Trees that stand 20-25 feet are good for one-story homes

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Question: Is it OK for me to prune my bougainvillea now? Last year, the nursery told me it was too early.

Answer: You sure can. Any time after our last chance of freezing temperatures would be fine. That is usually around the end of February.

Q: We live in Mesquite and have six mature mulberry trees. They provide excellent shade in the summer, but their disadvantages are starting to outweigh their shade service. How many roots can be cut off without damaging the tree? Can you recommend several good shade trees that do not have the above problems and would do well in Mesquite?

A: In removing roots, it depends on how close you remove them to the trunk. I would not remove more than a couple each year if they are close to the trunk. If further away, then you can remove quite a few, if not all.

By close to the trunk I am meaning that close is anything closer than half the distance from the dripline, or edge of the canopy, to the trunk. Further than this is not much of a problem. I would still do it gradually over a couple of years.

When cutting the roots, let the cut ends "heal over" for at least 48 hours before covering them with soil again. The chemical healing process starts immediately after cutting, and chemical barriers set up in 24 to 48 hours are enough to stop most soil problems from entering the tree providing the tree root is not re-injured.

It sounds as if you want a large tree that grows fast. Most fast-growing trees have problems, as well, and are probably not worth the planting in the long run. Large trees have one major problem, as well -- they use lots of water.

We see very few big trees growing in the desert unless they are near subsurface or surface waters. Adequate shade for people is produced by smaller trees and trees do not have to be big to produce good shade and comfort in a landscape.

If you have a one-story home, I would recommend that you focus on trees that stay in the 20- to 25-foot height range for shade.

Size is more important in water consumption than the type of tree selected. If your concern is to shade the house to reduce energy use in the summer, it is more important to shade the south and west walls than it is the roof of the house. The walls typically are more poorly insulated than the roof and shading the walls will be more effective in reducing energy use than shading the roof. This has been demonstrated in research at the University of Arizona.

There are alternative ways to trees in providing shade in the landscape. These are things such as patio covers and arbors and trellises that are covered in deciduous vines. They will use less water and get shade for you much faster than trees.

I am not against the planting of trees, but in the desert trees should be used conservatively as our desert here demonstrates for us if we are to conserve water.

Just remember, when selecting, big trees use more water than little trees. Select deciduous plants if you want warming from the winter sun. Evergreen plants are great for windbreaks and to provide visual barriers.

Q: My lemon tree is covered with aphids. What is the best treatment?

A: For aphids, try soap and water first or you can buy insecticidal soap and use that. That will only give you control for a couple of days until they repopulate. There also might be an ant problem associated with the aphids, so try to get them under control, as well, since the ants contribute to the aphid problem.

Next, I would try dormant oil if the soap control is inadequate. Dormant oils must be sprayed directly on the insects for control. The oils smother the insects, such as aphids and mites. You can use oils when the leaves are out and at temperatures up to about 90 degrees Fahrenheit. Check your label. Although I have used oils on days when temperatures were higher, I applied it early in the morning.

Lastly, try Neem oil if the dormant oil is unsatisfactory. All of these are organic methods or what we term "soft" pesticides.

Q: I have hybrid bermudagrass and it is doing badly under a large mulberry tree. Do you know of a grass seed I could seed there that would grow in the shade?

A: Hybrid bermudagrass is what we call a warm season grass and it goes dormant, or brown, in the winter here. For that reason, some people elect to overseed bermudagrass in the fall with a cool season grass like perennial ryegrass to give the lawn some winter color. Others do not overseed, but prefer the water-efficient look of a brown, dormant lawn.

Bermudagrass is the only warm season grass that will tolerate fall overseeding. Bermudagrasses are not shade tolerant at all and will not survive in shaded areas for very long. It is possible to grow perennial ryegrass such as Palmer or Prelude in a mixed stand with bermudagrass, and they have some shade tolerance and will out-compete bermudagrass in the shaded areas, but they need at least 50 percent sunlight to survive.

There are alternative warm season grasses that have some shade tolerance that you could use, such as St. Augustine grass and zoysiagrass. These have been used in a mixed stand of grass with bermudagrass to at least allow for some warm season grass to survive in semi-shaded spots. But if you are a hybrid bermudagrass fan due to its fine texture and manicured look, then you will not like the coarse look of St. Augustinegrass.

The shade from a mulberry will be too dense to allow the growing of lawn grasses under it unless the canopy is thinned probably annually to allow sun to penetrate to the lawn. If you want a green look under mulberry, then it would probably be best to select something other than a lawn grass in that area.

Q: This is an odd question. We have a new lemon tree in our back yard that is producing lemons. However, we are afraid to eat them because our dog urinates on this tree daily. Are the lemons safe to eat?

A: It is a bit strange but still a good question. Any time we live in dense urban locations where there is not as much room as in rural areas we generate unusual problems we have not been faced with before.

Dog urine is similar to any manure source as long as the fruit does not come in direct contact with the urine, such as by falling on the ground. Actually, even if the lemons fell and you washed the fruit thoroughly, you would be fine.

I would be more concerned with pesticides applied to the tree, which probably poses more of a potential health problem than dog urine. Pesticides are such a large part of our lives now we don't give them enough thought sometimes.

The tree cannot pick up anything unusual from dog urine and transfer it inside the fruit. The soil microorganisms break down the urea in the dog urine into nitrates which the plant picks up as a fertilizer source. The nitrates from urea are no different than the nitrates from a fertilizer. The plant cannot distinguish between the two.

Other elements in dog urine are handled quite nicely by the soil, which acts as a great filtering and buffering agent to protect us. Straight urea in urine is "hot" and can burn green tissue if it comes in direct contact with it.

Bob Morris is a horticulture specialist with the University of Nevada Cooperative Extension.