Pine will survive in rock mulch with plenty of water

: I recently moved into a new house with huge pine trees in front and back. I would like to remove the grass in the yard that surrounds the trees and replace it with rock. Will the pine trees be adversely affected by the rock cover?

A: They can handle rock mulch but may suffer when you pull out the grass if you aren't careful. When you remove the grass, you will need to put enough water back into the soil to keep the pine trees healthy. They will need a large amount of water to keep them in good health.

As you have figured out, the pine trees were surviving on the irrigation that the lawn was getting. In fact, the lawn would have to be overwatered to compensate for the water used by the tree. Their roots have spread into the lawn in several locations, probably where the lawn was getting the most water or where water accumulated in low spots.

Tree roots, when growing under a lawn, can spread a distance of two to three times the height of the tree. This includes into your neighbor's yard if the roots are receiving water all the way to the fence line and your neighbor's yard gets plenty of water.

Roots do not go out and seek water but they will grow best and most vigorously in the presence of water, fertilizer and air in the soil. Where these combinations are optimum, the roots will grow the best.

When you design your new landscape, put several shrubs, plants or groundcovers in areas relatively close to the pine tree under its canopy. These plants should be overwatered to somewhat to compensate for the water needed by the tree. I wish I could give you a hard and fast rule about how much water to apply but it will be hit and miss for awhile.

Some of the roots of the pine will die in areas where it no longer receives water while other roots will grow into areas close to drip emitters used for other plants.

Watch the new growth and the density of the pine tree canopies. The first sign of a lack of water will be needles that develop brown tips. You may also see needle drop. Secondly, there will not be as much growth during the next couple of seasons, which appears as a thinning of the trees canopy that allows more light through it.

Thirdly, the real danger sign, branches at the tops of the trees will die back. If this happens in the first season, it may be hit or miss whether the trees will make it or not.

Q: We have a large Canary Island date palm that has been in the ground for 41/2 years. The inside fronds seem to be turning yellow. We discovered that all of the irrigation was placed near the trunk and, as the tree has grown, it has squeezed off most of the lines. The palms were planted in pure sand. We read everything we could find on the tree and discovered that the trunk should not be getting watered. How far from the trunk should we place new irrigation lines? How many emitters should be used?

A: It sounds like you did some good detective work on the palms. Palms like to be near water, but not in it. Think of soil surrounding a desert oasis and where you might see palms growing.

If we look at palms that have grown in the wild here in Nevada, probably from seed distribution by animals like coyotes or birds, they grow best several feet from the water's edge. At this location, the roots have grown into the moist soil surrounding the water and found the right balance between moisture and air. If the soil is kept constantly wet, the plants die.

It is common for landscapers and even landscape architects to plant, or specify, that these palms be placed in pure sand at planting. There is a long history that has evolved into this way of planting, which, by the way I don't agree with.

In my opinion, palms should be planted, as much as possible, in the existing soil but amended with organic material. Palms will do very well in rock mulch and in soils that have low organic matter or low amounts of compost added but the soil must drain water freely.

When watering, the key will be wetting the soil thoroughly but allowing it to drain, but not dry completely between irrigations. The watering frequency would be similar to a fruit tree or other nondesert landscape or shade tree.

The roots are typically shallow so watering does not have to be terribly deep like it should for nonoasis desert trees. A typical watering schedule might be once every 10 days during the winter and increasing to twice to three times a week in midsummer.

The amount of water applied should be reflected by the size of the canopy. Your Canary Island date palm will require significantly more water at each application than the much smaller windmill palm, for instance, but the frequency of watering would be the same because both of their roots are shallow.

I would recommend that you start with about 30 to 40 gallons of water applied per each application. Vary the applications as the season dictates, watering more often as it gets hotter and windier. The emitters can be about 3 feet from the trunk and I would put them in at least four locations around the tree, perhaps more.

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.