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Most of your lawn care time is spent mowing. How and when you mow your lawn can have a great impact on weed growth. To optimize the health of your lawn and reduce weeds, adopt the following guidelines.

Ideal Mowing Heights For Common Tennessee Lawngrasses



MOWING HEIGHT (in) Minimum Maximum


Common Bermudagrass Hybrid Bermuda Centipedegrass Zoysia

3/4 to 11/2 1/2 to 1 1/2 1 to 2 3/4 to 11/2


Fine Fescue Kentucky Bluegrass Perennial Ryegrass Tall Fescue

2 to 3

Mow at the correct height. Mowing height can drastically affect the space available for weeds. Each type of lawngrass has an ideal mowing height range. Consistently mowing at an appropriate height allows the lawngrass to naturally close in or overlap, forming a closed canopy and reducing the space available for weeds. Lawngrasses have a maximum and minimum mowing height tolerance. Mowing above the maximum tolerance results in bushy growth (opening the canopy and providing space for weeds). Mowing below the minimum tolerance is the most common. Scalping, the removal of too much leaf surface, often results in a weak and weedy lawn. Mowing below the minimum tolerance does not leave enough leaf surface to support optimum growth of roots and new shoots. Repeated scalping often results in short, fine leaves; shallow rooting; and an open canopy. Thus, mowing lawngrasses at the ideal height can greatly reduce the space available for weeds to grow. Refer to the previous table for ideal mowing heights.

Use the correct mowing frequency. How often do you mow or how tall do you let your lawn grow before mowing? Generally, you do not want to remove more than 1/3 of the grass height in a single mowing.

For example, if you mow your tall fescue lawn at a 2-inch cutting height, then mow when it reaches 3 inches tall. If you mow your hybrid bermudagrass lawn at a 1-inch cutting height, then mow when it reaches 1.5 inches tall. The frequency at which you mow is dependent on rate of growth, not a set date. Removing more than 1/3 of the height will stress the grass, affecting optimum root and new shoot growth and subsequent canopy closure. If your lawn grows too tall between mowings, gradually remove the excess height by taking 1/3 of the height with several mowings rather than removing an excess amount in a single mowing.

Raise the cutting height prior to periods of environmental stress.

Mowing height determines the amount of leaf surface to support growth. In periods of environmental stress, such as dry weather, raise the cutting height to the maximum tolerance. For warm-season grasses, raise the cutting height in early fall to insulate soils against extreme low temperatures. For cool-season

grasses, raise the cutting height in late spring to promote root growth for improved summer drought tolerance and to help insulate against extreme high temperatures.

Mow with a sharp blade. Mowing with a dull blade usually tears the grass blade, exposing a large, jagged edge that is prone to moisture loss and disease entry. A clean cut allows the grass to recover quickly, maintaining a healthy canopy and neater appearance.

Alternate the mowing direction. Try not to mow in the same direction every time. Alternating mowing direction encourages upright growth in addition to distributing wear and reducing soil compaction.

Recycle clippings. Allowing small leaf clippings to drop, rather than bagging, cycles nutrients essential for lawngrass growth and survival. For more information on recycling clippings, refer to Extension PB 1455, Lawn Care to Reduce Landscape Waste.

Fertility and Liming

If lawngrasses are to achieve optimum growth and compete against weeds, the soil must be fertile, supplying the required mineral nutrients in appropriate amounts at the proper time. Through the year, lawngrasses have periods of active growth and periods of slowed growth, or dormancy. Nutrient demand is dependent on growth rate. Nutrients applied in excess or at the wrong time may be lost or captured by weeds. Soils in Tennessee may require additions of the primary nutrients nitrogen, phosphorus and potassium. Nitrogen is often soluble in water and mobile in soil. Nitrogen is also the nutrient required in the greatest amount by lawngrasses. Applied excessively or at the wrong time, nitrogen may weaken your lawn. Test the soil before applying fertilizer.

In addition to nutrients, your soil may need amending with lime. Most soils in Tennessee become acid (or sour) unless lime is applied. As a result, lawns become less hardy and weeds become more prevalent as the lawngrass is gradually thinned.

Assessing soil fertility, the need for lime and fertilization schedules for

lawns are explained in Extension PB1038, Fertilization and Management of Home Lawns.

Need for Additional Water

Water, whether provided by rainfall or supplemental irrigation, is essential for lawngrass health and survival. Generally, 75 percent or more of the weight of a lawngrass plant is water. Actively growing lawngrass usually requires 1 to

1.5 inches of water per week. In Tennessee, the total annual rainfall distribution is generally insufficient to meet this demand. You may decide to add additional water to your lawn by irrigating.

Irrigate your lawn early in the morning (e.g., 5:00 to 10:00 a.m.) to reduce disease potential and waste by evaporation. Irrigating in the afternoon increases the loss by evaporation. Irrigating in the evening increases the period of leaf wetness, often promoting disease. For best results, water deeply and infrequently. Irrigate until the soil is moistened to a 4- to 6-inch depth. Do not irrigate again until the appearance of the first symptoms of drought stress (e.g. rolled leaves and bluish-green color). This promotes deep rooting. Irrigating frequently for short durations often results in shallow roots, reducing your lawn's tolerance to drought and other stresses.

Irrigation may also promote the germination and growth of summer annual weeds. Summer annual weeds germinate on or just below the soil surface. Light, frequent irrigation may provide needed water for summer annual weeds and may not penetrate deep enough to benefit lawngrass growth.

Remove Fallen Leaves

Fall is the period when cool-season lawngrasses have the opportunity to recover from summer stresses. Fallen tree leaves may restrict light and limit recovery. Timely leaf removal will improve the availability of

light for optimum recovery. When a dense mat of leaves is not removed in a timely manner, lawngrass growth is weak and plants may die. After leaves are removed, the weakened health of your lawngrass provides space for fall-germinating weeds.

Selecting Lawngrasses for Open Areas

When establishing or renovating your lawn, select a lawngrass species or variety appropriate for your specific site and needs. Considerations include the level of care, soil type, exposure and location requirements. Tennessee is located in a transitional zone between northern cool-humid and southern warm-humid climates. Within the state, certain locations favor warm-season lawngrasses; others, cool-season grasses. Bermudagrass, zoysia and centipede are perennial warm-season lawngrasses. Warm-season grasses grow best during the spring and summer (optimum growth between 80 to 95 F). These lawngrasses lose color during winter dormancy. Perennial cool-season lawngrasses include Kentucky bluegrass, fine fescuses and tall fescue. Cool-season lawngrasses grow best during the spring and fall months (optimum growth between 60 and 75 F) and maintain color during the winter.

In addition to choosing between a warm- or cool-season lawngrass, choose a variety or a blend of varieties that are known to be adapted to your area. Contact your county Extension office for more information on recommended lawngrass species and varieties for your area.

Selecting Lawngrasses for Shade Areas

Within your landscape, there may be areas with different growth conditions. Some areas may receive full sun, while others receive very little if any light. Shaded lawns are often weak and thin due to low light intensity and limited energy reserves. This weak growth often results in an open canopy, favoring the invasion of shade-tolerant weeds.

In light- to moderate-shaded areas, choose a species or mixture of species that are shade-tolerant. To maximize light penetration, prune the lower limbs

Pseudoscorpion Tennessee

of trees and large shrubs as much as feasible (Refer to Extension PB 1163, Pruning Shrubs In The Landscape). In areas of intense shade, landscape with mulch or establish a shade-tolerant ground cover. (For more information refer to Extension PB 713, Landscape Mulching Materials, and PB1585, Annual and Perennial Flower Shade Gardening in Tennessee).

In general, cool-season lawngrasses are more shade tolerant than warm-season grasses. Cool-season grasses shaded during morning may wilt very quickly when exposed to full sunlight in the afternoon. Fine fescues (like red, hard, chewings and sheep fescues) are often tolerant of shade. For example, red fescue tolerates medium shade. However, this species has limited heat tolerance. Although tall fescue is usually more heat-tolerant than red fescue, this species is less tolerant of shade. Kentucky bluegrass is more tolerant of high temperature than red fescue. However, Kentucky bluegrass has poor shade tolerance when maintained in dense stands (For more information refer to Extension PB 1213. Managing Cool-season Lawngrasses in Shade).

Among the warm-season lawngrasses, bermudagrass is essentially intolerant of shade. Zoysia is able to tolerate light, open shade; however, shoot density may decline and color may fade.

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