New Research in New Windsor: A Turf-Tree Fertility Study

Posted by: Alison O'Connor, Horticulture Agent in Larimer County

I'm a sucker for punishment (but I like to say I have a "curious mind.").  I started another research project.  This time with the help of a few dozen Autumn Blaze maples in Windsor and Tony Koski.  There's a general consensus that when you fertilize your turf in a landscape, you're also fertilizing the trees.  Tree roots tend to be shallow (no more than 12" deep in most of our clay soils, but 6" deep in many situations) and need oxygen, so they are mixed right in with our Kentucky bluegrass roots.  Through an extensive literature review (I love the CSU Library), we found that very few replicated studies exist to look at how much fertilizer the trees are using compared to the turf.  Primarily because tree roots are rarely contained and the extent of tree root systems are unknown.

Enter the Autumn Blaze maples in the New Windsor Metro District.  I think they designed this place for us.  Ok, maybe not.  New Windsor is located just north of Windsor and it has these "beds" (medians) planted with a monoculture of bluegrass and Autumn Blaze maples (all the same age).  We are treating each bed as its own replication, which means that the roots for both the bluegrass and the maples are totally contained....and ours to study.  And there are enough beds to keep me busy for quite awhile.

Autumn Blaze maples as far as the eye can see

So what we're doing is pretty simple.  We're fertilizing each bed at one of two rates: 1 lb N/1000 ft sq or 4 lbs N/1000 ft sq.  Then each week, we're going to collect lawn clippings and analyze them for total nitrogen content.  Every other week, we'll pick a few leaves off a select number of trees and analyze them for total nitrogen and chlorophyll content.  What we hope to conclude is whether or not turfgrass fertilization supplies adequate nitrogen for trees growing in a bluegrass lawn.  We're looking at tree growth increments, leaf nitrogen and chlorophyll content and caliper.

Collecting turf clippings using a very sophisticated system
of a lawnmower and paint strainer bags

It's not that I don't support fertilization of trees...all I want to know is when you fertilize a mixed species landscape, do the trees receive adequate nitrogen to maintain growth...or is additional fertilizer necessary?  Or does the turf use most of the nitrogen?  It could change how we approach landscape fertility in the future. If we see no differences in tree response between the 1 and 4 pound nitrogen rates, then we'll know that most nitrogen is being used by the turf.  But if we see quantitative differences in terms of total nitrogen content and tree growth response, then we may conclude that supplemental tree fertilization is necessary.

So, what do y'all think?