What a coincidence! I generated this graph just about a week ago showing the basic watering plan for a lawn based on temperature.

{{gwi:119028}}

I'll quickly summarize for those graphophobes. If the temperature is going to be in the 100s for the week, then water once every 5 days. I know some of you are coming unglued at the idea of only watering once in that kind of heat. Keep reading.

I should have created that graphic years ago, but sometimes the simplicity takes time to make itself apparent. For years I have talked about "deep and infrequent" watering and gone on to define what deep means and what infrequent means. The graph does not define deep, but it is pretty good at picking the timing. DEEP means one full inch all at one time. YOU will have to measure how long it takes your sprinkler system or hose sprinkler to apply an inch. You can time that by putting out some cat food or tuna cans and turning on the water. Every system is different. Mine take 8 hours to fill the cans. My neighbor's takes 20 minutes. So there's quite a range. Whatever your system takes is the target which defines "deep" for you.

Let's say yours takes an hour to fill the cans. Then you can find the general high temperatures for your area in the newspaper, TV, or the Internet. Look at the graph along the bottom to find your temperature and look up and back across to get the watering frequency. If your high temps are going to be 80 degrees, then you should be watering once every 2 weeks.

But is this always the final answer? It is not quite this simple. My lawn is in 80% shade, my humidity runs from 50% in the day to 85% at night, my soil is crushed limestone about 4 inches deep, my grass is St Augustine mowed at 4 inches high, it was cloudy all last week, I have no concrete borders in the sun, and there is not enough wind down under our trees to make much difference to anything. But how do you get all those different factors into some sort of chart for your personal situation? You can't. It would be a 12-dimensional chart if you factored everything. Instead what you do is look at your grass. I would strongly suggest you hold the watering frequency strictly according to the chart, but if the grass wilts before the graph tells you to water, then water immediately but water longer (deeper) next time. Conversely if the grass looks great at the 2-week point, then wait a couple days and then water. Or if you really need to be on a schedule, you can water on the right day but water for less time. With my situation I normally water 3/8 of an inch. In the summer I boost it up to 5/8-inch. If we have prolonged drought coupled with high temperatures and very low humidity, then I will go to 7/8-inch per week.

Why do we water like this? For two reasons. One is deep watering promotes deep roots in your grass. If you, for example, watered 5 minutes every day, the roots would only be an inch long, or less. If you go on vacation and your daily watering system breaks, you might come home to a dead lawn. We see that all the time. But when you water deeply and infrequently, the roots are forced to grow longer to get the moisture out from deeper in the soil. If you go on vacation your grass already is "trained" to get water deeper in the soil, so you might not even water at all. I am going on 6 weeks without watering one of my lawns - of course the rain and clouds we had helped. The second reason for watering infrequently is to allow the surface of the soil to dry out completely. Weed seeds must have continual moisture for 3 days or more to germinate. If you water once and let it sit for a week or so, the weed seeds never get enough moisture to germinate. This will make your lawn care much more care free. No need for preemergent herbicide or post emergent later on.

Wow! That's an amazing graph! Does it matter what kind of grass I have? Or what zone I'm in? What about a similar chart for shrubs and bushes?

For normal turf grass, it does not seem to matter what kind of grass it is. If you had a lawn made of prairie grasses, then this changes considerably. With prairie grasses you might get away with deep watering once per month in the heat of summer.

Zones are only used to decide which plants to buy. They indicate a temperature you can expect to see in the winter and that determines whether a plant will survive. After the plants are picked, zones mean nothing. Having said that, there are many factors which go into how often YOU have to water your lawn. It has to do with directly sunlight, shade, clouds, soil type, soil organic matter, grass type, grass mowing height, grass density, wind, rain, and humidity. Rather than trying to factoring in all the variables, it is best to look at your lawn and make adjustments. I adjust the amount I water, not the timing.

Every shrub and tree is different, and you get into the same types of variables affecting the amount and frequency. I don't do shrubs.

Apparently the link to my chart was lost in one of the many changes to the forum. Here's the chart again.