Double-potting protects plants from heat, wind

Q: I just read your Review-Journal article from three years ago about double-potting. I’ve been trying to save a mandarin orange for five years that’s been in a pot for three years now. I had some old shade cloth that I double-layered and then stapled to the plastic pot to protect it from solar radiation.

A: I do recommend double-potting plants exposed to the sun for two reasons: The heat kills roots in the side of the container facing the sun (heat) and smaller pots blow over in the wind. Double-potting means placing a pot inside a pot.

In cooler climates, smaller containers blew over because of the wind. The production nurseries had two solutions: Put a layer of gravel in the outside container to prevent lodging or dig holes and put the potted plant inside a second container in the ground.

That was a lot of digging but you only had to do it once. To solve the roots growing into the ground problem, a twist of the container once a month severed the roots and solved that problem.

Some research points out another problem in hot climates. The side of the pot or container that faces the sun becomes so hot (approaching 180 degrees in 30 minutes or less) that roots on that side die from the heat. The upper limit for many plants is about 140 to 145 degrees.

Watering plants should be done early in the morning. Wet soil handles heat much better than drier soil.

To double pot in our climate, take the nursery container and place it inside a larger and more decorative container. Insulation, air gaps and decorative bark work well to fill in the larger gap between the nursery container and the more decorative larger container. In the spring, every three to five years (depending on the size of the container), pull out the container and repot the plant.

Q: The Southern Nevada Water Authority regional plant list shows plants with low and very low water needs. After much digging, I now know this comes from WUCOLs (Water Use Classifications of Landscape Species), a database of landscape plants at the University of California. Southern Nevada has no chart of water needs in gallons for very low water needs plants. Thank goodness you and Dr. Dale Devitt published “Managing Landscape Water Use in the Las Vegas Valley: Potential ET” as a university fact sheet in 2002. So, I know we have monthly values for plants in general for Las Vegas with an explanation. Has anyone come up with weekly averages on how much to give landscape plants?

A: I don’t know much about the SNWA’s regional plant list and its relationship to WUCOLs from the University of California. I understand WUCOLs were mostly generated from knowledgeable people agreeing on how much water to give different landscape plants, not from research.

I would remember, people manage irrigation; plants don’t. You can select plants that use less water, but if they are watered too often, then the applied water is wasted.

I suspect that lower-water-use plants need water applied less often due to their desert origin — local plants. This is why xeriscape focuses on native plants. Desert plants are not just cactuses and other succulents but can include small trees and shrubs.

That being said, plant water use increases fourfold between early January (midwinter) to our hottest months of June, July and August. This is talking about application frequency (days per week).

That means stepping up applied water through the spring and early summer (watering more often) and stepping it down (watering less often) when it starts cooling off from late September through December. This means that plants require about four times more water to stay healthy from midwinter to midsummer.

Daily watering is only done to lawns, flower beds and raised beds. The other landscape plants can handle watering three times per week or less often.

Q: Folks ask how much water to put on cactuses. I’m left with telling them there is no “Mojave Desert answer” except “not much” and “nothing in the winter.” You dance around this problem in Dr. Devitt’s and your new book, “Selecting and Maintaining Trees for Urban Desert Landscapes: A Mojave Desert Water Conservation Perspective.” I did look. If I were going to make an educated guess, I would put the value at half of the published values for low-water-use plants. Can you help me?

A: Nothing beats looking at the plants. Cactuses use water stored in their stems or trunk. They start to shrivel when they are running out of water but don’t die.

You are right. Native cactuses are not watered very often, probably four times a year. But when they are watered, they have water applied so that it wets all the roots. Plants that get bigger are watered deeper and wider because they have deeper and wider roots.

If your native cactuses are larger, then they should be given the same amount of water but over a larger area. For instance, native bunny ears opuntia from the Sonoran and Mojave deserts can get large — some can get up to 20 feet tall.

If they start getting larger then they need more applied water. Still, the applied water is applied four times a year but more is needed for each application. Add emitters but keep the minutes the same.

The University of Arizona recommends an area to water equal to the spread of their canopy. I think it’s excessive. At least half the area under the canopy is enough when they mature. At first, water them to the size of their canopy but as they approach half their maturity, water only half of that area.

There are cactuses from less hot places that have special needs. For instance, the San Pedro cactus should have some shade in the late afternoon but can handle the low temperatures of our desert.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.