Let’s talk about something I get asked about the most and find the hardest to answer: irrigation. As a gardener, you’ve probably asked how much you should water a plant when you bought it. I bet the response was something like “it depends.” Let’s spend some time answering some questions you might have. My goal is to help you understand why we irrigate the way we do in the desert. The answer will still be "it depends" but you will know what it depends on so you can figure it out.
First, understand that we water differently in the summer when it’s hot and the daylight hours are longer than in the winter when the days are cool and shorter. One main reason is evapotranspiration (ET), which is the sum of all processes by which water moves from plants, soil, and other surfaces into the atmosphere via evaporation and transpiration. If you want to learn more, here’s a simple breakdown: USGS Evapotranspiration and the Water Cycle.
Plants and soil release moisture throughout the day. Plant leaves release moisture into the air through their stomata, which are like pores—this is transpiration. The intensity of sunlight, humidity levels, temperature, and wind determine how quickly this moisture evaporates. Imagine yourself outside on a hot, dry, windy day, standing in the sun. How quickly does your sweat evaporate? All these processes combined are evapotranspiration.
When I first learned about ET and saw my plants wilting on a hot summer day, I thought I needed to spray water on them. The plants were losing more water than they could take up through their roots. They were dehydrated and losing moisture, so I needed to replace it. This was exactly what I was NOT supposed to do. Those plants were wilting as a defense mechanism—I was witnessing transpiration. When I sprayed water on the leaves, the plant thought it was raining, triggering a response to open its stomata. This caused the plant to lose even more moisture.
A second problem with spraying the leaves is that water droplets in the direct hot sun can act like a magnifying glass, burning the plant. Lastly, using overhead spray in the middle of the day loses more water to evaporation than watering in the early morning or evening. I was just wasting water and possibly hurting my plants. During a heatwave, if you have plants wilting, see if they recover overnight. If not, give them a deep soak in the morning.
An exception to the summer “no daytime overhead spray” rule is during monsoonal weather. We frequently get cloud cover but not the actual rain. In this case, you can simulate nature by spraying down the plant when there’s cloud cover and cooler temperatures. A tip from a customer: give a quick spray and then wait 20 minutes. This allows the plant time to register moisture in the air and open its stomata. Then go back and give a good soaking. The stomata will be open and can absorb all that moisture.
This leads to the question of when to water. We want to water when we will lose the least amount to evaporation. I have a smart controller that monitors the temperature and automatically adjusts irrigation times accordingly. Several water districts offer rebates on them. https://cvwatercounts.com/rebates/ On May 1st, I manually reprogram my start times from morning to night. Then on October 1st, I switch back to the morning. I experimented with this to conserve water when I opened the nursery, and I was able to reduce how many days I ran my irrigation by half. Running the irrigation at night keeps the moisture from evaporating, allowing the water to soak deeper and the soil to stay moist longer. Because you’re running the irrigation at night, it’s important to check for leaks. In winter, I prefer to irrigate in the morning. If you’re in an area that gets frost and use overhead spray, then start a bit later.
How long should you water? This is impossible to answer with a specific time due to too many variables. How fast does your soil drain? How much sun does the plant get? What kind of plant is it? If it’s native, do you want it to look natural or more like a landscape plant? What kind of irrigation system do you have? Native and desert-adapted plants want to dry out between waterings. They prefer a deep soak followed by drying out. How much they dry depends on the plant type. Creosotes want to dry out more than desert marigolds, and our native grape, which lives in riparian habitats, likes more water than either. This is why I recommend drip irrigation systems that water deeply but infrequently, not traditional systems that water multiple times a day for a few minutes.
In irrigated landscapes, you can use this basic guideline to determine how deep to water: ground covers, cacti, and flowers need to be irrigated approximately 12-18 inches deep; shrubs 2-3 feet deep; and trees at least 3 feet deep.
To decide how long to let your plant dry out, you need to know its natural habitat. When does it rain where it grows? Does it grow near water? In a wash? Foothills? If you don’t have access to this information, then irrigate again as soon as the designated depth for that plant has dried out. Watch the plant and see how it responds. If you’re overwatering, underwatering, or watering at the wrong time of year, the plant will let you know. There’s no substitute for knowing your plants.
With the new California water regulations, we all need to make changes in our landscapes and lifestyles. We can use this precious resource wisely and still have beautiful natural landscapes that benefit us and wildlife.
