We had a cooler than normal month of May, but temperatures have finally heated up and the 100-plus temps are likely to be around for a while. So now's the time to make adjustments to your watering schedule. That means increasing the frequency of your watering. It also means being sure to water long enough to deep-water your plants. That's the key to plants' survival.
The frequency of watering is based on the type of plant and the general category it falls into. The watering chart I have included lists the frequency of irrigation to maintain optimal plant health during the summer months. Even though desert trees like palo verde can survive on little or no supplemental irrigation, they will thrive with deep watering every 14 to 21 days.
As for deep watering, plants have a potential rooting depth based on their size and genetics. Most trees can produce roots to a depth of 3 feet. Shrubs send roots down approximately 24 inches. Landscape ground covers and herbaceous garden plants have a potential rooting depth from 12 to 18 inches.
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I have known homeowners who water daily, and their trees are in a constant state of wilt. The reason — they're not watering long enough. Having drip emitters running for 10 or 15 minutes, regardless of the soil type, will only wet the top few inches of soil. With 100-plus temps and bone-dry air, moisture is literally sucked out of the top layer of soil in a matter of hours.
This brings us to determining the duration of watering based on the desired depth of wetting. Because all irrigation systems — and for that matter all soils — are not created equal, it's impossible to tell you how long to run your drip emitters based on their output. Nor is it possible to tell you how many gallons of water to apply to a given plant per application. There are just too many variables of heat, humidity, sun exposure, wind exposure, soil texture and, finally, plant species. But there is an easy way to determine watering duration — I call it the dipstick method. Like measuring the oil in a crankcase, a stick or probe can be used to measure the depth of wetting in desert soils.
A metal or wood probe such as long screwdriver or wooden dowel rod, a 3-foot piece of rebar or a bamboo stake can be pushed into moist soil with little resistance. But when it hits dry soil, the rod won't go any farther. By pushing the rod as far in as possible and pulling it out, you can measure the depth of wetting. False readings may occur, most often if buried stones are encountered. Push the probe down in several locations to be on the safe side.
SUMMER WATERING SCHEDULE
(Based on daily maximum temps of 90 degrees or higher)
TYPE OF VEGETATION WATERING FREQUENCY DEPTH
Cacti, agave and yucca 10 to 14 days 12 to 18 inches
Desert trees and shrubs 14 to 21 days 24 to 36 inches
Desert ground covers and perennials 7 to 10 days 12 to 18 inches
Non-desert trees and shrubs 7 to 14 days 24 to 36 inches
Non-desert ground covers and vines 5 to 7 days 12 to 18 inches
Palms 5 to 10 days 36 inches
Citrus trees 7 to 10 days 36 inches
Roses and perennial flowers 3 to 5 days 18 to 24 inches
Seasonal flowers and vegetables 2 to 5 days 12 to 18 inches
Lawns 2 to 4 days 6 to 8 inches
Patio container plants 1 to 3 days to bottom of pot
Potted cacti and succulents 5 to 7 days to bottom of pot