here may be a battle brewing on your clients' properties between their trees and their grass. Trees and turf tend to be mutually exclusive in nature; you won't see many trees growing in the prairies or grasslands, and grass is not common on the forest floor.
Our urban landscapes represent an unnatural ecosystem in which we force two somewhat incompatible plant types together and expect optimum performance from each. Trees and turf compete for sunlight, water, mineral nutrients and growing space below ground. Turf roots typically outcompete tree roots and win the belowground battle. However, the dense shade of a tree's crown can be too much competition for turf, and trees win the aerial war.
Shade leads to reduced grass density, increased root competition and increased weed invasion. There are some varieties of turf that are somewhat shade tolerant, but this may be a partial solution, because shade-tolerant grasses tend to be less tolerant of wear.
Pruning for light penetration
Pruning to increase light penetration should be considered, keeping in mind that it is not a permanent solution. An important axiom to remember is that trees will grow into the voids created by pruning. Keep in mind the old rule of thumb not to remove more than one-fourth of the tree's foliage-bearing crown in a single pruning. If a tree is topped or thinned too much, it will be stressed and will probably produce many water sprouts (suckers) along its branches to compensate for lost foliage. This defeats the purpose of pruning to allow more light penetration.
It may help to "raise" a tree's crown to improve light penetration. Crown raising involves the removal of lower branches, and most tree species are tolerant of this pruning practice. Crown raising, however, does not significantly increase sunlight to the turf in most cases.
Some trees have a tendency to form surface roots, which can be a major problem in lawns. Besides ruining the appearance of the turf, they can interfere with mowing equipment, and can even become a safety hazard. Homeowners always want to know to what extent they can prune or remove tree roots without bringing about the demise of a tree. Since cut roots tend to develop more roots, root pruning is usually not a good solution.
The most simple maintenance recommendation is perhaps also the most important: mulch. Mulching the root areas of trees is probably the least expensive but most beneficial thing you can do to enhance tree health and minimize competition with turf. Mulch helps retain soil moisture, moderates soil temperature, and reduces competition from weeds. Organic mulch can help condition the soil and improve microbial activity.
Apply mulch about 3 to 4 inches deep, but do not pile it against the trunk of the tree. As far as the trees are concerned, the bigger the mulched area the better. Group trees together in mulch beds and extend the mulched areas as far out as practical.
There is a long-standing, but inaccurate, belief that trees must be "deep-root" fertilized. This belief is associated with the myth that a tree's root system is an underground mirror of the crown. Because most of the absorbing roots are actually in the upper few inches of soil, it makes little sense to place the fertilizer deeper.
If the lawn is being fertilized and trees are occupying the same area, the trees might not require supplemental fertilization. The key to any fertilization program is to base the application on the plant's needs. Soil and foliar analyses can provide the information required to make an educated decision about nutrient needs.
Mowing equipment and string trimmers can damage trees. Most people don't realize the degree of damage that can be caused by the bumping of a mower or the whipping action of the nylon string in a trimmer. A tree's bark can provide only so much protection against these devices. Young, thin-barked trees can be damaged almost immediately. In the worst-case scenario, the trees are eventually girdled and die. Those that are not killed will be stressed. The wounds may serve as entry points for diseases, borers or other insects. Many canker rot and root decay fungi have entered trees from wounds created by lawn and landscape maintenance workers.
Herbicides, especially broadleaf weed killers, are often used on lawns. Since most trees are broadleaved plants they can be injured or killed if high enough doses reach them. Homeowners must keep in mind that "weed and feed" fertilizers contain herbicides that can damage trees.
Achieving a balance
Trees and turf can peacefully coexist and even thrive together in a landscape. Armed with an understanding of how each affects the other, you can modify your clients' landscapes and adjust your maintenance procedures to optimize the growing conditions for both.
Sharon Lilly is the director of educational goods and services for the International Society of Arboriculture (www.isa-arbor.com).