Regional Gardening Reports :: National Gardening Association

In the Garden:
Southern California Coastal & Inland Valleys
April, 2004
Regional Report

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Deep watering fosters a glorious mix of nasturtiums, iris, and alstroemeria.

Deep Watering

The weather and the texture of your soil will determine how much water you'll need to apply to your garden, and how often. Heavy clay soils require less water than sandy loam soils. During periods of long, hot weather, plants need more frequent and longer irrigation than during periods with more moderate temperatures.

Teach your plants to grow deeply for moisture. In spring, for average soils, water deeply only every two to three weeks. By the time summer's heat arrives, plant feeder roots will be growing deeply for moisture, and the plants won't need watering more frequently than once a week during very hot spells.

One inch of water will soak down to different depths, depending on how heavy your soil is: 12 inches deep in sandy soil, 9 inches deep in loamy soil, but only 3 inches deep in clay soil. Plant root zones generally reach from 2 to 12 inches deep.

Clay versus Sand
If you have clay soil, since it is so compact you can get away with watering a little each day for two or three days to allow absorption down to the root zone. If you apply a lot of water at one time, you'll lose some to runoff because the soil can't absorb it fast enough. Clay soil will retain this moisture for a much longer period than sandy soil, which is very porous. So you have to be careful not to overwater, which can increase root rot problems.

Soil with a lot of organic matter is the best because it holds water but still has room for air that's need for good root growth.

Lawn Needs
Lawns are the greatest users of outdoor irrigation. It's important to make sure the roots are growing deeply and getting the moisture they need. A 2-inch mowing height will allow the lawn to retain some surface moisture, so you don't have to water as often.

Make sure irrigation drip lines, soaker hoses, sprinklers, and trenches are in place before root systems get too large.

To test how deeply your irrigation water is penetrating in your lawn, water for the usual length of time and then push a trowel into the soil until the blade is completely submerged. Push the soil clump to one side, or lift it out completely, and look at both the depth of the roots and the water line in the soil; it will be dark toward the lawn surface and lighter where it's dry.

The water line should be just past the longest roots. If it's not this far down, replace the clump, water again, and test another spot until the water line is below the roots. Adding all these irrigation times together gives you the correct amount for each watering. Don't water again until two-thirds of the root length is again dry. This may mean that you can double the time between waterings, and the grass roots will not suffer during the really hot portion of the summer.

Mulch the soil -- especially with organic matter such as leaves or grass clippings -- to temper the drying and heating effect of the sun, and your irrigation will be more efficient.

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