This time of year the #1 question we get is, “How much should I water?” It happens every year as the winter rains subside and spring brings in some drier weather. The short answer is: Lawns need 1 inch of water per week to stay green. That would be a combined total of rainwater and water from your sprinkler.
If you are happy with that answer, you can supplement your knowledge with an article from the Seattle Times, “How much water is too much for your lawn?”
If you really want the lowdown on watering lawn grass, the answer is… “Well, it depends.”
It depends on 1) grass type, 2) site conditions, 3) soil texture and structure, 4) evaporation, and 5) root depth.
Let’s start by combining #1 and #5… The type of grass you plant and the depth the grassroots can grow are a direct factor in when, if any, supplemental water will be needed throughout the year. In the Pacific Northwest, our primary grass seed mixes consist of Ryes and Fescues. [see footnote 1 for drought-tolerant grass seed mixes] These grass plants are very resilient and, if allow to, can grow roots as deep as 14 feet! Yes, that was not a typo… 14 feet. The problem is, most home builders do not know this and scrape off good topsoil to build your home and then replace it with a new material they call “topsoil” but isn’t at all like the native soils that were scraped off nor at the same depth. We have seen it over and over again where 2 to 4″ of “topsoil” was placed over hard and compacted construction soils and we are now expected to have green grass year round in an extremely limited soil depths.
Here is a picture taken by one of our organic peers, Hendrikus Shraven (Hendrikus website) He makes his own soil mixes and in 1 demonstration project he was showing how deeply he could grow grassroots in his soils. These roots are 4-1/2′ deep and he grew them in 3 months! That’s right… This is not a typo… 3 months.
Imagine for a moment if your grassroots were 4-1/2′ deep… Do you think you would need to supplement your lawn with water? Most likely not.
This brings us to #3, Soil texture and structure, or, “What the heck am I growing my lawn on?” We could do an entire day seminar on this topic and still not cover all the bases. But suffice it to say, the amount of water your lawn needs is directly related to the soil under your lawn. Sandy soil drains water quickly. Clay soil holds water for long periods of time. Organic matter holds and releases water with great efficiency. Soil biology is the way Nature breaks things down and builds the structure within the soil profile. The combination of Sand, silt, clay, organic matter, biological life, and depth (how much you have) will affect how much water you need to put down.
Here is a good place to start your research into what to do to grow healthy soils: “Growing Healthy Soils,” Seattle Public Utilities
It should be noted that all recommendations on watering include, “Water infrequently but water deeply.” What does that mean? If we want to put down 1″ of water a week and spread that out over 7 days, the water will only penetrate the uppermost part of the soil profile and there will be no reason for the grassroots to go looking for water. If we challenge the grass a little, they will send out their roots to try and find water and develop a deeper root system. So, we should try to water just once or twice a week and then do what is called “Cycle and Soaking.” The problem is, when we water infrequently, the soil builds up a crust layer and it takes a while for the water to penetrate it. So we want to water a little, wait, water a little more, wait and water once again. Most irrigation controllers have this feature built in. For a more in-depth discussion on this topic, read this article from a Texas A&M Professor: Texas A&M, “Cycle and Soak”
And finally we get to, 2) site conditions and 4) evaporation, or, “How much water is leaving the site before the plants can use it?” If your property is sloped, water will roll down it. If it is in full sunlight, the water will evaporate. Heck, it will evaporate even in the shade, just not as fast. Taking sloped sites into consideration is important to knowing how much to water. Watering in the early morning hours when the sun is low and the wind is light will increase the amount of water that makes it into your soils. It is estimated that you will lose no less than 15% of your water to the air (never making it to the ground) if you water in the peak heat of the day and when the wind is blowing.
#1 tip for reducing evaporation? Mow higher. 3″ at least. Mowing higher helps to shade the soil under the grass keeping it cooler even during the heat of the day. Mowing higher also increases photosynthesis, helping the grass perform better. And, of course, mulch mow your grass clippings. The plant took a lot of energy to produce its top grow. When you mow it off and haul it away you are removing a lot of energy from the system that created it. When you mulch mow, you are adding organic matter in a form best suited to the grass… itself. One of our basic priciples is… “A plant’s best food is its own foliage.” Meaning everything that plant needs it is producing. When we leave it in the grass clippings, we need to supplement less to the grass in terms of fertilizer. Plus, the mulched grass clippings add moisture holding properties and help to keep the soil cooler reducing evaporation.
Okay, so you made it all the way to the end of this article and you are ready to give it a try, but you keep thinking back to the very first paragraph… “water 1″ per week…” and you are wondering, “How the heck do I measure 1″ of water?” It’s called the “Tuna Can Test.” Literally, you get yourself 2 to 6 (the more the merrier) tuna cans, place them around your lawn and run your sprinklers for a set amount of time. Generally, 15 or 20 minutes is recommended. At the end of that time, you can measure the amount of water in the cans and do a calculation on how long you need to water to get 1″. If you water twice or 3 times a week, you can adjust each watering cycle to the appropriate amount of time.
There you have it in a nutshell… Pick the right grass seed mix, pay attention to your site conditions, apply practices to develop healthier soils, keep the soils cool, and focus on developing deep grass roots.
[Footnote 1] For our favorite “alternative” grass seed company, check out all the seed mixes you can get at Protime Lawn Seed PT Lawn Seed. They have a number of wonderful mixes including this seed mix, Fleur de Lawn (Fleur de Lawn) that we did for one of our customers. When they mow their lawn, it looks more like grass than the other plants in the grass seed mix. But they don’t like to mow it because it mows off all the flowers. Imagine that, not mowing your lawn every week! How fabulous. Oh, and as a bonus, they tell me they use 1/5th the amount of water they used to use on the common builder lawn that they inherited.