How Much Water to Apply

The root zone — the volume of soil that is “mined” by a plant’s roots — is a reservoir for water. Each time you water, fill up your soil’s reservoir and add a small amount more. This slight overfilling ensures that salts from irrigation water or fertilizer are leached below the root zone and don’t accumulate to the point that they damage plant roots.

To know whether you have watered enough, dig a hole a few hours after watering and check the depth of the wetted zone. If the soil below the root zone of the plants is wet, you have watered enough. As a rule of thumb, assume that the root-zone depth for lawns and annual flowers is 1 foot; for vegetables and perennial flowers, 2 feet; and for trees and shrubs, 4 feet.

Fortunately, however, you don’t need to dig a hole 4 feet deep to see if you have watered long enough. Time your water application, dig a hole to see how deeply the water penetrated, and use the results to calculate how long you’ll need to water to reach the depth you want.

Irrigation water is most conveniently measured in inches. An inch of water is enough to cover the area watered one inch deep. You can measure this by placing some straight-sided cans, such as tuna fish cans, under a sprinkler and letting it run for 15 minutes. The depth of water in the can, measured by placing a ruler in the can, is the number of inches of water your sprinkler applies in 15 minutes. Multiply that depth of water by 4 to find the amount applied per hour. By digging holes an hour or so after watering to see how deeply an inch of water penetrates, you can easily calculate how long to leave a sprinkler system on to fill the reservoir of each plant.

As a rule of thumb, gardens in humid or cool climates require about an inch of water a week; in hot, dry climates, they may require up to 2 inches a week.