Deep, infrequent irrigation and the 1/3 rule: historical perspective
By Andrew Hoiberg, PhD
How to best irrigate turf? Deeply and infrequently. How much turf should be removed during mowing? No more than 1/3 of the total blade length. Two seminal philosophies and recommendations in turfgrass management, but do you know where they originated? We will take a historical look at the research that formed these tenets of turfgrass management.
Deep and infrequent irrigation
Deep and infrequent irrigation is a concept that takes advantage of a turf plant’s tendency to root deeper when they are in need of water. The concept is that you apply a large amount of water (1+ inch) perhaps once a week (or when the grass starts to show drought stress) to wet the entire soil profile, and as the top dries out, the plant is forced to grow deeper roots to utilize the water further down in the profile. This seems particularly useful where irrigation is manual and time constrained, such as in home lawns. Instead of watering your Kentucky bluegrass (a species that is traditionally associated with being “water needy”) lawn a little every day, apply 1- 1.5 inches of water and wait until it shows drought symptoms (wilted, “purple,” footprints in the grass) and then repeat the heavy/deep application of water. After a few weeks of this practice and utilizing ET data, you can form an irrigation program to suit your turf’s needs during the season, adjusting as necessary.
Investigation of irrigation principles and practices has been around for some time. There are numerous references in early popular turf publications and some research articles from the 1930s. Welton et al. (1934) observed that rates of 1.5, 2.0, and 3.0x the normal (natural rainfall) all resulted in turfs that gradually became inferior to those that did not receive supplemental irrigation. Additionally, as the rate of water increased, the quality of the turfs decreased.
Early investigation (Sprague, 1931) of water logged soils showed that too much moisture will result in a poorly developed root system. This is caused by limited gas exchange and compaction from overwatering. Sprague concluded that a system of watering that delivers moisture to the upper 2 inches of the soil profile only will force the plants to confine their root growth to this thin, moist layer. He goes on to note that the smaller the supply of water during leaf growth, the smaller the leaves and the greater the cell wall thickness will be. Therefore, strengthening of the tissue will result and grass will be better able to withstand the wear during stressful periods.
For practices, it is suggested that watering should accomplish a depth the same as that of the root system. Sprague goes on to say, “periodic moistening to a depth of 4 or 5 inches is far more desirable than daily sprinkling which penetrates only 1 or 2 inches,” followed by an editor’s note that “Jno. Morley has said this for years,” (I am not sure who Jno. Morley is!). The article concludes that the ideal system for watering the golf course should involve only enough moisture for slow, but hardy growth; wilting should be avoided, but occasional wilting is better than risking supplying too much water which will result in a soft, tender turf that is susceptible to “injuries of many kinds.”
After further research, I came across another fundamental principle relating to plant physiology with regard to drought stress and irrigation. In experiments conducted by Meusel (1964), it was observed that the ratio of stomata to epidermal cells was increased by excessive watering, causing the overwatered plants to wilt faster than those provided with less water (two times per week versus six times per week).
Clearly, the deep and infrequent irrigation rule has been around for a long time and is firmly established into our collective turf management consciousness. Even if you never realized the history behind this practice, now you can rest assured that it’s backed by good science and has been providing the basis for irrigation management for turf managers for more than 80 years.
We’ve all heard the 1/3 rule pounded into our heads over and over again during our turf education and careers. But where did it originate and how many of you actually follow it?
The reasoning behind this rule is to limit the amount of stress that you place on turf plants when performing the destructive process of mowing. By removing only 1/3 of the leaf during a mowing event, you will limit the amount of photosynthesizing material removed and allow the plant to maintain reasonable growth by minimizing stress.
It is based on the correlation between the percentage of top growth removed and the subsequent effect this removal has on root growth. Root growth is severely impeded, and actually ceases for a period of time depending on the amount of top growth removed.
The research that first introduced this theory was conducted by Crider (1955), a USDA employee and conducted on pasture and forage grasses. Three separate studies were conducted within this experiment. The first, evaluating cutting leaf tissue at different intervals on smooth brome, tall fescue, and orchardgrass for cool-season species and Florida paspalum, king ranch bluestem, switchgrass, glue grama, and bermudagrass for warm-season. The second experiment examined the effects of cutting on root growth in the field on weeping lovegrass and breadgrass. The last experiment examined different percentages of top growth removed on Rhodes grass for a single cutting and Kentucky bluegrass, smooth brome, and Rhodes grass for multiple cuttings. Although these were not examined in a turfgrass setting, the results are, in my opinion, still applicable to today’s turf managers as they have been cited in popular textbooks on the subject (Christians, 2003) and there is many parts of grass physiology that extend to a majority of species, whether they be forage, pasture, or sports fields.
The research proved exactly what was theorized: cutting too much leaf tissue off will result in the stoppage of root growth and place unnecessary burden on the turf.
I have often witnessed or heard of turf managers growing grass long during the summer when the fields are not in use only to come back to the fields mere weeks before traffic begins again and remove far more than 1/3 of the top growth to get the stand back to “traffic height.” We know this puts a tremendous amount of stress on the plant and is not the right strategy for preparing the stand for player traffic.
The 1/3 rule has been around for decades and is known to minimize plant stress and should not be ignored. As a side note, it almost seems like it has become cliché to mention the 1/3 rule, but it is all too often forgotten or brushed aside for convenience. Consistent mowing height and frequency will always put your fields in a more likely position to succeed the various other stresses that inevitably occur.
Andrew Hoiberg, PhD, is Vice President, Research & Development, for Calcium Products, Ames, IA.