TREES AND TURF

    Woody plants and turfgrasses are critical components of design
    plans for homes, offices, and parks. Trees and turf offer distinct
    personal, functional, and environmental benefits. Personal
    preferences for color, fragrance, and form should complement the
    functional properties of size, shape, density, and placement of plant
    material.
    We’ve all seen thinning grass under large shade trees, large surface tree
    roots that cause safety hazards and mowing obstacles, young trees that
    don’t seem to grow, and tree trunks badly damaged by lawn mowers or
    string trimmers. All of these undesirable effects can be caused by trees
    and turf growing too closely together. 

    Turfgrasses provide many of the same environmental benefits as trees.
    They
    • change carbon dioxide into the oxygen we breathe
    • cool the air by changing water into water vapor
    • stabilize dust
    • entrap air polluting gases
    • control erosion
    Turfgrasses, in addition to being environmentally beneficial, are attractive in
    formal and informal designs. There are many advantages to combining trees
    and turf in the landscape.
    Selection
    When trees and turf are used in the same areas, extra attention must be
    given to plant material selection in addition to the usual hardiness, climatic,
    and soil needs. An effort should be made to make the trees and lawn
    compatible. Grass is generally a sun-loving plant. Most grass species will
    not grow well in areas that get less than 50 percent open sunlight; however,
    new varieties with improved shade tolerance are being introduced. Consult
    your garden center specialist or sod producer for recommendations of
    shade-tolerant grasses for your area.
    In areas where the lawn is the primary design feature, select woody plants
    that do the least damage to grass growth and maintenance. The woody
    plants should be small, have an open canopy (to allow sunlight to penetrate
    to the ground), or have a high canopy. Select trees that do not root near the
    soil surface; surface rooting is most serious where shallow topsoil or
    composted clay soils are present. Remember, tree roots get larger as the
    tree gets older.

    Competition

    Trees, shrubs, ground covers, and lawn grasses all require sunlight, water,
    and rooting space for growth. Each plant in the landscape competes with
    neighboring plants regardless of type or species. Some even produce
    chemicals that are exuded from roots to restrict growth of nearby plants.
    For each plant to do well, it must have adequate space. Because perennial
    woody plants increase in size each year, they require additional space over
    time. The landscape design should provide adequate space for these plants
    to mature.
    While shade is the biggest, most obvious problem trees create for turf
    growth, a tree’s roots also contribute to poor turf performance. Contrary to
    general thinking, most tree roots are in the top 2 feet of soil. More
    important, the majority of fine, water absorbing roots are in the top 6 inches
    of soil. Grass roots ordinarily occupy a much greater percentage of the soil
    volume than tree roots and outcompete them for water and nutrients,
    especially around young trees. However, grass root density is often much
    lower in areas where trees were established first. In these situations, tree
    roots compete much better for water and nutrients and prevent or reduce
    the success of establishing new turf.

    Competition is especially important when transplanting, seeding, or sodding.
    The newest plant in the area must be given special treatment and must
    receive adequate water, nutrients, and sunlight, which frequently means
    that competing sod should be removed from around transplanted trees and
    shrubs or that some of the lower branches should be removed from existing
    trees above a newly sodded lawn. In any case, do not do any tilling around
    trees.
    Mulching is an alternative to turf around trees, and its use eliminates
    potential competition. A 2- to 4-inch layer of wood chips, bark, or other
    organic material over the soil under the drip line is recommended because it
    • helps retain soil moisture
    • helps reduce weeds and controls grass
    • increases soil fertility when mulch decomposes
    • improves appearance
    • protects the trunk from injuries caused by mowing equipment and
      trimmers that often result in serious tree damage or death
    • improves soil structure (better aeration, temperature, and moisture
      conditions)
    Maintenance Practices
    Maintenance practices for trees and turf are different. Because tree and
    grass roots exist together in the upper 6 to 8 inches of the topsoil,
    treatment of one may damage the other. Fertilizer applied to one plant will
    also be absorbed by the roots of a nearby plant. Normally that is good, but
    excessive fertilization of either trees or turf can result in tree crown or grass
    blade growth greater than desired.
    Many herbicides or weed killers that are used in turf can cause severe
    damage to trees when misapplied. Misapplication can occur on windy days,
    causing the drift to fall on nontarget plants, or on hot days when the
    herbicide may vaporize and diffuse into the air. While most herbicides do
    not kill tree roots, some, such as soil sterilants and a few others, do.
    Herbicides that can cause tree damage have statements on their labels
    warning against using the product near trees.
    Watering of lawns is beneficial to trees if the watering is done correctly.
    Trees need, on average, the equivalent of one inch of rain every seven to
    ten days, depending on the species. Frequent, shallow watering does not
    properly meet the needs of either trees or turf and can be harmful to both.
    Turf growing under or near trees should be mowed at the top of its
    recommended mowing height. Mowing off no more than one-third of the
    grass blade’s height and letting the clippings remain on the lawn does
    much to ensure a healthy and vigorous lawn. In an ideal situation, tree and
    turf maintenance would be handled by the same individual in order to
    maximize the benefits of all maintenance practices.
    Special Situations
    • Placing fill dirt around existing trees. Fill dirt frequently is added around
      existing mature trees so that a level or more visually desirable lawn can
      be established. Fill dirt changes the ratio of oxygen to carbon dioxide
      around tree roots and the roots may subsequently die. Consult a tree
      care expert before adding fill or constructing soil wells around tree trunks.
    • Establishing lawns around existing trees. Preparation of a seedbed for
      lawns requires disruption of the upper 4 to 6 inches of topsoil. This soil
      contains the feeder roots of trees. Damage to tree roots often results in
      declining tree tops.
    • Lawn watering in arid sites. Homes are sometimes built in woodlots. In
      arid regions, the watering that is required to maintain grass is especially
      damaging to dryland trees. Excess water at the tree trunk encourages
      growth of fungi that can kill trees.
    Thin turfgrass growing around trunk-scarred weak trees does not need to be
    a common sight in the landscape. With proper planning, proper plant
    selection and placement, and reasonable management, the many and
    varied benefits of both trees and turf can be readily achieved.