Wet Behind the Gears

There are few subjects that stir up more dispute than proper irrigation of turfgrass. It seems that every “expert” has a different opinion on the subject and that they will fight almost to the death to defend it. Basically the controversy boils down to the “light and frequent” argument vs the “deep and infrequent” argument, which is further complicated by:

  • The time of year and its effects on root growth phase
  • Water conservation concerns
  • Whether the turf is newly established
  • Mowing height
  • The soil type

Let’s start with what we do know: if you are establishing new grass from seed or sod, light and frequent irrigation IS necessary for rooting in. No ifs, ands, or buts. Generally this will be necessary for at least a month to six weeks during the growing in phase.

Second, the BEST time to water is between midnight and eight AM: this way you are timing watering with the potential presence of dew, thus providing the shortest leaf wetness period possible to reduce disease without sacrificing irrigation water to evaporation by the sun.

The depth of irrigation is far more important than the frequency. No one ever checks to see how deep that water is going, or how long the moisture persists at that depth. Both of these factors depend on soil texture. Optimum irrigation frequency maintains soil moisture at a depth of eight to ten inches such that soil forms a ball that will remain intact after being tossed five times and will stick slightly with pressure in medium to fine textured soils (higher in clay and silt). If you have a sandy soil, you can use a screw driver barrel as a moisture meter: moisture should extend along the barrel for 8-10 inches on a daily basis. When this is no longer the case it is time to water again. The amount of water required for this measure for all types of soil is between 1-1.5 inches as measured in a rain gauge.

Now here’s where the hairy part comes in: the speed with which the soil wets up in contrast with the output of your sprinkler system. Clay soils wet very slowly. The potential for runoff before the soil wets to the depth required to keep the whole root system hydrated due to the speed of water output is a very real problem. You may have to set the system to do several consecutive irrigations on the SAME DAY to reduce runoff events. Water that runs off will NOT be available to your turf. By the same token, keeping the system running for prolonged periods in clay soil leads to saturation and death of roots through lack of oxygen, while lightly running the system every other day means that the soil never wets up to the proper depth and the dry areas below the soil surface no longer support root growth.

Sandy soil wets up very quickly, and will thus dry out more quickly. Again, running the system for a prolonged period will result in water being lost through continual drainage. Light, frequent irrigation will result in water not reaching the proper depth to maintain the entire root system. The solution is once again to monitor how long it takes the soil to wet up to the proper depth using an individual sprinkler system, and then how long it takes that soil to dry to the point that there is no longer consistent moisture on the barrel of the screw driver or in a soil sample taken at various points in the yard. This is the point where the sprinkler should kick on again.

Now we need to throw another wrinkle into the system: mowing height. Simply put, short grass equals short roots. The shorter you mow the grass, the equivalently shorter the roots become. If your customer is buzz cutting his or her lawn, then you will be FORCED to use shallow, frequent irrigation to keep that upper layer continually moist. In other words, the growing in period is never finished. The same is true of turf grown on severely compacted soils. If the grass is mowed at 2.5-3”, then your best strategy for turf health is to keep that deep root system intact by irrigating as deeply and infrequently as possible. The infrequent part of the equation is to save water, to prevent saturation and suffocation around roots, and to promote mild water stress in the turf plant. As it turns out, mild water stress prevents the plant from elongating rapidly (less mowing), but it still produces food at the same rate. The extra food is stored and provides a buffer against summer stress.

A final issue is the natural cycle of root growth of the turf plant. When soil temperatures are high in the summer, the plant ceases to make roots and many of the roots die off. Your best option is to sample the turf in the summer to find out what depth the roots are mostly at, and recalibrate your irrigation schedule to accommodate the shallower roots, if necessary. Unless the root system has become extremely shallow over the entire lawn, causing dry out between irrigations or rainfall events, this probably will not be necessary. Again, compaction can cause soil to heat up more quickly and hold heat longer than uncompacted soils, and clay soils are more likely to become compacted than sandier soils.