Some plants can tolerate rock mulch

Q: I have moved into a house that is about 3 years old with a wide variety of plants. I have read your articles against rock mulch but I don't know which of my plants can tolerate the rock mulch that is already there and which ones I should pull the rock away from and use wood mulch.

I come from Northern Utah and do not have a clue how to handle these types of plants and all this rock.

A: I am not against rock mulch but it should not be used for everything and not all plants should be planted into rock mulch. My personal philosophy on desert landscapes focuses on using appropriate mulches for plants that can tolerate those mulches.

Generally speaking, those plants that originate from arid or desert environments can handle rock mulches. Those plants that do not originate from arid or desert climates probably should not be planted in rock mulch. There are always exceptions, which never helps when you are trying to make rules.

The problem is that we intermingle desert plants with nondesert plants and then we use rock mulch around everything. Those plants that can tolerate the rock mulch do all right over time.

Those plants that are not true desert plants tend to decline and fail over time. Plants such as bottle brush, photinia, Carolina cherry laurel, Japanese blueberry, butterfly iris, mock orange and others usually begin to decline four to five years after they are planted as the soil amendments used at planting are depleted.

If you want to learn how to handle nondesert plants in rock mulches, please visit my blog and search this topic.

The other reason for decline would be improper watering. Make sure to get your irrigation intervals and the amount that you apply each time to be adequate for the plants. Improper watering will cause plants to look terrible as well.

Q: My problem is with my Japanese blueberry trees. Leaves seem to fall prematurely, browning and yellowish. One of the tree's bark is peeling off and appeared to be dry. There are no visible insects but I do not know what to look for anyway. I have spent so much money on them (and would hate) for them to die. Please help.

A: Japanese blueberries planted in a desert environment is like a square peg in a round hole; it will fit but you have to use a hammer. Japanese blueberries require a soil heavily amended at the time of planting and organic mulch on the soil's surface. It will do terribly in southern or western exposures in full sun or in rock mulches.

If you planted this Japanese blueberry from a 15-gallon container, then it will require about 15 gallons of water each time you irrigate. The amount of water must increase as the plant gets larger from year to year.

This can be accomplished by adding minutes to your existing irrigation schedule at each watering or adding additional emitters. The frequency in the application of water, but not the number of gallons per application, will vary from season to season.

There is generally a schedule for each season, which means you should increase the number of times you irrigate per week about four times each year. These schedules will coincide with about Dec. 1, Feb. 1, May 1, mid-June, mid-September and finally Dec. 1, which completes the seasonal cycle.

Because your plant did not have the fullness that you would like, I would assume it is because of improper irrigation, which also may lead to infestation with borers. Pull off the loose bark you see and look for damage . This would include sawdust under the bark and perhaps elliptical exit holes from the trunk under the damaged area. If the damage is more than halfway around the trunk, then I would replace the plant.

Q: I read the post on your blog on oleanders with interest. I too have an issue. My five oleanders are mature (at least 20 years old). When we moved here a year ago they had not been watered. When the water was turned on and major landscaping installed, many of the leaves turned yellow and dropped. However the trees blossomed and continued to do fine.

During the rains, the gardener turned off the water system and sadly forgot to turn it back on. The water is, of course, back on but I had to replace many bushes, although not the oleanders. However, they have begun to turn yellow again and are dropping leaves. There is still a majority of green leaves and the trees are about to burst into blooms. They have been fertilized, as has everything on the property, but I am baffled. Would you be so kind as to give me your advice on what is happening and why?

A: Oleanders are so extensive worldwide that no one is sure where they originated. Their climate of origin is important because that determines under what conditions these plants perform best. But, we do know quite a bit about how oleanders behave with and without water.

We know that they are very drought tolerant. This means that when there is limited water, they can survive. Many plants can't do this. Normally when drought-tolerant plants first experience a lack of water, the leaves will drop and the canopy will become sparse. If the water continues to be sparse, the leaves they produce will be few and smaller and there will be little new growth. They have to have a sparse canopy to survive.

However, when water is present they have the potential for using a large amount of water and are not low-water users. They respond to this water by growing more, setting new leaves that are quite a bit larger and shedding the old ones. The plant itself becomes denser and flowers profusely .

Leaf drop is very characteristic in response to a drought. It also is characteristic when they receive water again after a drought. It also is characteristic of older growth to shed some leaves on older wood.

So, in a nutshell, going from drought to abundant water you can expect some leaf drop. And when oleander grows normally, expect some leaf drop as the plant matures but not typically as much as during dry/wet periods .

n The University of Nevada, Las Vegas' Research Center barbecue, plant sale and garden talk will get under way at 6:30 p.m. Aug. 23 with the barbecue and cactus sale. Garden talks featured will be "Getting to Know Your Soil" by Dr. Dale Devitt of UNLV and "The Colorado River and the Future of Water" by Dr. Tom Piechota. For directions or more information, call the UNLV biology department at 702-895-3853.

Bob Morris is a horticulture expert living in Las Vegas; he is on special assignment in the Balkh Province, Afghanistan, for the University of California, Davis. Visit his blog at xtremehorticulture.blogspot.com.