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Weigh cost of watering large trees against need for shade

Q: We have pine trees on our street that are 20-plus years old and 50 feet tall. We obviously want to ensure their survival as we try to conserve water. Approximately how many gallons of water would each tree require each month?

A: Pine trees filter the light reaching the ground. This light filtering by trees decreases the air and soil temperatures and the need for air conditioning during our hot summers. But big trees do use quite a bit of water. The cost of watering these trees must be balanced with the need for cooling.

The exact water requirement for pine trees is difficult to estimate because the amount of water that pine trees require has never been determined in our desert climate. I can approximate what they need.

Trees are not static in their water use either seasonally or with age. As any tree gets older and larger, it requires more water. A tree uses more water when the weather gets hotter seasonally and windier.

The amount of water a large tree requires should be applied minimally to a depth of 3 feet to at least half the area under a tree’s canopy or it might blow over in a strong wind. Large trees like pines use this stored water until their next irrigation.

In January, pine trees will probably require about 2 inches of water; February, 3 inches; March, 4 inches; April, 6 inches; May, 8 inches; June, 9 inches; July, 9 inches; August, 8 inches; September, 6 inches; October, 4 inches; November, 3 inches; December, 2 inches. The amount of water to apply varies with the weather as well. Over the course of a year, pine trees require about 4 to 5 feet of water applied so that the soil is wet 3 feet deep.

Change the irrigation clock at least quarterly as the need for water increases; more water when it gets hotter and decreasing amounts as it gets cooler. Once the watering depth (minutes needed on the timer) has been established, this seldom changes. To add more water to a plant, it is better to increase the number of emitters as the plant gets larger rather than to increase the minutes on a clock.

Your other alternative is to prune large trees to keep them smaller. Pruning trees so that they are smaller also saves water. This type of pruning is best scheduled during the cooler fall, winter and spring months.

But the irrigation frequency (number of times to water each week) should be changed as it gets warmer or cooler. Increase the days of watering each week as the temperature gets warmer (irrigation frequency) rather than increase the minutes (irrigation depth).

Q: The root systems of our pine trees have been covered with grass. It appears that we must remove lawn grass in accordance with AB356. Will that adversely affect the amount of water used for the trees due to heat and evaporation?

A: When lawns are removed there are two major problems for trees growing in it; tree root depth should be changed so that they grow deeper, and water use should be decreased. Cool-season grasses use about 8 feet (actually 80 to 90 inches) of water every year depending on the weather. Large pine trees may use about 3 to 4 feet less water than a lawn covering the same area.

Water use is seldom additive. If you grow trees in a lawn, their combined water use will not be 80 plus 40 inches. It will be something less than that but how much less must be measured.

Trees grown surrounded by grass do not have deep roots because of the frequent application of water needed by lawns. Tree roots that use this lawn-applied water will be in the top 12 inches of soil.

That must change when lawns are removed. This can be done by training new root growth to grow deeper. This helps stabilize large trees in wind, decreases the frequency of water applications, and increases their tolerance to heat.

Training tree roots to grow deeper takes time, perhaps as much as two years, but it can be done. This is accomplished by decreasing the frequency of applied water (watering days of the week) and increasing the amount of time the trees receive water. Do this during the cooler spring and fall months rather than the hot summer months. Make sure these trees get water distributed thoroughly under their canopies.

The amount of cooling that grass provides has been equated to an air conditioner. One acre of grass equals the cooling of a 70-ton air conditioner.

Grass cools the area due to plant evaporation from the leaf blades called “transpiration.” A lawn’s cooling effect due to grass (growing in full sun at air temperatures of 105 degrees will be about 95 degrees. The temperature of concrete and asphalt in full sun on the same day was 165 degrees. That’s a difference of 70 degrees.

Replacing the grass with a surface mulch, whether rock or wood chips, raises the temperature but not as much as you might think because of the trees’ shading. If trees are removed, then there will be no more shade, and the surface temperature will be anywhere from 160 to 170 F in full sun during the summer.

When selecting a mulch to replace well-watered grass, pick the lightest color you can handle. Light-colored mulch reflects more light (cooler) than dark-colored mulch by about 5 to 6 degrees.

Q: What is the best pomegranate variety to grow? I planted Parfianka and Salavatski varieties of pomegranates. Let’s see how they do this winter.

A: It depends on your preference and their use. Some varieties have a pure red rind (skin of the fruit), others are yellow, and some are nearly black. Some varieties have less bitter juice than others. Some have softer seed that can be swallowed while other varieties have hard seed that is usually not.

The two you mentioned are Russian varieties that came from the warmer “stan” countries that were part of the former Soviet Union.

Winter freezing is another difference. Winter temperature tolerance ranges from about 20 degrees to about 15 degrees. Some of the Russian pomegranates were damaged during some brutally cold winters during some of my trials done from 2006 to 2010 in North Las Vegas.

Parfianka seems, so far, tolerant of our cold winters. What is nice about Parfianka is its red rind (overall red skin) and red arils, the interior juicy part. It resembles Wonderful pomegranate in and out but is sweeter with softer seed.

What is different in the variety Salavatski is its purported winter cold temperature tolerance. It is one of the more cold-tolerant pomegranates marketing experts say. The fruit does not get as deep red as Wonderful or Parfianka, and the juice is tart, similar to juice from Wonderful. I sense that its major advantage is cold tolerance.

Q: This past spring, I planted a small Meyer lemon tree that had four lemons on it. All four lemons are bright yellow now. When is the best time to harvest them?

A: Meyer lemon is usually harvested from December to January. These lemons are round and turn orangish in color when ready. These fruit should be harvested before the tree starts flowering in early spring.

Meyer lemon is not a true lemon such as the varieties Lisbon and Eureka, but a hybrid crossed with some sort of orange in it. That’s why the fruit turns orangish when ripe during cold weather, but the juice tastes like a lemon and why it is classified as a lemon.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.

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