Richard L. Duble, Turfgrass Specialist
Texas Cooperative Extension
Text and images copyright © Richard Duble.
Mowing is the primary cultural practice in turf management. Without regular mowing even a fine turf quickly becomes just another weed patch. Good mowing practices enhance a sports turf more than any other cultural practice. Density, texture, color, root development, wear tolerance and other aspects of turf quality are all enhanced by proper mowing.
Mowing is primarily a function of the growth rate of grasses. Since grasses continually interact with their environment, the growth rate changes in response to environmental changes. Thus, the turf manager must recognize the need to change mowing practices accordingly. For example, during prolonged periods of drought stress, it might be advantageous to raise the mowing height and reduce the frequency of mowing. Similarly, following the application of fertilizer, it may be necessary to increase the frequency of mowing to avoid excess accumulation of leaf clippings. The skill of the turf manager at making these adjustments determines the quality of turf maintained under his supervision.
Although growth rate largely determines the mowing schedule for a particular site, other factors need to be considered when planning mowing practices. The type and size of mower, frequency of mowing, mowing height, mowing patterns, the management of grass clippings and the maintenance of mowing equipment all need to be considered when planning mowing practices.
On closely mowed turf such as that on sports fields, golf greens, fairways,and some lawns, the reel mower provides the best cut. However, the quality of cut is influenced by the mowing height, the number of blades on the reel, the rotational speed of the reel and the forward speed of the mower. The typical reel mower has 5 to 7 blades and is suitable for mowing at heights of 0.5 to 1.5 inches. At lower mowing heights, the turf would develop a wavy or rippled appearance unless the reel was powered to revolve at a high rate of speed. At mowing heights below 0.5 inch, 9 or more blades per reel or a hydraulically-powered reel are required to produce a smooth cut. The typical golf green mower has 11 to 13 blades on a relatively small diameter reel.
The "clip" of a reel mower is most readily defined as the "distance" the mower moves forward between the time that each reel blade engages the bedknife. Most conventional homeowner type reel mowers have a clip of about one inch. For optimum smoothness of cut with respect to ripple or corrugations, a reel-type mower should have a clip approximately equal to the height of cut. Ground-driven reel mowers have a constant clip and can only provide a smooth cut above a specific height and will show obvious ripples below that height. Power or hydraulic driven reels provide a variable "clip" and extend the application of a given reel mower.
Vertical differences between the peaks and valleys of a reel cut are always present because the "valley" is the height of the bedknife cutting edge and the "peak" is the height to which the grass springs back after being cut. The horizontal distance between peaks, or between valleys, is the clip. When the vertical distance between the peak and valley is minimal the resulting cut will "appear" to be smooth. Again, this condition exists when the clip is no greater than the height of cut.
The specific clip of a reel mower is dependent upon the number of blades and the rotational speed of the reel and is independent of reel diameter. The tip speed, however, does increase in direct proportion to an increase in the reel diameter when rotation speed is constant.
If we consider a reel with a certain number of blades, the "clip" can only vary with the rotational speed of the reel in relation to ground speed. With the same number of blades, the faster reel will produce the shorter clip length.
The relationship of "clip" to "height of cut" affects not only the finished appearance of the turf, but the ability of the mower to cut at heights of two or three times the clip distance. A mower with a 1-inch clip will not cut as well at a 2-inch height as a mower with a 2-inch clip.
There are three basic requirements for acceptable reel mower performance : (1) the bedknife must be exactly parallel with the reel; (2) the reel must make only light contact with the bedknife and (3) the cutting edges of the reel and bedknife must be sharp and straight. A sharp, properly adjusted mower will cleanly cut a piece of paper. Backlapping is the standard procedure used to restore sharp edges on the reel and bedknife.
From time to time, either in the field or after reel service, it may become necessary to reconfirm the reel to bedknife contact. Newspaper can be used to help measure the degree of contact along the length of the bedknife. By placing the paper tangent to the reel cylinder between the blade and bedknife, the paper can be used as shim to bring each of the reel blades in equal contact with the bedknife. The paper should be pinched, but not cut. Next, place the paper perpendicular to the reel cylinder and refine the adjustment to just cut the paper. Check at least 3 points along the length for each blade.
At taller mowing heights, 1.5 inches and more, the rotary mower is generally used. At these taller heights, the quality of cut may not be as important and the rotary mower cuts tall grass, weeds and seedstalks more effectively than the reel mower.
The rotary mower cuts grass by impact of the blade against the grass at a very high velocity. The first one half inch of the blade's leading edge cuts the grass. The additional cutting edge length enhances the mowing performance and quality of cut.
Theoretically, the concept of clip also applies to rotary mowers. Rotary mowers are generally designed so that the blade tip cuts the grass at least once every two inches.
In addition to the cutting performance of the rotary blade, good dispersion of grass clippings is also important. Good dispersion is determined by the blade speed, blade design, housing depth, baffling and other factors that affect airflows and velocities.
From the point of view of the blade, there are three irregularities which can decrease quality of cut. These regularities are a warped, twisted, or dull blade. A warped blade is bent in such a way that the blade tips are worn on one or both sides. The situation must be corrected. The twisted blade leaves the cutting edge above the flat surface it is being checked on. With the body of the blade parallel to the flat surface, the blade edges should lay flat on the surface. A twisted blade consumes additional horsepower, and causes shredding of the grass blade tips.
To properly sharpen a rotary blade, material should be removed from the top surface of the blade only, trying to maintain the original bevel. If the lower edge of the blade is ground or filed, the cutting edge will be above the heel of the blade, resulting in beating, shredding and browning of the grass.
In addition to blade irregularities, damaged housing or improperly mounted engines or decks, bent crankshafts or spindles will result in uneven heights of cut.
Another impact-type cutting unit is the flail mower. The cutting unit on a flail mower consists of a number of small blades attached to a horizontal shaft. As the shaft rotates, the blades are extended by centrifugal force. Each blade becomes an independent, freely revolving, cutting unit. Thus, if the blade strikes a hard object, such as a rock or piece of metal, the blade recoils without damaging the mower or creating a dangerous projectile. The quality of cut still depends on the sharpeness of the blade.
The single most important aspect of turf management is the frequency of mowing a turf. If there is a "secret" to turf management, it is frequent mowing-the more often, the better. However, mowing is time consuming and expensive. Therefore, we often compromise between what is best and what is practical.
Golf greens exemplify fine turf and are usually mowed daily. But, those that are mowed twice daily are even finer. However, golf greens make up only a small percentage of the total acreage of a golf course and efficient mowing equipment makes frequent mowing practical. Fairways, however, constitute a large percentage of a golf course and more expensive equipment and greater time is required to mow fairways. Thus, fairways are not mowed as often as greens. That doesn't change the fact that fairway turf would be better with daily mowing.
Likewise, the sports field manager who mows his fields three or more times per week will have a finer turf than one who mows less often. The same can be said for the homeowner. The homeowner who mows every 5 days will have a finer lawn than the one who mows at 7 to 10 day intervals.
Several factors influence the recommended mowing frequency. Of course, growth rate (and factors that affect growth rate) and mowing height have the greatest influence on mowing frequency. As a "rule of thumb", mow often enough so that no more than 30 percent of the leaf is removed at any one mowing. By following that rule, photosynthesis is only marginally affected by mowing. However, if 50 percent or more of the leaf is removed by mowing, several days are required to restore photosynthesis to its previous level. In contrast, if only 10 percent of the leaf is removed by mowing, the effect on photosynthesis is insignificant. Therefore, to maintain optimum growth, frequent mowing is required.
In addition to minimizing the effect of mowing on photosynthesis, frequent mowing also helps to maintain a high percentage of the leaf surface-a requirement for healthy root development. Following severe defoliation (as a result of infrequent mowing) energy reserves (food) stored in the grass plant are needed to restore leaf surface. Consequently, root growth may cease for several days since leaves always take priority over roots for energy reserves following defoliation or dormancy. Thus, mowing frequency, which determines the extent of defoliation, has a significant impact on residual leaf surface and root growth.
Growth rate and mowing height must both be considered when deciding on a mowing interval. If we follow the "rule of thumb" and remove only 30 percent of the leaf surface, the lower the mowing height, the shorter the interval between mowing. For example, consider a growth rate of 0.1 inches per day and a mowing height of 1 inch. In 10 days the grass will be 2 inches tall. Then, the mower would remove 50 percent of the leaf when cut back to 1 inch. The turf would need mowing at 5 day intervals to stay within the "30 percent rule of thumb". The same grass mowed at .5 inch would need mowing at 2 or 3 day intervals to stay within 30 percent. Without exception, the shorter a grass is mowed, the more frequently it must be mowed.
Besides mowing frequency, the turf manager's use of fertilizer has more affect on growth rate than any other cultural practice. High rates of nitrogen fertilizer promote vegetative growth beyond that needed for maintenance. Thus, mowing frequency must be increased as the application of nitrogen increases. During summer months, nitrogen fertilization should be kept at a maintenance level.
Environmental factors also influence mowing frequency through their effect on growth rate. Temperature is the environmental factor that has the greatest impact on growth rate. When temperatures favor growth, mowing frequency needs to be increased. Conversely, when temperature retards growth, mowing frequency can be reduced.
In warm season grasses, growth (leaf extension) begins at about 65°F. As temperatures increase above 65°F, the rate of leaf extension increases, assuming other requirements (light, moisture, nitrogen, etc.) are met. Consequently, mowing frequency needs to increase as the season progresses from early spring to mid-summer. To compensate for the increased growth rate and maintain the same mowing frequency, mowing heights are raised as the season progresses.
The photoperiod (daylength) also influences mowing frequencies. During summer months (long days) leaf growth is upright; while during spring and fall (short days) leaf growth is more decumbent. Consequently, mowing frequencies can be reduced during spring and fall months.
Grass variety and turf use have the greatest influence on mowing height. Other factors such as mowing frequency, shade, mowing equipment and the season of the year need to be considered, but grass variety and turf use are the limiting factors. Tall fescue, for example, performs best at mowing heights between 2 and 3 inches. Tifdwarf bermudagrass and creeping bentgrass perform best at mowing heights below .5 inch. In general, the bermudagrasses perform best at mowing heights of 1 inch or less, while other warm season turfgrasses perform best at mowing heights of 1 to 2 inches. For the cool season grasses, bentgrass performs best at mowing heights of .5 inch or less, while bluegrass and ryegrass performs best at a 1 to 2 inch mowing height.
The parameters that determine mowing heights for grasses are often fixed by the use of the turf. Golf greens, for example, must be mowed below .25 inch to provide the smooth, consistent and fast surfaces that golfers expect. Sports turf for baseball, football, and soccer needs to be mowed near the 1-inch height to reduce interference with the ball and provide fast playing surfaces. Grass bowling greens and tennis courts must also be mowed very short. In contrast, roughs on golf courses are typically mowed at a 2 inch height to penalize the golfer for hitting into the rough.
Grass areas that are mowed infrequently, but where appearance is important, must be mowed between 3 and 4 inches in height to prevent scalping. Roadsides, lawns around commercial and industrial sites, and some park areas often meet these requirements.
On lawns, grass species, frequency of mowing, shade and the type of mower determine the optimum height. The turf manager must realize that the shorter a turf is mowed the more often it must be mowed to prevent scalping. As a result, the homeowner may compromise between the height best suited for the grass and the height best suited for his frequency of mowing. For example, bermudagrass provides the finest turf at mowing heights below 1 inch which requires mowing at 5-day intervals or less. If the homeowner can only mow on a 7-day schedule, the mowing height must be raised to 1.5 inches to prevent scalping.
On shaded sites, mowing heights need to be raised 30 to 50 percent to compensate for the more upright growth of leaves under reduced light. Grasses growing in shade typically have long, thin, spindly leaves and higher mowing heights help maintain leaf surface to carry on photosynthesis. If shaded lawns are continuously mowed short, the grass gradually thins out and weeds invade the turf.
The type of mower the homeowner uses may also influence mowing height. Rotary mowers do not usually cut below a 1-inch height and perform best at heights between 1.5 and 3.0 inches. On the other hand, the reel-type mowers cuts best at 1.5 inches or less.
The season of the year may also influence recommended mowing heights. In early spring, close mowing is recommended to control thatch and to increase turf density. Also, the more prostrate (decumbent) growth habit of grasses in the early spring allows for close mowing. Mowing heights may be raised slightly during summer months to reduce the frequency of mowing and to reduce watering. Higher mowing heights in the summer also help compensate for the more upright growth of leaves during long days. Mowing heights may be raised again in the fall on warm season grasses to reduce the risk of winterkill from low temperatures. Then, in the spring as the grass breaks dormancy, the lower mowing heights may be used to remove excess residues and promote early green up.
Using St. Augustine grass as an example, the homeowner may mow the lawn at 1.5 inches in early spring to remove excess residues and promote early recovery of the lawn. By mid-summer the mowing height might be raised to 2.0 to 2.5 inches to reduce mowing frequency. Finally, in areas of the South where winterkill occurs, the mowing height might be raised to 3 inches in the fall to provide more cover and insulation for the stolons and crowns of St. Augustine grass during winter months.
Mowing patterns are often neglected, but they, too, can have a significant influence on turf quality. Lawns that are mowed in the same pattern each time because of convenience develop problems during stress periods. If grass clippings are not collected from such lawns, clippings tend to accumulate in rows along the path of the mower. During severe drought or cold stress the thatchy rows are the first to show injury. By changing the mowing pattern each time the lawn in mowed, the grass clippings are uniformly distributed and the problem does not develop.
On golf courses and sports turf mowing along the same path and direction each mowing produces grainy turf and non-uniform playing conditions. Even on golf course fairways where it is most convenient and much faster to mow the length of the fairway, it is advantageous to occasionally mow across the width of the fairway.
Mowing the same direction between alternate yardage stripes on a football field may be done for the visual effect, but the direction should be reversed occasionally. Likewise, mowing a baseball field or golf green in a checkerboard pattern is fine for the effect it produces, but the direction of mowing should be occasionally reversed.
Management of grass clippings is critical to the quality of turf produced. On very dense, closely mowed turf areas such as sports fields and golf greens, grass clippings are routinely removed for aesthetic purposes and to prevent interference with the ball. On turf areas mowed at .5 inch or higher, clippings do not need to be removed if they do not interfere with the use of the turf. If the turf is mowed at appropriate intervals and clippings are uniformly dispersed, the clippings do not present a problem. In fact, by leaving the clippings in place, the nutrients they contain are recycled through the turf and the need for fertilization is significantly reduced. By catching and discarding grass clippings each time a turf is mowed, about 100 to 150 pounds of nitrogen per acre are removed each year.
On large turf areas where it is not practical to remove clippings it may be necessary on occasion to mow several times in one day or several days in a row to uniformly disperse the clippings. This is often necessary following several days of rain or following an application of fertilizer. Grass clippings should not be left in such volume that appear unsightly or smother the grass.
If a homeowner does not want grass clippings left on the lawn, they can be used for compost in a garden or as a mulch around plants. But, other than for aesthetic purposes, there is no need to routinely remove grass clippings from lawns.