David M. Kopec, Extension Turfgrass Specialist

Many homeowners and landscapers frequently will ask, A why can't I get grass to grow underneath my trees? Well, there are many reasons why and sometimes it's not the grasses fault. Sometimes, no matter what you do, the grass does not persist. This is often the result of an environment which will not support grass growth. Areas such as this are better off in gravel, mulch or under story flowers.

SHADE AND SHADE TREE COMPETITION:

Shade is the lack of direct full sunlight which does not reach the lawn. Shade can come from buildings, walls and landscape trees. Shade (or lack of direct sunlight) poses several problems for turf. One is the duration of light (intermittent shade throughout the day) and second is light quality (the range of wavelengths that plants need to make food). Therefore, when investigating the use of grasses for shade, ask yourself what is the shade condition.

(1) Is it constant shade?
(2) Is it intermittent (filtered shade) through a treeline?
(3) Is there full sun and for how long and at what times of the day?

Competition from shade means here, tree roots in the lawn. Trees will extract water wherever they can get it. Here it's from the lawn irrigation. Although turf in shade does not use as much water as turf in full sun (to keep its leaf temperature regulated) an appreciable amount of water applied to the lawn can be taken up by tree roots. Invasive tree roots develop on many species of landscape plants when the opportunity arises!

The second form of stress from shade is the loss of green plant cells in the turf leaf. The grass plants then do not make enough food to satisfy the normal growth rates of new shoots, new leaves, roots and rhizomes (where applicable).
GRASS RESPONSES TO SHADE

Turf grass plants are essentially struggling and stretching to reach for sunlight in the shaded condition. This is why the turf becomes leggy and the distance between the leaves and the stems becomes wider. Thus, the internode length becomes longer and for the same mowing height there is less leaf area available to capture the already low levels of sunlight. This is why it is important to raise the mowing height in the shade.

Secondly, grass leaf blades become finer (more narrow) when growing in shade. This is just the opposite of most dicot (broadleaf) plants! The narrow leaf response is part of the stretching for light response.

Turf grass plants growing under shade conditions have low food reserves. Therefore they are not as tolerant to traffic stress, heat, cold or excess soil moisture stress (drought or flooding)!

Mother nature has provided us with a few grasses which grow in shade in special environments. When we impose these grasses in unfavorable environments , they fail. When we try to grow grasses that are not shade tolerant, we are doomed to failure. Ask yourself, why do I need grass in the shade? Many times a mulch, stone or gravel or just soil maintained with an edge can delineate the shade line from the rest of the lawn.

Listed below is the shade tolerance of the most popular turfgrass species used in Arizona.

LOW ELEVATION (300-3000 FEET)
GRASS SHADE TOLERANCE
Bermudagrass: seed and sod types none-very low
Zoysia grass: seed and sod types fair - moderate/partial shade
St. Augustine: seed and sod types moderate
Buffalo: seed or sod types none - low
Tall fescue: seed or sod types fair*
*specialty use grass = small lawns only
Bermudagrass does not like shade. It will not persist in shade. Although occasionally when you will see bermudagrass in shade, it is leggy, weak and thin. It must be mowed high to survive (2.25-2.50 inches).

The best choice for shaded lawns in the low desert is St. Augustine. It is available as sod. From sod pieces you can make plugs. St. Augustine grows slower than bermudagrass, but faster than zoysiagrass.
Note that each of the warm season grasses are so different from each other in leaf width and growth habit, that they do not mix together in a lawn. Bermudagrass growing in full sun next to St. Augustine under a mulberry tree produces an abrupt change in color and overall appearance. One of them will look like a weed.

Also note that each of these grasses will grow in the full sun. Therefore a lawn which has both full sun and heavy shade will support either St. Augustine or zoysiagrass.

MID-ELEVATION (3000-4200 FEET)

You certainly find bermudagrass at these elevations, but the higher you go the shorter the normal growing season is. Shade just adds to increase problems at these elevations.

GRASS SHADE TOLERANCE
Bermudagrass: seed or sod none
Tall fescue: seed or sod fair - good
Kentucky bluegrass: seed or sod fair*
Buffalograss: seed or sod none - low
* variety dependent

HIGH ELEVATION (4300-7500 FEET)

GRASS SHADE TOLERANCE
Creeping red fescue fair - moderate
Kentucky bluegrass fair*
Perennial ryegrass none/temporary only
Tall fescue fair - moderate
Hard fescue none - low
Chewings fescue none - low
Poa trivialis fair
* variety dependent

At high elevations, only cool season grasses are essentially used for lawns. On a species (grass-type) level, creeping red fescue is moderately shade tolerant. Tall fescue is moderately shade tolerant. For Kentucky bluegrass, certain varieties are more shade tolerant than others. Shade tolerant varieties of KBG include; Glade, Nuggett, Liberator, Quantum, Compact Chateau, Coventry, Moonlight, Brilliant, Showcase and SR2000.

Perennial ryegrass is a quick fix for heavy shade. It will germinate, grow and make a cover for about a year and then thin out drastically.

KBG seed mixtures include perennial ryegrass and creeping red fescue. This is done to achieve quick establishment (from ryegrass) and have some shade tolerance (creeping red fescue).

When planting a mixture in a large shaded lawn, you will achieve better results by including a shade tolerant KBG, as opposed to relying on the creeping red fescue alone.

Poa trivialis (rough stalk bluegrass) is a grass which will tolerate wet sites, either in sun or shade (at mid to high elevations). This is a speciality grass available as seed. Plant it the same time as you would KBG. Rough stalk bluegrass should be planted by itself. This is because of its light green color and lax (soft) leaves.