1" Of Water Per Week and Other Lawn Watering Myths



How much should you water your lawn? That's a very common question and one that gets asked frequently and there are some common answers. They may have started out as rules-of-thumb but have been repeated so frequently as to at least verge on the edge of being classified as myths.

So lets go over why you shouldn't go by these suggestions and then cover how you should really water your lawn. Bear with me, it's pretty easy, the only hard part is understanding why you can't let someone else tell you how to water your lawn. There's even a tool to tell you how long you need to water your lawn. The hardest part is probably reading through this, I know I can be a bit long winded. Hopefully it's worth it.

Lawn Watering Myths

One inch of water a week

This is the one I see repeated the most. You want to water established grass deeply and infrequently and 1" of water a week seems like it should accomplish this right? Well yeah, in many cases it does but it's too broad a statement to follow it blindly.

Different types of soils hold different amounts of water for different amounts of time. For instance clay soil will hold water longer than sandy soil and it's not like there are two types of soil. Everyone's soil is different with differing water holding properties.

Different types of grass also need different amounts of water then you have to consider that a lot of thatch build up may be keeping a good bit of water from penetrating deep into the root zone and there are factors that will determine how much water the grass and soil loses. This combination of moisture lost through evaporation and plant transpiration is called evapotranspiration (ET). Evapotranspiration is effected by the weather. Hotter drier weather increases ET. Thatch also plays a role in evapotranspiration. While thatch can soak up water making it necessary to water longer, it also acts as a mulch and reduces evaporation so you don't need to water as often.

Even something you wouldn't think would play a role is the condition of your mower blade. I've said this repeatedly, one of the best things you can do for your lawn on so many different levels is keep a sharp blade. In a paper Mower blade sharpness effects on turf (Steinegger et al. 1983) mowing kentucky bluegrass with a dull blade used 1.3 times less water than mowing it with a sharp blade. That's right less water. The damaged grass wasn't as healthy and wasn't taking in as much water. So don't think you'll save on water with a dull blade, you'll just have bad looking grass all around.

So unless someone knows exactly what type of soil you have, what your weather is like, how much sun your lawn gets, how much thatch you have, how sharp your blade is, what type of grass you have, how deep your roots are, they can't tell you how much water your lawn needs. 1" of water a week could be 2x as much or more or maybe not even half the amount you need.

30 minutes per zone once a week

This one I've seen different variations of where it's X minutes per zone every Y days or something similar. This is poor advice. In addition to all the issues mentioned above in the 1" per water a week discussion, you have the added variables that go along with your sprinkler system. When I've seen this advice given there is no description given of the type of irrigation system the person has. People seem to give advice based on what works for them even though chances are it won't work as well for you.

There are vast differences between sprinkler heads/nozzles, the installation arrangement and the water pressure (PSI) delivered to the heads. All that will determine how much water is actually delivered.
Looking at a typical line of sprinkler nozzles from the same family the precipitation rate can vary from 1.43 in/hr to 2.05 in/hr at the same pressure using the same family of nozzles.

It gets worse when different types of sprinkler heads are used which is common. For example, you might use gear driven sprinklers for large areas and sprayers for smaller areas. Different zones could have different precipitation rates that could be 0.4 in/hr to over 2 in/hr. For one person 30 minutes of watering might mean 1/4" of water, for another it might mean 1/2".

How much and how often to water your lawn

So I've ranted about myths, evapotranspiration, given you some numbers about precipitation rates of sprinkler heads you may not be familiar with and you might be thinking that I'm going to give you some advice that's too hard to understand or to difficult to implement. Nothing could be further from the truth.
The best advice I've ever heard regarding lawn watering came from Roger Cook in an episode (probably many episodes) of This Old House or Ask This Old House. Water your lawn as much as it needs when it needs it! Simple huh? OK that didn't clear things up, Roger went further to indicate you need to water down to the root zone and then water it when the lawn shows signs of drought stress and I'll expand on some of the things he said with what I've learned to help you figure out how to water your lawn.

How long should I water my lawn?

As Roger Cook explained, you need to water down to the root zone. This should be pretty obvious. Plants absorb water through their roots so having the soil around the roots moist is important. Watering below the root zone is just wasting water.

These are the steps to determine how much water your lawn needs. It's best to run this test after a few days without rain and no rain scheduled for the next day either.

Step 1 - Determine root zone depth: You need to know how deep your roots go. You can take a square spade and make a U-shape cut in your lawn then pull it back and see how far down the roots extend. An easier way is to use a soil probe (aka soil sampler) as pictured to the right (click picture for more details.) A soil probe saves a lot of time especially if you have multiple sprinkler zones. It also comes in handy when you're taking soil samples to send in for a soil test and doesn't affect a large area of your lawn.

You can also see the profile of your soil to determine how much thatch you have. The one pictured to the right takes 12" deep cores. It's not much more expensive than smaller soil probes that only go down to 6" which isn't deep enough for most lawns. If you're willing to spend a little more this soil sampler is a little longer and looks easier to use.

If you don't have at least 6" of the soil dry at the top wait for it to dry out more before performing the rest of the test.

Step 2 - Water your lawn for 15 minutes: Run your sprinkler system for 15 minutes in each zone. You can program them to all run one after the other. You won't be seeing how far it went right away.
It's a good idea to use some tuna cans or rain gauges to see how many inches of water your sprinklers put out in 15 minutes. You'll use this information along with your rain guages to make adjustments to your sprinkler time so that you don't waste water.

Step 3 - Measure how deep water penetrated 24 hours later: After 24 hours come back and see how deep the water penetrated. To figure out how long to water your lawn divide 15 by the number of inches of moist soil times the root depth of your lawn. For example, if the root depth is 8" and the soil was moistened to 3" deep: 15/3 * 8 = 40 minutes. If your root depth is 12" 15/3 * 12 = 60 minutes.
To make things easier, just enter the values in the form below:
Depth to root Zone:
Depth of Moist Soil:
?minutes
This is how long you need to water your lawn. It doesn't matter what time of year it is, spring, summer or fall. The only time you need to recalculate is if there's a significant change in your root zone depth (if you're lawn is still being established for example or you're moving from a shallow infrequent watering schedule) or there's a significant change in your thatch (dethatching).

How often should I water my lawn?

Well, like Roger Cook said, only water it when it needs it and he says you can tell when it needs it when it looks limp and when you walk on it you can see your footsteps because the lawn doesn't spring back. How often this happens depends on the weather, the amount of rain fall and evapotranspiration but when your lawn needs water you'll see it.

You don't need to keep your soil moist throughout the root zone. In fact it's better to let the top dry out. This will minimize chances of diseases forming as well as dry out shallow rooted weeds and prevent new weed seeds from germinating. You don't want your grass to completely dry out, but it's good for it to get a little thirsty. Water it when about 30-40% of the lawn starts to show signs of drought stress.

If it's been a while, there hasn't been any rain but you're not sure, take a peek at the soil and see if the root zone is moist. It doesn't have to be completely wet but if it's totally dry you obviously need to water.
You can also view this post that contains a tool from the Northeast Regional Climate Center that will use rain fall and temperature to help tell you when you should water your lawn.

What if you're watering by hand?

If you're watering by hand using a hose and sprayer, you're not going to have the consistency that someone with a sprinkler system does but you can still apply the same principles and try to water your lawn as evenly as you can.

If you are interested you can put together some simple lawn sprinkler systems yourself and use an automated sprinkler timer that connects to your hose for less than $100. Use the link for more information.
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6 comments:

  1. Here is a video about how long to water, although it is based on Florida sandy soils.
    http://irrigationrepair.com/How_Long_To_Water_Your_lawn.html

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  2. The above comment is obviously spam so don't follow the link.

    Instead of deleting it I decided to comment on it. The video in question doesn't give you the information contained in this site. It basically just tells you how to adjust your sprinkler timer.

    In the video they say that you can do a water audit but they say you have to pay someone a lot of money to do it. What I described in this post is instructions on how you can run your own water audit without having to pay some irrigation company that is spamming internet blogs.

    Took a look at the rest of the site and it is not very impressive. For good advice on sprinkler systems including DIY installation, maintenance and repair visit irggigationtutorials.com.

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  3. I posted your link on a forum because I thought other's might find it interesting. A post by andy10917 responded that it was bad advice if you have shallow roots because it will keep them shallow. Is that true?

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  4. Glad you liked what I wrote enough to link to it. Got your comment about the edger on the other post. Makes me feel better since I actually have one sitting in my garage now! Just haven't had a chance to use it yet with all the rain. I gave it a try today but the soil was too damp.

    This wound up being pretty long so it's posted across multiple comments.

    Funny that person got angry over what I wrote. Just goes to show whenever you challenge someone's long held beliefs they don't take it well. Doesn't matter if it's politics, religion or something as simple as watering your lawn.

    If I hit a nerve with some people, that just means I'm glad I posted this information. Hopefully more people read it and less people stop thinking they can tell someone how much water they need over the internet. It does more harm than good.

    He's wrong on a lot of points. I'll touch on as many as I can.

    First and foremost, this is a deep and infrequent watering plan. You're just letting your grass tell you how much and when to water, not some person online that is just repeating something someone told him, who was told by someone else, who was told by yet another person until who knows where it came from. Who do you think knows what your lawn watering time should be? Your grass or someone that might live 100's or 1000's of miles away?

    Within the first couple of paragraphs I stated it was a rule-of-thumb and with all rules-of-thumb they are not to be taken as fact. What I tried to do here is provide an easy way for people to learn how and when to water their particular lawn that is better than using generic advice. I've fallen into this trap and wouldn't be surprised if I recommended the 1" of water per week myself.

    Continued next comment....

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  5. I love how people think they can train grass through watering like it was a dog. You can't (at least not just with water) and saying that will probably also piss him off. There are many factors that determine root mass and water is just one of them. Other things such as soil compaction, soil structure, nutrient availability especially phosphorous are some of the other things to look at and probably the things I would look at first. If you do have shallow roots because of frequent light watering you can still use what I suggested to get your roots to grow deeper, you just need to check your root depth and adjust your watering time more frequently. That was mentioned in my post but I'll try and make it clearer.

    Let me try and explain why. If you noticed, I recommended you don't check how far the water went right away. Why? Because soil particles don't absorb water but soil does. Just like in a sponge, water moves in between the gaps. Have you ever tried to wet only half a sponge? What happens? The water moves through the pores and the entire sponge becomes moist. Leave the sponge on the counter a while and you'll notice the top is dry and the bottom is still wet. Why? Gravity pulled the water down and evaporation on top dried up the sponge. If you checked the soil a couple of days later you'll see the moist soil is even deeper and the top drier. In sandy soils you might not want to even water as deep as the root zone since water passes through it faster but this blog is geared to the northeast/mid atlantic region where sandy soil isn't very common and I don't know enough about it to make that assessment.

    Through gravity and capillary action water will spread throughout the soil and mostly move down deeper into the soil on it's own. That's why there's such a thing as ground water and it constantly gets replenished from rain passing through many feet of soil. If you water past the root zone all you are doing is putting water out of reach of the roots. Nutrients get carried along with the water so all the money you spent on fertilizers and amendments are going to go down with it adding polution to ground water. That's a major concern for people who don't have public water and rely on well water.

    Continued next comment...

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  6. Let's examine how plants grow too. Plants have a purpose. Their purpose is to grow until they seed so they can reproduce. That's it. That's what they're trying to do and we're forcing them to do what we want them to do. They want to put all their effort into producing seed using the least amount of effort possible. If there's enough moisture and nutrients in the upper part of the soil, that's all the roots they will grow. Growing deep roots is what we want to help grass survive the summer and look nice. Grass is happy to go dormant, we're the ones that don't want it to.

    Grass also doesn't have a sense of smell or other sensory perceptions as far as I know. You can't put water just a little bit out of reach and roots will chase after it so the idea that letting the root zone dry out and the roots will follow it is another part of this myth.

    If you put water past the root zone, by the time the plant decides it needs more water, that water will be well out of reach of new root growth. Remember, water moves down into the soil over time. The water that is there is just below the root zone now was in the root zone before the plant needed it. The water that was below the root zone is still going to be below the root zone.

    Green parts grow up, root parts grow down. Plain and simple. If you let the grass get thirsty like I suggested the grass is going to put more effort into root growth to try and find water. What direction are the roots going to grow? Down, because that's the general direction roots grow barring unusual circumstances. When it grows down it's going to find some of the water that percolated down since the last watering. There should be some there, The upper parts of the soil loses water through evaporation as well as what the grass has absorbed. If the plant can't get enough water it's easier for it to attract microorganisms that will help it get water and nutrients than it is to grow deeper roots so that will probably happen first depending on the level of available phosphorus where the roots are. It's a very complex ecosystem and plants are adaptive.

    Now picture a plant or a tree that has a shady side and a sunny side. The sunny side will be more full because the plant concentrates growth there, where it's easier to get what it's looking for. Same thing with roots. If you let the top part dry out and the bottom is still moist that's where the grass will focus it's root growth.

    The key part of deeply and infrequenty in terms of root growth is the infrequently part. To get a plant to do something it doesn't normally do you have to stress it and you stress it by depriving it of water in this case.

    If you're really trying to grow deeper roots because you've been on a shallow infrequent watering schedule the only difference you need to make is let the grass dry out more. You keep the amount of water the same but first you check other possible causes such as lack of phosphorus, compaction, hard pan and so on. Then check on how deep your roots are getting and adjust the water accordingly. Just plug in the new variable in for root depth in the form. How deep the water penetrates should be about the same unless you did something else like dethatch.

    Ok written more than I expected so I won't even go into how root development in grass is greater in spring and fall. Two seasons where trying to water deeply and infrequently will be hard considering the amount of rain we usually get in this part of the country.

    ReplyDelete