Q.
We have on our street in Las Vegas pine trees that are over 20 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.
The exact water requirement for pine trees is going to be difficult because water use of pine trees has never been quantified in our desert climate. Even if they have, those amounts are “ballpark amounts” and close approximations. Every site is different. Every year is different.
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| Mature pine tree about 35 to 40 feet tall and 50 to 60 feet wide. The area to water is at least 25 feet in diameter to stabilize the tree and spread the water it needs out. Can a smaller tree do the same job? It may use less water. |
Approximate Water Need
I can approximate.
We do know that as the tree gets larger it needs more water to grow healthy. Its annual water use is probably between 4 to 5 feet (48 in. to 60 in) of
water applied under its canopy and surrounded by bare soil or mulch. This is approximately what it might need when its not surrounded by grass. Grass of mixed landscapes makes it cooler and it uses less water BUT not good for pine roots and its health.
Increase Water Estimates: Plant Need + Salts + Water Lost When Irrigating
If the tree is being watered by city water (because city water, not most well water, has salt in it) then it will
need to be increased by about 20% to compensate for mixing in Colorado River water with it. Then, on top of all that, you must figure in losses due to imperfections in the
irrigation system. If you are using drip irrigation, that water loss will be
about another 5 to 10% beyond what it needs.
Water needed by the plant
+
Las Vegas’ salty Colorado River water (20% more)
+
Water lost by the irrigation system (varies from 5 to 45%)
I will just guestimate what pine trees need. Add another 25% on top of that to compensate for Colorado River salts and another 5 to 10% if using drip irrigation. The water lost each month depends on the weather
so I will give you approximate values. If it is a colder than normal month, the
trees use less. If it is hotter and windier than normal, the trees use more.
Irrigate Following the ET Curve
The
water use will follow a bell-shaped curve like the one below. These amounts relate best to tall fescue lawns.
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| Annual water use of adequately watered tall fescue in Las Vegas, Nevada. (Month 1 = January, 12 is December). Water needed by green and growing tall fescue lawns is about 1/10 of an inch of water each day in both December and January. This adds up to about 75 to 85 inches (7 to 8 feet) of water use per year. Pine trees don’t use as much water as this tall fescue lawn so its water use is about 4 to 5 feet of water (48 to 60 inches) applied under its canopy each year. Water use is in inches of water use per day when the sun is shining and very little wind. The larger the tree, progressively, more water is needed by the tree. As trees get bigger, they need more water. |
Monthly Water Use
January
tall fescue lawns in full sun use about 0.1 inch per day; so pine trees may use about 2 inches in 30
days if applied evenly under the canopy.
February
a tall fescue lawn uses 0.14 inch per day; so pine trees may use about 3 inches
Following
the same reasoning:
March
0.19 inch per day; 4 inches
April
0.28 inch per day; 6 inches
May
0.36 inch per day; 8 inches
June
0.41 inch per day; 9 inches
July
0.40 inch per day; 9 inches
August
0.35 inch per day; 8 inches
September
0.28 inch per day; 6 inches
October
0.22 inch per day; 4 inches
November
0.15 inch per day; 3 inches
December
0.1 inch per day; 2 inches
Watering a Mixed Landscape
First of all don’t. Mixed landscapes of trees and grass are difficult to water…. physically. Second, trees are better off NOT growing in grass. Their roots are better off and their roots will grow deeper. When trees are NOT in a mixed landscape, the trees can get the right amount of water and the water can be applied more easily.
Trees interfere with applying water to grass as well. They get in the way. Whenever possible separate trees and lawns. Trees and lawns should be on separate valves anyway. Water in our area is just going to get saltier and saltier.
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| Salt accumulation from the irrigation water on pine trees on this golf course slowed the growth of this tree on its lower limbs (about halfway up). The hybrid bermudagrass (growing beneath it) was fine. |
Mixed Landscape of Grass and Trees
In a mixed landscape of grass and trees, lawns probably uses about 6 to 7 feet of water (a bit less due to shade and cooler temperatures of grass) on top of the pine water use. It depends on the
temperature, sunlight, wind and humidity it receives but having two plants
receiving water for the same area, although not additive, does make for
higher water use than either plant alone.
 |
| Mixed landscape in southern Nevada. |
Grass in full sunlight uses 7 to 8 feet of water. Pine trees use perhaps 4 to 5 feet more water if pine trees alone are grown in
the same area (grown in bare soil or mulch). Grass is cooler than bare soil in the summer so water use will
go up when it is removed (to maybe 6+ feet) but still not as much as needed by the
grass alone (8 to 9 feet). Water use of plants in mixed landscapes can be like whack-a-mole.
What To Do?
Follow the ET calendar when watering.
Give more water to bigger plants. Taller plants are watered wider, deeper, and less often.