Doing the Math for Lawn Watering

Note:  the following was written prior to water rationing.  It is impossible to water a lawn properly under water rationing regulations when, for example, it is illegal to water more than three days a week.

How often should I water my lawn?
No question is uttered with more fear and trembling than this. No other question begs for an answer that, in a few words, will keep a clear ecological conscience, minimize a water bill and still keep the grass green.
When I tell people that, in July, they may have to water their lawn 10 times a week, the response is one of disbelief. The calculation, however, is quite simple. In July, our common lawn grasses require 2 inches of water per week. A compacted lawn can only absorb 0.2 inches of water before runoff occurs. Thus, 10 waterings, each of which applies 0.2 inches of water, are needed on a weekly basis, during this time of year, for compacted lawns.
How do I know if my lawn is compacted?
If you do not regularly aerate your lawn and do not apply compost or some other organic material on a regular basis, your lawn is compacted to one degree or another. Most of us are disinclined to regularly aerate and broadcast compost over our lawns, so most of our lawns are compacted.
How long should I leave the sprinklers on?
In the Valley, most small lawns, of 1,000 square feet or less, and even some larger ones, are watered by spray-head sprinklers. Spray sprinklers, in typical spacing patterns, deliver 0.2 inches of water in about five minutes. On larger lawns, where slow-moving, gear-driven rotary sprinklers are utilized, it takes approximately 15 minutes to deliver 0.2 inches of water.
There are only seven days in a week. How do I schedule 10 waterings?
One approach is to water five days a week, with two waterings on each of the five days. Here it is almost essential to have an automatic, remote control sprinkler system with a time clock.
Converting a manual system to an automatic one is not difficult. All you need to do is turn off the sprinkler line water main, unscrew the existing valve stems, and screw in automatic actuators; unless you have very old or rusted valve bodies, there is no need to change them. Run wires between the actuators and the appropriate terminals in a sprinkler time clock. Do not plug the time clock in until all the wires have been connected. Where no power outlet for a time clock is available, or where it is not practical to lay wires, you may wish to consider battery-operated valves. Be aware that battery-operated units are not as reliable as remote control systems.
What is the best time of day to water?
Early morning is the best time to water, between 4 and 9 a.m. Lawn grasses, like virtually all other plants, conduct photosynthesis, their food-making process, when light is available to leaves and water is available to roots. Photosynthesis proceeds less effectively when a plant is under stress. On hot days, stress is likely to set in before the morning is over. Ideally, a plant should have water available to its roots as soon as the sun comes up. If you follow the five-day-a-week watering schedule, with two waterings per day, set your sprinklers to water your lawn at 4 and 8 a.m.
What kind of lawn grass – called “turf” by lawn care professionals – should I plant?
The most popular type of lawn grass in the Valley is tall fescue, sold under name brands such as “Marathon and Medallion.” It is a cool-season grass, which means in practical terms, that it stays green all year. It also stands up well to foot traffic, but should have at least half a day of sun. A problem with cool-season grasses – perennial rye and Kentucky blue grass would be other examples – is that they do not recover from dieback. If you go away for a long weekend and your sprinklers malfunction, you might have some dead spots on your return. These will have to be reseeded.
Bermuda grass, which needs nearly as much water as tall fescue to look its best, will recover from dieback. Its rhizomatous roots go down as deep as 20 feet and will regenerate new grass blades soon enough. Hybrid Bermuda, similar to that used on some putting greens, has finer blades than ordinary Bermuda grass but is trickier to maintain.
For shady locations, St. Augustine grass is the preferred choice because, like Bermuda, it has rhizomes, which allow it to recover from dieback. Like Bermuda, it is a tropical or warm season grass, and undergoes a period of winter dormancy during which it turns brown. St. Augustine does not tolerate heavy foot traffic. A cool-season grass mixture for shade usually includes rough blue grass, creeping red fescue and perennial rye.
How often should I aerate and spread compost on my lawn?
As often as you like. Aeration is a process where 2-3 inch cores are removed from the lawn and soil beneath it. This allows water, air and nutrients to reach roots and keep the lawn actively growing. Some people advise against summer aeration, claiming that it dries out lawns, but I have not found this to be true. Spreading a thin layer of compost or Nitrohumus immediately after aeration, or any other time, is always beneficial. Compost encourages the growth of aerobic soil bacteria that break down old grass roots, further relieving lawn compaction.

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