Sulfate damage common in desert soils

The soils in Las Vegas present some very difficult conditions not only for gardeners but also for our homes. Some of the cracking of walls, foundations, corrosion of aluminum or steel and collapse of soils can be attributed to the chemical nature of our desert soils. Natural chemicals in our soils, when they come in contact with water, can cause chemical reactions to occur corroding concrete, steel and aluminum used in our buildings.

High levels of sulfates, found mostly in the form of gypsum, can have very serious consequences for foundations, sidewalks, driveways and patios. Even things such as the bottoms of patio doors can show signs of being eaten away over time. Time can vary depending on the types of building materials used, construction techniques and how wet the area is kept.

The first sign of sulfate damage is the corrosion of concrete or some metals in wet areas. Signs such as pock marking, crumbling, or what appears like dissolving concrete can be some of the first. Even though contractors can use corrosion resistant forms of cement when making the concrete, such as type five or type two modified, time and the sulfates takes its toll even on resistant cement. Basically desert landscaping and limiting the contact of water to corrosive materials is recommended.

For more information on this subject and ways you can deal with it, call our Master Gardeners at 257-5555 and ask for our new fact sheet on corrosion of concrete and ways to deal with it.

Question: Is now a good time to plant oleanders? Are the plants you buy this time of year sufficiently unstressed to plant successfully? How and when should I prune the oleanders?

Answer: Now is the best time to plant most plants, including oleanders. Oleanders are tough plants and will take most stresses without even flinching. Just make sure that when you plant them that they are replanted into the ground at the same depth as they were in the container.

Secondly, put an irrigation well around them so that when you hand water them with a hose during the first couple of weeks the water has a place to stay around the plant and really sink in deep. After that they can survive on whatever irrigation system you have provided them.

Also remember that most dwarf oleanders are not as cold-hardy as their bigger cousins, the standard forms. They may be damaged or killed at temperatures in the low 20s.

The best time to prune them is just before new growth in the spring. Take out one quarter or one third of the plant every two or three years. Select the largest stems and cut them off at ground level. This will cause new shoots to form and fill in at the base and give you a show of flowers from top to bottom, as long as you don't use a hedge shears during the rest of the year.

Q: I have a pear, a peach, an apricot and a fig tree. I am a new resident in Las Vegas and I can't tell the different seasons. There only seems like two. When do I spray the dormant oil spray? When should I give the fertilizer? Can I use the tree spikes? Will the leaves fall?

A: All of the fruit you mention should receive dormant oil three or four times during the months of December and January, up to just before leafing out. Avoid spraying during bloom time. You can actually apply dormant oil after leaves come out but before it gets hot.

I would estimate that you will apply your fertilizer and dormant oils at about the same time, through mid-January. Fertilize lightly again after you harvest if you are using granular fertilizer.

You can use fertilizer stakes if you want. They work well and are convenient if you have just a few trees. Make sure you use enough. They will last several months so you don't need to fertilize again after picking if you use them.

All of the leaves will fall when we get temperatures near or below freezing. Usually this would be typical any time from mid-November to about Dec. 15. It depends on the weather.

It is important to mulch your fruit trees with organic mulch several inches deep and it should extend at least to the drip line or ends of the branches. The fig should have plenty of foliage covering the fruit or you may have lots of fruit drop. I would prune back some of the ends of the branches of the fig facing the south and west to produce more leaves and shade the fruit. This will eliminate the early fruit produced at the beginning of 2005 on those sides pruned, but you should still have fruit produced later on branches produced in 2005. Figs like lots of water and fertilizer.

Q: I have ants in my three fruit trees. I want to get rid of them but I don't want to use an ant poison that might get into the fruit.

A: The ants are going into the tree for a reason. This is probably because of aphids which are common in the cooler parts of the year. They will probably stop going up there when the leaves drop and the aphids hide for the winter.

They will return in the spring when the leaves and aphids return. If you can live with it, I would just ignore them. If you can't then I would trace them back to their nest somewhere in the ground close by and then bait the nest with ant bait. There are several available to pick from that you could use in the soil without spraying or contaminating the fruit trees. This also puts the poison at the nest rather than spraying it or applying it over a large part of your yard.

I can't give you names of chemicals specifically since there are several under several trade names, but the retailer or garden store representative should be able to direct you. It takes a few weeks to wipe out a colony but is more effective than using highly toxic pesticides that only kill the foraging ants, not the colony.

You can use sticky barriers around tree trunks to aid in controlling. Ants patrol trees defending these pests from attack by their natural enemies, in order to maintain their supply of honeydew excreted by the aphids.

Q: How long do I water my new 15-gallon trees and plants before I can cut back to a few days a week? My landscaper says that I have to water them every day for at least two weeks and then cut back.

A: The plants should be watered with a hose the first few times to settle the soil around the root system. Water should be added to the planting hole at the time backfill was added. It will help if there is a basin around the plants to hold the water unless you run the water around the plants very slowly so it sinks into the ground.

You don't really need to water daily for two weeks unless you think the soil is not a good soil. I think the gardener was trying to emphasize the importance of keeping the rootball moist to help reduce transplant shock. Watering frequently will help to remove salts from the soil and is a safety precaution if salts are present.

Right now you could get by watering them two or three times a week, once you really soak them a few times, as long as they are not desert plants. Desert plants can go longer between irrigations. But it is important to water thoroughly and deeply each time, then hold off until the soil begins to dry before the next irrigation. The difference in watering desert versus nondesert plants is not the amount of water applied but how often water is applied.

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