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.

What’s in the Worm Juice?

A couple of weeks ago I mentioned that I would be taking a look at the leachate that comes from vermicompost. Here is the worm house, owned by Master Gardener Meleah Maynard, from which this leachate came. This is a picture from when the house was new — it now has multiple floors.

It has been running for a few years now, and the “ingredients” that she puts in, mostly table scraps, are pretty typical of what anyone would put into compost. She reports that it produces about a gallon of leachate every 2-3 weeks. The leachate from this house has the following properties:

  • pH – 8.5: That’s a high pH for soil, but for a fertilizer added every week or two it’s fine.
  • Nitrogen – 1120 ppm: That’s high for a fertilizer.  About twice the concentration I’d use if I were applying a liquid fertilizer to my plants at home. The nitrogen is present mostly as nitrate, which is a good thing.  If the nitrogen were present primarily as ammonium, that might cause problems.
  • Phosphorus – 22 ppm: That’s a good/appropriate concentration of phosphorus for most plants. It’s much less than we apply when we use a typical garden fertilizer. Potassium – 5034 ppm: This is an order of magnitude higher than we’d apply for most plants using a liquid fertilizer.
  • Calcium – 279 ppm: This is a reasonable amount of calcium.
  • Magnesium – 211 ppm: This is reasonable amount of magnesium.
  • Sodium – 634 ppm: I’d like to see less sodium, but this shouldn’t cause a major problem.
  • Other elements present included Iron, Copper, Manganese, Zinc, Molybdenum, and Boron, all at levels less than 1 ppm.

So what’s my conclusion? I think that, based on the nutrients and nothing else (no trials), this could be a great liquid fertilizer if it were used properly. I’d recommend diluting it somewhere between 1:1 and 1:5 worm juice : water before applying it, and I’d only apply it once every week or two. If you want to use it, try it on something that you’re not too concerned about first, just to make sure that it doesn’t do anything too terrible (It shouldn’t, but I believe in caution).

Powers of the Mind


A couple of days ago I read a journal article which seemed to show that certain individuals could, using some sort of mind powers, called biofield treatments, influence the growth of plants.  You can read the article here.

In case you were wondering what goes through my mind when I read something like this, let me tell you:  The first thing that enters my head are skeptical thoughts.  I try to get rid of these quickly though, because I believe that, as a scientist, it is my job to critically evaluate the science behind the paper without letting my own preconceived notions influence me.  It’s also important to remember when reading a paper like this, which challenges preconceived notions, that this paper has gone through a significant review process.  This process does not guarantee that the paper is perfect, but it does mean that some other scientists somewhere have concluded that the paper is worth something.

OK, so now you know what goes through my head.  Next question, after reading the paper am I convinced that powers of the mind can actually make plants grow bigger and have greater yields?  The simple answer is no.  There are a lot of things that are going on here that are just odd and which raise questions, and without answers to these questions I find it difficult to believe that everything is occurring exactly as indicated in the article.  Yes, something appears to be going on, but whether it is due to “biofield treatments” isn’t clear.  To begin with, I’d like to have soil tests showing the nutrient status of the soil prior to the experiments.  I’d also like to see a nutrient analysis of the foliage of the plants at the conclusion of the experiment.  It is odd to have added the nutrients that the researchers added to test plots and to see no effect – unless a biofield treatment was used.  It also seemed odd to me that plants wilted when there was drip irrigation there.   And it seemed odd that the fertilizers used weren’t described better.  There were other things I was interested in knowing too, but I won’t bore you.

Another thing I noticed is that one of the authors of the article is actually a member of the foundation which paid for the research to be conducted — and is, in fact, the corresponding author (in other words, the author who you should contact should you have any questions).  This isn’t “against the law” or anything, but it is odd.

As a scientist it is my responsibility to acknowledge the possibility that these biofield treatments had some effect on plant growth, but to actually convince me that they did you need to write an article that is rock solid with no opportunity to say “But what about….”.  Right now this paper just doesn’t do that.  Too many odd things going on.

Be a Part of History!

Here at the Garden Professors we pride ourselves on being on the cutting edge of technology.  In fact, we’re so tech savvy we didn’t even whine when FaceBook foisted a new homepage format on us for no apparent reason.  So it’s only fitting that we offer you, Mr. and Ms. Garden Professor Blog reader, an opportunity to participate in the first ever landscape horticulture research project designed by social media.


Here’s the deal.  My current research project on water and nutrient management of trees in container production has left us with over 100 ‘Bloodgood’ London planetrees in 25 gallon containers.  What I need from you are ideas for a study plan on what to do with the trees next.


Of course, as with any major research project, the first step in the rigorous scientific process is to come up with catchy acronym for the study.  I propose “the SOcial MEdia DEsigneD TRansplant ExpErimental Study” or SOME-DED-TREES for short.  Needless to say, I am willing to consider alternatives.  In any event, we have a unique opportunity to investigate post-transplanting growth, development and physiology of landscape trees.


So here’s what we have: Approximately one hundred,  2” caliper trees, grown in containers in a standard mix of 80% pine bark and 20% peat moss.  Trees have been grown for two years in essentially standard nursery culture – daily irrigation and 60 grams of Nitrogen per container.  The subject of the original study was fertilizer source; half the trees were fertilized with Osmocote and half received the same amount of nutrients from organic fertilizers. After two years we have not seen any difference in growth or foliar nutrients between the treatments. Nevertheless, I will need to include the prior treatment as a blocking variable to eliminate any potential confounding effects.  Beyond that it’s wide open.  We could have 6 treatments x 2 blocks x 8 trees = 96 trees.  I strongly suspect in the final analysis the block effect will be non-significant and we can consider there to be 16 replicates, but life is full of surprises.


So, what tree establishment or tree care question is burning a hole in your brain?  “Shaving” or “butterflying” container rootballs?  Fertilizing at time of transplant?  The latest biostimulant?  Crown thinning at time of transplant?  Frequency of post-planting irrigation?  Width of the planting hole – how wide is wide enough?  Send me your suggestions and we will set up a poll to vote for the top choices.

Out of the lead frying pan and into the phosphate fire

A recent NYT post reports that adding fish meal to lead-contaminated soils will cause the lead to bind to phosphate found in fish bones.  As the article explains, this chemical reaction results in the formation of pyromorphite, “a crystalline mineral that will not harm anyone even if consumed.”

Given my concerns about excessive phosphate loading in urban soils, I contacted Dr. Rich Koenig, an urban soil scientist and chair of WSU’s Crop and Soil Science department.  I wondered, at least, if the article should have directed people to have a soil test done first to determine how much phosphate was already in the soil before adding more.

Dr. Koenig was likewise concerned that the application rate of fish meal was probably far higher than what plants would require, thus increasing the risk of phosphorus leaching and runoff. He also referred my question to Dr. Jim Harsh, a soil chemist in his department familiar with the process described in the article.

And as Dr. Harsh pointed out, it’s important to make lead less susceptible to uptake by people and other animals exposed to contaminated soils.  However, he’s unconvinced that phosphate is the best choice; in fact, research by Dr. Sally Brown (cited in the NYT article) and others (including Dr. Harsh) have found that compost containing iron is also able to bind and immobilize lead.  The advantage of high iron compost is that iron will not leach into nearby waterways, nor cause the same kinds of plant toxicity problems, as phosphate can.

Thanks to both Dr. Koenig and Dr. Harsh for their quick and informative responses on this topic.

Interveinal chlorosis mystery

Today my family took our annual 4th of July weekend hike.  We ended up on a fairly new trail through the Robe Canyon Historic Park.  It was a gorgeous day and we saw all manner of plants and animals.  The highlight of this trail is an old lime kiln; bricks and other remnants of early settlers are scattered around the area.  The kiln closed in the 1930’s.  (The hot link embedded in the park name leads to a 2004 article about the trail and the history of the site.)

Ever on the lookout for interesting plants or plant problems, I found many of our native species with definite signs of interveinal chlorosis.  This is indicative of a foliar deficiency of iron or manganese. These forest soils are rarely deficient in either nutrient, and they also tend to be acidic (meaning that it’s easy to take up iron and managanese; alkaline soils inhibit uptake).

So why are these native plants, naturally growing on native soils, showing iron and/or manganese deficiency?Answer on Monday!


Fast food is unhealthy for plants, too

In early December, I posted my thoughts about fertilizing crops vs. landscapes.  An anonymous reader asked if we could follow up by discussing the relationship between excessive fertilizers and plant susceptibility to pests and disease.  It’s taken a month to get the scientific literature (and my act) together, but here it is.

There are decades’ worth of articles about the direct relationship between increased nutrient availability and increased susceptibility to pests, disease, and disorders.  One of the earliest articles linked the incidence of celery blackheart to over-fertilization.  Since that time, researchers have found similar causal relationships in vegetable crops such as rice, onions, and soybeans, ornamental crops including poppies, and perennial orchard crops such as nectarines.  Unfortunately, there’s been no research on landscape species.

Happily, the way plants react to excess nutrient levels is generic – so we can apply the findings in the agricultural literature to landscape situations.  Just like kids and candy, plants will greedily take up all the available macronutrients their roots can find, especially nitrogen and phosphorus.  (It makes NO difference is the fertilizer is organic or inorganic.)  Plants in highly nutritive soils respond with lush vegetative growth – and fewer flowers, by way.  Less metabolic energy is put into protective compounds, so these succulent new leaves and shoots are prime targets for all kinds of unwanted plant-eaters and foliar pathogens.

As with so many things in life, moderation is the key.  For routine landscape needs, use woody mulches rather than fertilizers and nitrogen-rich composts.  This “slow food” approach not only benefits your plants, but provides ideal habitat for mycorrhizal species, which have been shown to help restrict root uptake of excessive nutrients, while assisting with uptake of less available ones.

Fertilizers, crops and landscapes

Last week Jeff wrote about the dangers of using “balanced” fertilizers, especially in reference to phosphorus content.  Comments quickly followed about using fertilizers in many situations – on farms, in container plants, on trees – and so on.  One of the latest comments came from Nick and began “I don’t usually recommend fertilizer for perennials or woody plants to consumers. In most cases they aren’t needed.”  And this leads into today’s topic.

Many of the horticultural practices we use in our gardens and landscapes have, unfortunately, been derived from agricultural crop production.  Whether you’re growing a field of wheat, garden tomatoes, or containerized shrubs your goal is maximizing crop production.  By its nature, this is an unsustainable practice because it requires continual inputs of water and nutrients at higher levels than would naturally occur.

But this is not how you should care for landscape trees and shrubs, and why Nick’s comment was a good one.  You don’t need to routinely add fertilizer to these plants; they don’t need it to grow normally.  What we should be doing in landscapes is preventing nutrient deficiencies.  Once you have a soil test in hand, you’ll know what nutrients may be too low (or too high) and how soil pH will affect that.  For most of us, this may involve occasionally adding one of a few nutrients (most commonly nitrogen), or perhaps acidifying the soil to improve nutrient availability.

 How do you know when to add nitrogen to established landscape plants?  Let your foliage do the talking. If leaves are uniformly yellow, small and sparse, you might have a nitrogen deficiency.  This will be most common in the mid to late summer, when plants are growing most rapidly and competing with one another for resources.  Be sure this symptom is wide-spread, however.  If it’s just one plant showing deficiency symptoms, it’s probably not a landscape issue.


Dog Spots

There is a general misunderstanding among the gardening (and yard owning) community about dog spots.  It seems that some people believe that dog spots occur because of a high or low pH or because of some sort of poison in a dog’s urine, but that really isn’t the case at all.  Dog spots occur because of something that I pointed out a few weeks ago in another post.  Urine contains a lot of nitrogen.  When a dog pees on your lawn that extra nitrogen isn’t used and so ends up being poisonous to the grass which is peed upon.  You’ll notice that around the periphery of a dog spot the grass is particularly bright green.  This is because the extra nitrogen helps the grass in that location rather than poisoning it.  If you want to get rid of a dog spot the best thing to do is to follow your dog around carrying a five gallon bucket of water and pour it over the spot as soon as the dog pees — this should stop the grass from dying, but will probably not get rid of all the extra nitrogen and so you’ll end up with a bright green spot instead of a brown one.

Balanced Fertilizer and Peak Phosphorus

When I first started reading extension recommendations for plants around 15 years ago now, I read a lot about “balanced fertilizers”.  Today I still see balanced fertilizers recommended, but not nearly as frequently, which is a good thing.

At this point you may be asking yourself “what is a balanced fertilizer?”  A balanced fertilizer is one that has three numbers on its label which are the same — such as a 10-10-10 or a 14-14-14 (Nitrogen, Phosphorus and Potassium are the three nutrients indicated by
these numbers).  Although it might seem like equal numbers would mean equal amounts of these nutrients, these numbers actually indicate percent Nitrogen, percent P expressed as P2O5 (in other words, if you took all of the phosphorus present in the fertilizer, made it into P2O5 and then added it back to the fertilizer and figured out what percent of the fertilizer that made up — that would be the second number) and percent potassium expressed as K2O (basically the same as the phosphorus example you saw previously).

But here’s the problem.  The amount of phosphorus, and often potassium, that is added when you use a balanced fertilizer is typically out of line with the amount that the plant needs.  This is because fertilizers are usually applied based on the amount of nitrogen that a plant needs.  The reason that a balanced fertilizer was usually recommended was that phosphorus and potassium levels in these fertilizers is high enough that they provide all of those elements that a plant needed without being toxic to the plant.  The problem is that, while the levels of these nutrients added might not be toxic, they are in excess of what is usually needed.

OK, so we’re adding excess phosphorus and potassium, what’s the problem?  Well, for the potassium the problem isn’t usually that big a deal.  In fact, a fertilizer bag with the first and third numbers equal may be what’s called for in many cases — fruits and nuts in particular often like a higher level of potassium.  Additionally, the world has a pretty big store of potassium so we’re not likely to run out any time soon.  Phosphorus, on the other hand, is a little bit different.

Phosphorus is a bit more hard to find in large quantity than either of the other elements in a bag of fertilizer.  Because of this it is often the element that limits the growth of plants, for example algae.  When phosphorus runs off into a lake or other body of water it can allow algae to go crazy and use up all of the oxygen in the water killing fish and other creatures (actually it’s the dead algae that do this — bacteria use oxygen while breaking the algae down).  Most of you are familiar with this and know that it’s the reason why Minnesota and now Wisconsin have banned the use of phosphorus fertilizers on lawns without a soil test.

But here’s what you may not know.  Most of the phosphorus which we use for fertilizers comes from rock phosphate.  Rock phosphate is mined in only a few places around the world, Florida being one.  Just as it is a foregone conclusion that we will run out of oil someday, it is also a foregone, but lesser known, conclusion that we will run out of rock phosphate.  Estimates are that we will reach “Peak Phosphorus” (in other words maximum phosphorus production.) in 20 years or so and that we will run out in 50 – 100 years.  Here’s an interesting article on the situation  http://www.foreignpolicy.com/articles/2010/04/20/peak_phosphorus

So conserve our natural resources and skip the balanced fertilizer.  When asked for a general use fertilizer recommendation I usually recommend something with a ratio of roughly 5-1-2, with a higher potassium content if you’re growing fruits or nuts.