Teaching plant roots to grow deeply for water will lessen irrigation needs during hot weather. The weather and the texture of your soil will determine the amount and frequency of irrigation to apply to your garden. Heavy clay soils require less frequent irrigation than sandy loam soils. During long periods of hot weather plants need more frequent irrigation than during periods with more moderate temperatures. However, excess irrigation that keeps the soil soggy will increase root rot problems.
Mulching the soil with a 4-5 inch layer of organic matter such as leaves, straw or grass clippings will temper the drying and heating effects of sun and heat, allowing irrigation to be more effective with less frequency and quantity. Mulch also deters weed seed germination, and will break down gradually to provide constant nutrition to plant roots.
To test how deeply your irrigation water is penetrating, water for the usual length of time, then push a trowel into the soil its full length. 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'll be dark where it's moist and lighter where it's dry. The water line should be just past the longest roots. If it hasn't gotten 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 of time for each watering - at least during that part of the season.
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 plant roots will not suffer during the really hot part of the summer.
With permission from Yvonne Savio, UCCE MG LA County