Yet the health and extent of the rooting system is at least 50 percent of what is important to growing healthy plants. Roots that are brown/black instead of white are suffering from ill effects of some environmental (physics), chemical, or biological impact. And, rooting depth is an essential piece of knowledge needed in order to be a good turf manager.

The latter is the source of discussion in this article, especially as it pertains to mowing, temperature, fertilization, and irrigation practices.

Mowing

Turfgrass root depth declines with declining mowing height. For example, mowing Kentucky bluegrass (Poa pratensis L.) at a height optimum for its physiological needs (2-3 inches) results in maximum rooting depth of about 4-6 inches.

Reducing the mowing height by half, to accommodate sporting activities, results in rooting depth loss of about half. Although some species, such as creeping bentgrass (Agrostis stolonifera L.), tolerate very low mowing – rooting depth is drastically impacted with this practice.

It is not uncommon to see rooting depths as little as 1-2 inches on golf greens cut to 0.1 inches. Raising mowing heights, especially on golf greens, can reduce, but does not eliminate, the shrinking effect during the heat of the summer. With the use of vibrating rollers in between mowings, green speed can still be maintained even at higher heights. Mowing frequency can also have a major impact on root depth and health. Removing more than about 1/3 of the shoot results in root stunting. This practice “shocks” the plant and root growth essentially ceases for 10-14 days, resulting in more rapid root-depth decline.

Temperature

The effects of mowing are especially pronounced during hot weather. Roots tend to “shrink” during the hottest part of the year. The cause of this is due to the plant’s constant battle to grow new aboveground shoot tissue.

The products of photosynthesis are largely spent by the plant to replace cut blades and, as a result, the carbohydrates stored in roots are used up through the summer.

This effect is reversed as cooler fall temperatures signal the plant to slow shoot growth and lay up stores in roots to survive the winter. The difference in root depth between the end of summer and the end of fall can be dramatic – often two-fold.

Fertilization

Although the species being grown, seasonal temperatures, and mowing practices are the main impacts on root depth, excessive nitrogen fertilization tends to promote shoot growth at the expense of root growth. Nitrogen is generally the most limiting factor in the level of turf greenness.

Unfortunately, the attitude of “if some is good, more is better” often prevails. Although ample nitrogen often yields short term color benefits, chronic over application of this nutrient results in poor root depth (as well as negative environmental impacts due to nitrogen leaching to ground water and greenhouse gas emission to the atmosphere). Adequate (not excessive) amounts of slow or controlled release nitrogen fertilizer should be applied to insure a steady supply without experiencing the typical roller coaster of highs and lows of this nutrient.

Fertigation is another alternative, although irrigation uniformity commonly results in significant spatial differences in application rate. Soil and tissue analysis and color evaluation will help a manager fine tune the nitrogen regime.

Other nutrients are typically less important for rooting depth related issues, although potassium can also be a problem – especially on very sandy soils. Again, it is important to insure a steady supply.

Irrigation

Fertilization and irrigation interact with regard to rooting depth. Excessive irrigation will leach nutrients out of the rooting zone, especially nitrogen and sulfur, but also potassium and others in very sandy soils. The shorter the rooting zone the more likely that nutrients will leach out of reach.

Irrigation frequency and amount of water applied will also directly impact rooting depth. The advent of the automatic sprinkler system has resulted in poor irrigation management. Turf is often watered too frequently, which promotes shallow rooting (Figure 1). Turf should be watered to the depth of rooting and then allowed to dry down. In fact, it is a good management practice to slightly water stress the turf repeatedly in the spring (stress is indicated by a lack of “bounce back” when walked upon and a slight gray color). This practice alters the hormonal signaling in the plant, which results in more of the plant’s energy being applied to root development.

Alternatively, irrigating with more water than the soil can store in the rooting zone is a waste of water and money associated with irrigation, fertilization, and pesticide application (some pesticides will leach through the soil similar to nitrogen).

Although it is true that the evaporative force of the sun will draw soil moisture upward, there is a severe limit on the effectiveness of this force for turfgrass. On hot days, the water supply in the top few inches of soil quickly depletes due to evaporation and plant water use.

Water being drawn upward by evaporation is generally insufficient to meet plant needs once the soil is mostly dried out. Deep rooted plants aren’t as affected by this because they can draw on subsurface moisture, but turfgrass roots tend to be very shallow compared to other plants and, therefore, are not able to rely on stored subsoil water.

Managers need to measure rooting depth monthly or quarterly and only apply enough water to fill the root zone (Figure 2). In order to do this, the soil texture must be determined by sending it to a soil testing laboratory. The soil texture analysis will show the percentages of sand, silt, and clay. Some labs will also report the plant available water unique to the soil being tested.

Alternatively, the results can be inserted into an equation to calculate this value (such as this one available at http://staffweb.wilkes.edu/brian.oram/soilwatr.htm).

Note that the plant available water is generally reported in inches of water per 12 inches of soil and, therefore, the rooting depth needs to be factored in since turfgrass roots will not be able to utilize all of the water to this depth.

For example, a soil with 20 percent sand, 54 percent silt, and 26 percent clay holds 2 inches of plant available water per 12 inches of soil. If the turfgrass roots were 6 inches deep, the amount of water the soil will hold in the rooting zone is one inch (6 in. soil x 2 in. water ÷ 12 in. soil = 1 in. water).

In this case, one inch of water should be applied with each irrigation (less if all of the plant available water has not been used since the last irrigation). It is also important to insure that the soil will allow infiltration of this amount of water during the time it is applied.

Otherwise, changes in application rates may need to be made or the irrigation event may need to be started, stopped, and then restarted after the surface soil has drained somewhat. Adjustments may also need to be made to account for irrigation uniformity problems.

Bottomline

Turf managers shouldn’t ignore the below-ground portion of their stewardship. Shoot health is evaluated almost daily. Evaluating roots periodically is equally important and will result in reduction in problems and maintenance costs.

Editor’s note: Hopkins is a certified professional soil scientist and associate professor at Brigham Young University and can be contacted at 801-422-2185 or email at Hopkins@byu.edu. Christensen is a recent graduate of Brigham Young University and can be contacted through email at ryanchri11@me.com.