Select trees for home’s scale, plant with purpose

A: You might try contacting the Las Vegas Valley Water District or the Southern Nevada Water Authority. I know they have recommended plant lists for conserving water. These lists are made from educated “guesses.”

At the University of Nevada Cooperative Extension we have tried to stay away from recommended plant lists because there is very little research that has documented how much water ornamental plants actually use. In our research, we demonstrated that two mature Southern live oak trees can use as much water as an 1,800-square-foot Bermuda grass lawn. That could be equivalent to about a 1,200 to 1,400-square-foot fescue lawn.

When selecting trees for water conservation, decisions should be based upon two criteria: the mature size of the tree and its place of origin.

Large trees are never water conserving even though some may be regarded as moderate in their water use. In all cases, bigger trees use more water than little trees.

Next, consider if the trees originate from dry climates, but note that not all trees originating from dry climates are low in water use. Take the mesquite, for instance. Mesquite is a tree considered to be a phraetophyte, or a tree that grows abundantly when water is present but has the capacity to use less water when it needs to. Usually you find them in deserts or dry locations growing in or near washes or arroyos. After decades of growth, desert mesquites are typically small with very little new growth.

In the landscape, we usually give them abundant water. They respond by growing quickly and get quite large in just a few years. However, when mesquite trees can’t get the water they would like, they begin to look ratty. They drop their leaves, lose the shade cover they were valued for, don’t grow as much and may even have branch dieback under extreme drought. It can be a vicious circle because as trees get bigger, they demand more water to continue looking good.

When selecting trees, keep their mature height in proportion to the home. If you have a one-story home, your trees should have a mature height around 20-25 feet maximum. For a two-story home, you may want some trees for shading and they should reach perhaps 40 feet maximum. They should be placed in the landscape where they will be valued. If it is for shade, then place them where the shade will do the most good.

The fewer landscape trees and large shrubs on the property the lower your water use will be as these plants mature. So trees and shrubs should be selected and planted judiciously. In the desert where water is precious, each plant should have a purpose in the landscape.

Deciduous trees should ultimately be used to create shade for outside living areas and cast shade on the south and west walls of buildings to help reduce energy costs.

Another important point, but one that is seldom talked about, is the compressor on the air-conditioning unit; it should receive shade as well.

Q: I am sending you a picture of my African sumac tree that is about three years old. It has not grown well and, last week, I decided to try to stake it better. When the tree holders (stakes) were taken off, the tree just bent over. It is not strong at all. So, the tree was restaked and secured really well. I have no idea what is causing it to be so limp and so small three years after it was planted. Do you think the tree is getting enough water? It is being supplied by a small drip system.

A: Thanks for the picture. (Readers getting my newsletter will see your picture.) The problem started when this tree was growing in production nurseries and continues with staking it in the landscape. Let me explain.

Production nurseries are where trees are propagated and grown until they are large enough for sale. They push the trees’ growth to their maximum and remove lower limbs early in their lives to produce the canopies and for easier maintenance. They stake the trees so the trunks cannot move.

The worst things you can do to a tree are stake it so that the trunk cannot move and remove small shoots along the trunk.

The result is a trunk that has nearly no taper to it. Taper is the difference in the trunk diameter when measuring between the bottom of the trunk and the trunk at the lowest limbs. When you look at a trunk of a properly grown tree, the trunk diameter should be much larger at the base than just below the lower limbs.

When a tree trunk has good taper, it will stand upright on its own and is much less likely to snap in high winds. When a tree is grown so that the trunk produces almost no taper, it cannot stand on its own and does exactly what you describe.

So, now that you have purchased this tree , what do you do?

There are two things that will add taper to a tree trunk: movement or swaying of the tree back and forth and the presence of small stems all along the trunk.

The first thing you’ll want to do is reduce the volume and weight of the canopy, which lessens the stress on the trunk and decreases the “sail” effect due to wind. To reduce the canopy, remove entire limbs back to their source. You do not want to just simply sheer the canopy back. That will create all sorts of problems later on. I would remove about 50 percent of the canopy’s volume.

Secondly, I would try to stake the tree as low on the trunk as I could so that it can sway but not snap in a wind. As the tree trunk gains taper, move the staking lower on the trunk.

Thirdly, if any shoots grow from the trunk, do not remove them until they reach the diameter of a pencil.

Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.