Some years ago, when the township stopped using the dump on Alexander Road, the area became a distribution place for mulch. Residents and contractors brought discarded leaves, branches, and other tree residue there for “composting,” that is the grinding up and natural breakdown of the material into a uniform organic substance that could be used on garden beds as mulch to “condition” the soil and help it retain moisture.
People could go to the “compost station” and take modest amounts away in their own containers for nothing. A larger amount had to be paid for. But because of all the free mulch, many people started to use it in ways that were not appropriate, such as piling it up around the trunks of their trees — even mature trees.
As we approach one of the best times of year to plant a tree (fall is another good time), it would be good to remember some of the things to avoid if you want your new tree to grow normally. One of the worst things you can do is to pile up a bunch of mulch like a volcano around the base of the trunk.
Examples of the bad things that can happen abound in this area, mainly in the form of large over-grown bulges in the bark near the ground. When in contact with the bark of a tree, mulch has the effect of stimulating abnormal local expansion of the bark through galling or callousing. Such a condition is very harmful to the tree since it provides a haven for rodents and insects such as termites, which frequently attack the tree’s tissue in the affected area.
Some people believe that mulch has the effect of retaining moisture, especially in prolonged dry spells, so it must be beneficial. Mulch does retain moisture, and if used properly can be beneficial. But the problem is that trees do not absorb moisture from the ground near their trunks, so there’s no point in putting mulch there. Although mulch at that location may stimulate the growth of roots near the trunk, additional roots at that location are not beneficial, since they are ineffective in supporting the tree, say, in a high wind.
Trees get most of their needed water from the ground near or even well beyond the “drip line,” which is directly below the outermost leaves extending from the canopy of limbs above ground. That’s where the tree’s tiny moisture-absorbing rootlets are found — way out at the ends of the main roots and beyond. If you see a diagram of a tree’s “circulation system,” you will see that it extends underground farther out from the center than its branches do from the trunk above ground. The rootlets at the ends of the roots are what supply most of the needed moisture and nourishment to the tree.
If there is pavement near the tree, don’t worry. As long as there is some unpaved area near the tree — even just on one side — the roots will find it and get the moisture. Many trees grow perfectly well in the city even when nearly surrounded by paved streets and concrete or stone sidewalks. One of the largest trees in Princeton grows in front of the Methodist church on Nassau Street at the corner of Vandeventer Avenue. Except for a small lawn area near it in front of the church, it is surrounded by paving in all directions.
This all applies to a tree of any age. Even a newly planted tree with a trunk that is only a few inches in diameter will quickly send out a root system underground that extends as far out as the branches do above ground. Putting a pile of mulch around the trunk is neither desirable nor necessary, and, in fact, leads to the abnormal growth of the bark mentioned above.
If the tree is planted in a lawn, it’s a good idea to have a thin layer of mulch extending a foot or two out from the trunk to help limit how close you bring your lawn mower to the tree so you won’t hit and damage it. It also helps keep the weeds down. But a layer a couple of inches thick is plenty for this purpose. You can also plant ground cover and other attractive plants in the area around the tree. But never pile mulch up against the bark.
There was a time when one of the first things a new property owner did was to purchase a good garden book that gave instructions on how to plant almost anything, including trees. That’s what we did, and over the next few decades we planted around 20 trees — both deciduous and evergreen — on our one-third acre. We never used excessive mulch, and they all grew normally. Except for a few that were blown down or damaged in storms, they have all survived very well. A couple of oaks I planted in the early 1960s are now around 50 feet high. Mulch was reserved for our vegetable garden, flower beds, and the shrubs near the house where it belonged.
Property owners and businesses that use lawn services to take care of mowing and raking sometimes mistakenly assume that the services also know about the proper care of trees and the use of mulch. But that is not necessarily true. Lawn services are good at mowing, raking, and trimming grass, but they are not necessarily gardeners or tree experts, so you should make sure that they really know how to take care of your trees before you ask them to. And that includes the proper use — or non-use — of mulch.
I am indebted to Ron Slinn, vice chair of the West Windsor Shade Tree Commission and former forest economics professor at Duke University, for reviewing this column.
