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Woody plants and turfgrasses are both 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 affects 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% 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 (trees that 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 is
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 the neighboring plant 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. Since 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 out-compete 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. This 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 2-4 inch layer of wood chips,
bark, or other organic material over the soil under the drip line is
recommended because it:
• Helps to retain soil moisture
• Helps to 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, and
treatment of one can unintentionally damage the other. Because
tree and grass roots exist together in the upper 6-8 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 this is good, but excessive fertilization of either trees or
turf can result in tree-crown or grass-blade growth greater than
that desired.
Many herbicides or weed killers that are used in turf can cause
severe damage to trees when misapplied. This can occur on windy
days, causing the drift to fall on non-target 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 the equivalent of one inch of rain every
seven to ten days. 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 1/3 of
the grass blade's height and letting the clippings remain on the
lawn will do 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 is frequently 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 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 4-6 inches of
topsoil. This soil contains the feeder roots of trees. Damage to
tree roots often results in declining tree tops.
• Creating tree root buffers with turf. A sufficiently wide strip of
turfgrass between trees and hard surfaces such as building
foundations, sidewalks and roads can help to reduce the potential
damage caused by tree roots as well as provide an area where
water and nutrients can soak into the soil and be beneficial to
both turf and trees.
• Lawn watering in arid sites. Homes are sometimes built in
woodlots. In the West, 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.
For Additional Information
This brochure is one in a series published by the International
Society of Arboriculture as part of its Consumer Information
Program.
