Q. How much fertilizer do I need to feed my large tulip tree in my front yard? It lost a limb during a storm and it ripped some of the bark off the side of the trunk.
A. You have two options, and neither involves you trying to scatter dry fertilizer on the turf around the tree.
The turf would rob most of the fertilizer before it ever made it to the tree roots. The feeder roots of most trees are located in the upper 18 to 24 inches of the soil profile. Turfgrass roots usually extend about 4 inches down in the soil.
Many folks still believe that trees have these taproots like a carrot that hold the tree upright. In fact, some trees might have a taproot at the beginning, but the feeder roots take over the job of supporting and taking in water and nutrients. This is why trees that have experienced environmental stress – where the root system has been compromised and diminished, trees planted improperly, or trees planted in areas where the root zone area is small – are often the first to blow over in storms.
If you have the resources, then hire an arborist to deep-root fertilize the trees in the fall about every three years or so. An arborist has the proper equipment to provide enough slow-release fertilizer – mixed with a few hundred gallons of water – to get the fertilizer into that 18- to 24-inch depth so the roots can take it in efficiently. There is some disagreement over whether tree fertilization is best during the spring or fall.
In my opinion, fertilizing during the fall is optimal. Can trees still be fertilized during the spring? Sure, the key is using slow-release fertilizer.
My do-it yourself method of fertilizing large trees mimics the natural way trees fertilize themselves in the wild. Harvard University and the Morton Arboretum are two places that have used this method. Simply run the mower over the lawn after leaves have fallen to the ground. Rake up the shredded leaves and place them in a large circle under the tree you want to fertilize. Large meaning a 15-foot circle of leaves around the tree. Just pile them over the grass. Pile them at least 12 inches in depth but keep the leaves at least 12 inches away from where the trunk of the tree meets the ground. Over time the leaves will decompose, and the leaf mold tea created when it rains will fertilize the tree.
Will this method hurt the grass? Probably not in the short term. I actually welcome the chance to have less grass to mow, so I usually place newspapers in a circle around the tree before I place the leaves. For no cost I have created a natural mulch under the tree to control weeds, fertilize the tree and reduce the lawn area.
It is a trifecta win.
The Plant Medic, written by Ricky Kemery, appears every other Saturday. Kemery retired as the extension educator for horticulture at the Allen County branch of the Purdue Extension Service.