity.

Another close shave…

Life is full of surprises. A case in point is some recent work on our Social Media Designed Tree Establishment Study (SoMe-Ded-TrEeS). One of the objectives of the project was to determine the impact of root-ball manipulations to remove circling roots on container-grown trees. When we planted the trees (‘Bloodgood plane trees in 25 gal. containers) two years ago, we ‘shaved’ the outer roots on one-third of the trees, ‘teased’ apart the circling roots on one-third, and planted the rest as-is (‘pop and drop’ in Linda’s vernacular).

Rerearch assistant Aniko Gaul shaves a root system  with a pruning saw before installation 2 years ago
Rerearch assistant Aniko Gaul shaves a root system with a pruning saw before installation 2 years ago
Research Technician Dana Ellison teases apart a root-ball before installation
Research Technician Dana Ellison teases apart a root-ball before installation

Two weeks ago our campus nursery staff dug 12 of the trees for us using their tree spade – four from each of the three groups. We then borrowed an air-spade from MSU Campus Infrastructure and Planning and we excavated the root-balls to determine the amount of new root growth into the surrounding soil. (‘We’ meaning my technician Dana Ellison and Nicole Rowley, one of our new undergraduate research assistants)

Dana takes a turn with the air spade
Dana takes a turn with the air spade
Nicole excavating roots with the air spade
Nicole excavating roots with the air spade

The results of the root exam were a surprise to me. Going in to this exercise my expectation was that the roots that were ‘teased’ to remove girdling roots would fare the best – and they did have visibly fewer circling roots and more new root growth than the ‘pop and drop’. But the biggest surprise was the marked improvement in rooting of the ‘shaved’ roots.

control root systems
control root systems
'Shaved' root systems
‘Shaved’ root systems

To be honest, I had some reservations about the shaving treatment. Just based on geometry alone, shaving off the bottom and outer 1” of roots around a 25 gal. tree removes over 20% of the roots; and this proportion is even greater is you consider the proportion of fine, absorbing roots. In the sampled trees, however, shaving essentially eliminated all circling roots. Even more impressive was the amount of new root growth out of the bottom of the root-balls. NOTE: In the photos below the red-dashed lines indicate the approximate dimensions of the roots when the trees were planted two years ago.

Root system on untreated control tree
Root system on untreated control tree
Root system on 'teased' tree
Root system on ‘teased’ tree
Rot system on 'shaved' tree
Rot system on ‘shaved’ tree

We (again meaning Dana and Nicole) trimmed all the roots that extended beyond the original root-ball and separated roots based on whether they came from the side or the bottom of the root-ball. They (the roots, not Dana and Nicole) are in the process of drying in the lab and will be weighed shortly. Based on the volume of material in the bags, however, it is clear the shaved trees will be the ‘winners’. Another example of why it pays to keep an open mind when doing research.