Figuring out how much water your landscape needs

People are nurturers, and because watering feels like nurturing, it's the part of gardening that most people love. But combine love of nurturing with lack of knowledge about watering decisions, and you have the ingredients for water waste and unhealthy plants.

How much you water depends on the root depth of your plants, the soil type you have, and the application rate of whatever delivers your water, whether it be sprinklers, drip, or you standing at the business end of a garden hose. The frequency and quantity of watering depends on many factors as well; try a good watering followed by a waiting period to see when it's time to water again. Then use this result as your base schedule for the season (refer to the following section to see how to read plants for signs of water need).

Observing your plants

You don't have to gather a lot of data and crunch a lot of numbers to see that plants need water. Some simple signs in the weather, in the soil, and in the plants themselves will tell you most of what you need to know about when to water.

^ Watch plants for symptoms of drought stress, such as loss of shine on leaves, slowing of growth, and dropping of older (that is, lower) leaves. If you see any |fojl of these signs, check your soil. If the soil is dry 6 to 12 inches down, water the plants, and adjust your controller (or the calendar in your mind) to water more often. If plants show stress and the soil is moist, reduce the frequency of watering. (Oddly enough, the symptoms of too much water and too little water are the same.)

Also pay attention to the weather. If it's windy, hot, or dry, go see whether your l»NG/ plants need a drink. In fact, if you're thirsty, your plants may be thirsty too.

Don't let plants dry out to the point where they wilt. Wilting is hard on them. Using evapotranspiration to your advantage

Evapotranspiration (also known as ET) is the combination of evaporation of water from the soil and the transpiration of water through the leaves of your plants. It's a way of indicating how much water your landscaping is using in any given weather situation. Many things affect ET, including temperature, humidity, wind, cloud cover, size/age/condition of plants, and time of year.

You can save a lot of water by matching your irrigation to the ET. You can access current ET data by calling your water purveyor for a local resource. Then you can reprogram your controller to match the watering times to the current ET. If the ET for the past week was 1.2 inches, for example, you can use the scheduling data you develop in your water audit (refer to the later section "Perform a water audit") to adjust your watering time to replace the 1.2 inches — no more. I show you how to do this in the next section.

If you get a smart controller, of course, you can ignore this section; the controller itself will make the adjustment, using data from local weather stations.

Soit conditions: Precipitation rate versus infiltration rate

Each soil type absorbs water at a particular speed known as the infiltration rate. Clay soils, which are made up of tiny, tightly packed particles, are the slowest to absorb water. Sandy soils, with their big chunky particles and numerous air spaces, drink up water fast. Water goes straight down in sandy soil rather than spreading out as it does in a clay soil. Loam soils are in the middle. Table 9-1 shows the differences in infiltration rates.

Table 9-1

Infiltration Rates of Soil Types

Soil Type

Infiltration Rate (Inches per Hour)


0.2 to 0.4


0.5 to 1.0


1.0 to 2.0

Your irrigation system delivers water to the soil at a particular speed known as the precipitation rate. As you may guess, the precipitation rate needs to be slower than the infiltration rate of your soil; otherwise, you'll have runoff.

If you use sprinklers, keep these figures in mind:

  • Spray heads apply 1 to 4 inches per hour.
  • Rotor and impact heads deliver 0.2 to 1.0 inches per hour. (These heads are better for heavier soils because they apply water much more slowly.)

To prevent runoff, design your system so that its precipitation rate is lower than or equal to the infiltration rate of the soil. It isn't the end of the world if you can't, however; you simply can use your automatic controller to apply water in short bursts, with rest periods in between to allow the water to soak in. (The precipitation rate of drip systems is low enough that they won't create runoff in even the heaviest soils.)

Paying attention to water holding capacity

Soils also differ in how much water they hold and in how fast they dry out (which is referred to as water holding capacity). In sandy soil, you need to water every 2 to 7 days, depending on the type of plants you have and the weather conditions; in loam soil, the interval is 3 to 11 days; and in clay soil, it could be 3 to 15 days.

Performing a water audit

The basic idea of a water audit is to determine whether your irrigation system is operating at peak efficiency and to figure out what improvements or adjustments you need to make to fine-tune the system. Then you make those changes and develop a schedule for operating the system so it keeps the plants healthy without wasting any water. You can get help from your local water purveyor or perform a water audit yourself.

A simple, effective water audit will take you about an hour to complete. The audit can be used for a permanent sprinkler system or with a low-tech system like a sprinkler at the end of a hose.

Homeowners Guide To Landscaping

Homeowners Guide To Landscaping

How would you like to save a ton of money and increase the value of your home by as much as thirty percent! If your homes landscape is designed properly it will be a source of enjoyment for your entire family, it will enhance your community and add to the resale value of your property. Landscape design involves much more than placing trees, shrubs and other plants on the property. It is an art which deals with conscious arrangement or organization of outdoor space for human satisfaction and enjoyment.

Get My Free Ebook

Post a comment