Moisture meters don’t indicate how much water is needed

For those of you following my desert landscape design class, I have scheduled a second eight-week, class beginning Saturday, July 7, in the afternoons. There are only a few seats remaining before it will be full. The next landscaping design class will be offered in late October.

Q: I am using an inexpensive soil meter to help judge when to water. I redid my landscape to include cacti, ocotillo, agaves and other types of desert plants. Do I let my meter peg to 1 before watering again or is 2 or 3 OK?

A: I have promoted the use of moisture meters for helping gardeners “get a handle” on when to water plants. For nondesert plants, like many of our trees and shrubs, let the soil moisture drop to about 50 percent to 60 percent before watering again. If the meter is divided into 10 equal units, this would be about “6” on the meter’s scale where “10” is wet and “1” is dry.

Desert plants that are not cacti, such as mesquite, paloverde, acacia and leafy succulents, let the meter drop a little bit lower before watering, about 40 percent to 50 percent or about 5 on the same scale.

Cacti are in a category by themselves. I think you are about right. Let the soil moisture drop to between 2 and 3 before watering again. Because cacti store water inside their modified stems and leaves, they give a visual indicator when their water is running out; the outside shrivels or gets smaller. That is a dead giveaway it’s time to water.

Ocotillo should not be included in the same watering regime as cacti. Water it more often. Treat it more like a paloverde or acacia when watering.

When I am irrigating nondesert plants like fruit trees in a soil that is new to me, I take three readings near the tree in different locations and calculate the average. I don’t let any readings drop below 5, and I will water if their average is about 6. After I get the “rhythm” of seasonal water use, I seldom need a soil moisture meter after that.

The other thing I do is push the probe of the meter into the soil slowly. I want to measure soil moisture near the surface of the soil and watch how it increases as the probe goes deeper in the soil. This technique helps me understand how fast water is leaving the soil in a better grasp of soil moisture.

There are two questions that need answers when watering: when to water and how much to apply. Soil moisture meters help mostly with the “when” to water.

I judge how much water to apply by pushing a thin metal rod, such as ⅜-inch rebar, into the soil. It slips into the soil easily when it’s wet. For trees and shrubs, apply enough water to penetrate 18 to 24 inches deep. For lawns, flowerbeds and vegetable gardens, 12 inches deep. Two-foot tall shrubs, vines and groundcovers, 12 to 18 inches deep.

These are inexpensive meters and not meant to last forever when pushed into our soils. Once you get the feel for when to water, you won’t need it often. Only occasionally when you aren’t sure.

Q: My two lacebark elm trees grew well for the first five years but are now turning yellow and dropping their leaves. They are planted in my lawn plus I planted flowers in the soil around the trees a few years back. It would seem I’ve made several mistakes. Are they worth saving if I’m converting to desert landscaping?

A: Lacebark elm is one of my favorite large trees for our area. They can get big, 40 to 50 feet tall, so I question their use in landscaping for a single-story home. They serve a better purpose in parks and commercial landscapes that have large areas or tall buildings.

These trees have few insect pests, few diseases, survive in lawns, do well in desert landscapes, they are not “trashy” trees, handle our alkaline soils, tolerate poor growing conditions like parking lots and streets. Not only that, it’s a beautiful tree to look at. What is there not to like?

I think the biggest mistake you made with these trees is planting the flowers at their base. Those trees can handle growing in a lawn but not with flowers planted around them.

Flowers require frequent watering. Frequent watering and wet soils near the trunk usually lead to a disease called collar rot. Collar rot attacks the tree crown (where the trunk meets the soil) and causes the trunk to rot. This rotting of the trunk chokes the top of the tree and restricts water to the canopy causing the leaves to turn yellow and drop.

If you want to see if this disease has attacked your tree, gently remove the soil 1 inch deep from around the crown. Take a penknife and scrape the bark away from the trunk in three or four small locations in this area. The wood under the bark should be white. If it’s tan, brown or water-soaked, the tree has collar rot.

A liquid drench around the base of the tree with an agricultural fungicide like Subdue might save the tree during cooler weather. But with this summer heat coming on, I doubt you can save it.

Most large trees do not survive a landscape conversion from lawns to rock anyway. Tree roots become accustomed to lawns and send out roots in the soil in a very different pattern from what they need to survive in a desert landscape.

Making the conversion from lawn to desert landscaping frequently causes large trees to have severe death of tree limbs because of their root pattern in response to lawn irrigations.

Q: Fully mature California pepper and African sumac tree roots have come up in my lawn. I know they are looking for water, but the edging has risen where the trees are growing. Can I cut the roots that have erupted the edging without overly damaging the trees?

A: I am guessing that water for these trees is applied to dry areas surrounding them as well, but these trees are also close to the lawn. Water is abundant in the lawn, and usually fertilizer as well, so this is where the roots have flourished. Roots grow toward the lawn under the edging. As they get older, roots increase in diameter which lifts the edging.

You should have no problem cutting off roots near the grass surface. Consider installing barriers to force their growth under the edging. But make sure the tree has access to plenty of water and fertilizer away from the lawn.

Put drip emitters under the canopy but on the opposite side away from the lawn. Apply fertilizer to these areas as well. Encourage roots to grow away from the lawn.

Generally, trees can lose up to 50 percent of their small roots without serious consequences. Removing large roots can be a different story. Tree roots are also used to support the tree so make sure that removing these roots will not cause it to blow over in a strong wind.

If these are larger roots, cut them with a clean pruning lopper or saw and keep soil away from these open tree wounds for at least 48 hours.

Summer is not a good time to do this job. Add drip emitters and fertilizer to the other side of the tree, away from the lawn, this summer. Wait until the end of September to remove tree roots.

Q: I saw that you were recommended using compost as a fertilizer around plants instead of fertilizer from a bag. Can I apply compost to plants in the summer?

A: Yes, you can, but be careful. Not all compost is the same. The value from most compost is in soil improvement that encourages plant growth. There is some amount of fertilizer in all compost, but some types of compost contain more fertilizer value than others.

Never apply compost so that it is in direct contact with plants. If you are using compost as a fertilizer, apply it in a ring around the plants about 12 inches from their main stem trunk. This distance provides a degree of safety for plants during the heat of the summer.

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

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