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.

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.

For Mulch

Posted by Bert Cregg

Just a quick note up front that today’s post is a little data heavy, so if you’re still adjusting to this weekend’s time change; be advised.

A few weeks back Jim Urban wrote a post entitled ‘Against mulch’ on the Deep Root blog. The principle reasons he cited for his position were: 1) Mulch floats and can clog drains and releases “lots of phosphorus” as it breaks down, and 2) work by Gilman et al. that suggest that mulch does not reduce evapo-transpiration. We discussed the Gilman et al. paper ad nauseum here already so I’ll stick to the other points.

Most organic mulches float, it’s true. However, if mulch is repeatedly washing from a bed into a drain this suggests a problem with the design as much as anything. Second, I’m not sure what constitutes “lots of phosphorus”. Branch and stem tissue of hardwood trees is about 0.1% P. If we use just the bark as mulch, the P concentration is about 0.2 to 0.3%. Is that ‘lots of phosphorus”? I don’t know. I suppose if you put enough it down and allow it wash into a drain it could be.

So let’s stick to what we do know about landscape mulch. Linda has written the most comprehensive review of mulch out there and it demonstrates the benefits of mulch. Nevertheless I’d like to add some recent observations of my own to the discussion. These come from follow-up measurement on some studies that we have already published on shrubs and conifers. But I think our new data are important because they demonstrate the long-term benefits of much on tree and shrub growth.

2006 Conifer study. In 2006 we installed a trial to compare several different weed control strategies for newly planted conifers. Weed control, either by hand, Vis-pore mulch mats or 3” of coarse wood chips, dramatically increased tree survival.
swmrec mulch survival

After 8 growing seasons, trees that had the wood chip mulch or mulch mats had significantly greater caliper than trees that were not mulched.
swmrec mulch caliper

2004 shrub study. In another trial we compared the effect of various mulch types (wood chips, pine bark, hardwood bark) on growth of common landscape shrubs (golden globe arborvitae, Runyan yew, ‘Tardiva’ hydrangea, cranberrybush viburnum, and arrowwood viburnum). We re-measured heights of the shrubs study a couple of weeks ago (nine growing seasons after installation). To keep things simple here I’ve lumped the mulches together and simply compared mulched vs. un-mulched.

After nine years mulching increased height growth for all shrubs except the arborvitae.
mulch 2013 ht

Even more interesting is that the growth benefit of mulch extends beyond the establishment phase. If we start at age 4 and look at the relative growth rate for the past five years (i.e., growth increment for past 5 years / height at 4 years) we see that mulch continues to provide a growth advantage for all shrubs except the arbs.
mulch RGR

As I said at the outset, a little data heavy today but I think this is an import point. There is a lot of discussion these days about proper planting techniques but I think after-planting care often gets overlooked and mulching is an important part of that. That’s why I’m for much.

Are natives the answer? Revisited

I started to leave a comment on Linda’s Friday post regarding Seattle Public Utilities proposed building codes regarding “Healthy Landscapes” but decided I’d weigh in with a regular post.  Linda honed in on the 75% native requirement but there are lots of things to make one scratch their heads in the proposed codes.

Existing invasive plant species shall be removed and no invasive species planted.
No mention of how invasive plants shall be removed.  Heavy-duty herbicides? Armies of school children forced into slave labor? Slow-moving ground-fire? Goats?

75% of all new plantings will be native to Western Washington.
So where did 75% come from?  Sounds like a number that was pulled out of the air.  How is 75% defined?  75% of plants? 75% of the area?  And how does this foster “Healthy Landscapes”?  If I have a 2 acre landscape and plant an acre and half of salal or Oregon grape I’ve met the requirement of 75% but have I increased species diversity or structural diversity or contributed to a “Healthy Landscape”?

A vegetation plan must be submitted for review.
By whom?  What happens if they (whoever ‘they’ are) don’t like it?

Existing native plant species shall be protected whenever possible.
Sounds reasonable but what about existing non-invasive non-natives?  Could a homeowner be required to cut down a 40-year-old red maple?

And on and on we could go.  Let me state clearly, I’m not against native plants.  Quite the opposite – I grew up in western Washington and have a passion for PNW plants since my high school days.  Since moving to Michigan I’ve written articles and given talks promoting natives here as well. http://www.hrt.msu.edu/assets/PagePDFs/bert-cregg/GoingNative.pdf

Nonetheless, I think many in the native plant movement hurt their cause by parroting the same old lines without ever critically thinking about what they’re saying.  Repeating a lie often enough times does not make it the truth.

Let’s critically look at some of the reasons for planting natives according to the Washington State Native Plant Society:

Native plants are adapted to our climate of wet winters and dry summers.
True. But so are lots of non-natives.  Adaptedness is a function of the environment in which plants have evolved; whether it’s native or exotic.  There are many climates around the world that are similar to the PNW and can produce similarly adapted plants.

Require less water than most non-natives once they are established.
Once again, adaptations such as drought tolerance are a function of the climate under which plants evolved.  There are many exotic species that are more drought hardy than western Washington natives and likely to use less water.

Resist native pests and diseases better.
Sometimes. But unfortunately the days of worrying only about native pests are in the distant past.  Exotic pests are here and they are here to stay.  Dutch elm disease, white pine blister rust, emerald ash borer, chestnut blight, Japanese beetle, the list of exotic pests is long and getting longer.  Native does not mean pest-free.

Improve water quality by needing less fertilizer and no pesticides.
OK, here’s where I get confused.  The reasoning in Doug Tallamy’s book, Bringing Nature Home, is that native insects don’t feed on exotic plants, therefore if we plant exotics, native food pyramids will collapse and it will be the end of life as we know it.  So… if native insects won’t feed on exotic plants, why would exotics require more pesticide use?

Save resources and encourage a sense of Stewardship.
Ok, now maybe we’re getting somewhere.  Not sure why stewardship is capitalized here but if they mean a ‘sense of place’ or a ‘connection to the natural environment’ then I can buy it.  Many native activists, including Tallamy, run away from this argument – apparently it doesn’t sound scientific enough – but it’s one of the best we have.  Washington state has some of the most incredible plants anywhere.  They should be celebrated and promoted and planted.  In my mind, the biggest reason for planting natives – along with carefully selected non-natives – is to increase overall biodiversity.  When I mention biodiversity I am speaking broadly; species diversity, structural diversity, age-class diversity, and landscape diversity.  When we look to the future we have no idea what lies ahead. We don’t know what new, exotic diseases or insects are looming on the horizon. Most of us expect climate will change but no one can say with certainty how.  Plants cannot evolve as fast as climate will change or as fast as new pest will be introduced. The only way to deal with this uncertainly is to spread the risk through diversity – this includes natives, exotics, and even interspecific hybrids.

Every survivor has a story

It’s been suggested, not unfairly, that the Garden Professors are sometimes a little ‘Tree-Centric’.  As a forester and tree physiologist by training, I’m probably the guiltiest among my co-conspirators on that count.  But occasionally I do notice things less than 10’ tall and lacking a single, woody trunk.


When weather permits I like to take my lunch down to the MSU annual trial gardens behind my office here at the Plant and Soil Science Building.  Every summer the annual gardens are awash with the color of impatiens, geraniums, petunias, and other annuals.  This time of year, however, it’s bulbs that steal the show.  This spring there was a display of bright pink tulips that was especially striking.  Wandering by the beds recently I noticed a tag in one corner; ‘Susan Komen mix’.  For those that aren’t aware, Susan Komen lost a long, brave battle with breast cancer in 1980. The foundation founded by her sister, Susan G. Komen for the Cure, is the leading advocacy and fundraising group for breast cancer awareness and research in the US.  Komen for the Cure also provides services and advocates for our country’s two and half million breast cancer survivors.

In many ways, tulips are a fitting symbol for breast cancer survivors and the brilliant pink show made me think of a cancer survivor I know, my college girlfriend, Lisa.  I remember learning about her cancer nearly two years ago.  We had followed our own paths after our undergrad days and contact was sporadic as we each set out on careers and started families.  I missed her at our 30th high school reunion but found her number in the reunion directory and rang her up. The words hit like a punch in the stomach, “I underwent breast cancer surgery and treatment this past winter.”  I was stunned but in the next instant knew her trademark grace and humor were still intact.  “Yeah, when the reunion notice came out I had just lost both my boobs and all my hair.  Ya know, I just wasn’t up for all the ‘Hi, how are ya’s?’”  Today, Lisa remains cancer-free though after-effects of radiation treatments linger.  Through the wonder of Facebook we now keep up on each other’s awesome and talented kids.  She’s looking forward to two year’s cancer-free in July and then the all-important 5-year mark beyond that.

So, what does all this have to with landscape horticulture?   My friend and colleague Art Cameron concluded a talk at our Master Gardener College last Saturday with a quote: “To plant a garden is to believe in tomorrow.”  So much of what we in do horticulture is an expression of our faith in the future.  When we plant tulips in the fall we know they must endure dark, cold days each winter in order to bloom the next spring.  Breast cancer survivors and their families have to endure more than their share of darkness to see brighter days. We’ve become a society obsessed with instant gratification and the old cliché ‘stop and smell the roses’ seems trite and shopworn to a lot of folks.  But gardens connect us to the earth and to each other and provide the perfect place to take time and reflect on things that really matter.

According to the American Cancer Society nearly one in eight women in the US will develop invasive breast cancer in their lifetime.  To learn more, read breast cancer survivor stories, and to support breast cancer research go to www.komen.org

Blue Spruce Blues

One of the roles I’ve evolved into over the past decade as an extension specialist at MSU is that of ‘the Conifer Guy’.  Conifers are great and fascinating plants.  The oldest trees in the world are conifers, the largest trees in the world are conifers, and some of the most interesting (at least to me) landscape plants are conifers.  Unfortunately, for a variety of reasons, in the Upper Midwest we have gravitated to Colorado blue spruce more than just about any other conifer.  Part of this over-reliance on blue spruce in the landscape is driven by production (see, Linda, I’m not always an apologist for nurseries).  Growers want to grow what they know and what’s easy to grow.  As a nursery tree, blue spruce is a reliable performer that is well adapted to a relatively wide range of site conditions.  Of course, growers also want to grow what they can sell, and there always seems to be a steady demand for blue spruce.  In many neighborhoods it appears that there is an ordinance that every other tree has to be a blue spruce.  So what’s the issue?  In the Great Lakes region, blue spruce often look pretty good when young.  However, as trees age they become susceptible to several major pests, especially cytospora canker and gall adelgid.  So all those shapely blue Christmas trees that were planted 10 or 15 years ago are now a bunch of ratty-looking messes.  So what’s the solution for blue spruce burn-out?  Clearly landscapers and homeowners need to think beyond blue spruce and look for a greater variety of choices.  Here are three to consider.

– Serbian spruce Picea omorika  Whenever I’m asked to suggest a conifer, Serbian spruce is usually one of the first trees in the conversation.  While the color may not be as striking as a blue spruce, Serbian spruce still has impressive needles in its own right – bi-color with dark green on the upper side and silver below.  Adding to Serbian’s charm is its graceful weeping habit.

– Swiss stone pine Pinus cembra The late, great conifer expert Chub Harper used to remark, “I never met a cembra I didn’t like.”  Chub’s fondness for Swiss stone pine was well founded.  Here is an understated, consistent landscape performer.  Few pests, dark green needles and stately upright form.

– Korean fir Abies koreana  It would be a stretch to consider Korean fir an alternative to blue spruce.  While Korean fir is more broadly adapted than many of its pantywaist cousins in the genus Abies, it will still do best on the Holy Grail of moist, well-drained slightly acidic soils.  Nevertheless, Korean is tougher than the average fir and is a conifer with some character and worth a shot.  Korean firs are often heavy cone producers, which can add an interesting element of color.