How to Reduce Water Use in a Condominium HOA; Part 2.

 In How to Reduce Water Use in a Condominium HOA; Part 1 you determined what plant water use category you are in; 

Landscape water use categories

0-2 feet of water covering the irrigated landscaped area (nice job!)

3-4 feet of water covering the irrigated landscaped area (not bad)

5-6 feet of water covering the irrigated landscaped area (too much water used, scale back!)

Cranking the Water Use Down

If you are not happy with your landscape water use, or want to save even more water than you have, first consider eliminating trees above the roofline of the condos or at least pruning them lower. Are large trees important? Yes they are but perhaps not nearly as important as the cost of the water they need to survive.

These pine trees are planted too close but the shade they produce can't be denied. As these pine trees get larger, the cost of watering them may be worth more than the shade they produce. It will be up to you but consider removing them after replacement trees have been established.

Big Trees Use More Water than Little Trees

It may sound comical but people buy trees that grow 50 feet tall and only need a tree that is 15 feet tall (single story homes). Examples are pine trees. Sure pine trees may not use as much water as a mulberry but it still uses water. The bigger it grows, the more water it needs. Plant or keep trees that shade the walls and windows on the south and west sides of a building.

This hot, west facing exterior wall has four small shrubs that create no shade on the wall but still require water. Their distance apart (about 10 feet) would provide the water needed to plant and locate three small trees or shrubs (ten feet tall) that would shade this hot wall. As the plants we chose to replace them got larger, more water should be added. Add one more small tree (patio tree) to provide shade for the patio area walls and door.

Selecting "Backbone" Trees and Large Shrubs

Selection of "backbone" trees and shrubs should be hardy, deciduous, and survive temperatures to at least 20F during the winter. What are backbone trees and shrubs? These are trees and shrubs you don't want to lose to winter freezes. The trees and shrubs should be deciduous because we want the sun to warm this condo in mid-winter. If you want to save even more water, use small deciduous desert (xeric) trees and water them separately from the more frequently watered mesic plants. 

Landscape "Negative Space"

The fewer plants used and the smaller they are will, for the most part, determine how much water your landscape uses. Deserts use less water by NOT growing plants. I call not planting.... "negative space". Where are plants needed? Where are plants NOT needed? Leaves these spaces open. Creative use of negative space is challenging. In the wetter, eastern US lawns were used to occupy negative space. In the desert we must be more creative; boulders, changes in rock size (texture), changes in elevation, artwork, wall paintings...all can be important when not using water.

Dry washes can use textural changes along with plants. Rocks don't use water.

Open spaces don't use any water and the plants used were desert in origin (xeric) so they aren't watered very often. I would substitute small trees near the windows.

Textural change in the mulch and wall paintings add color and interest to an otherwise somber area and don't use any water. What is the water use here?

Planting in Desert Soil

Locations of ten foot tall trees or shrubs should be 4 to 5 feet from the home. Taller trees and shrubs can be planted further away. 

Soil amendments should be lightly mixed with the soil at planting time along with water. 

Planting holes should be wide and not necessarily deep unless there is a clear drainage problem. If a slight drainage problem exists, plant on a mound of amended soil rather than digging deep.

Water should be applied away from the foundation of the home. In the desert, plant roots follow where water is applied.

Trees and shrubs should be fertilized once in the spring. Two times at most. Showy plants are fertilized three or four times a year and when they flower best.

Trees use water. Bigger trees use more water. Lots of big trees use lots of water

How Much Water Do Pine Trees Use?

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.

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.

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.

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?

Use fewer and smaller plants to do a similar job.

Follow the ET calendar when watering.

Give more water to bigger plants. Taller plants are watered wider, deeper, and less often.

Estimating How Much Water Landscapes Use

 When you estimate a plant’s water use, the plant must be growing all by itself. When plants are grown together the roots intermingle, their branches might intermingle creating shade and wind diversions, not only that but its water use is confused with any neighboring plants close by! When you estimate a landscape water use, use the total sum of all your plants. It will be a little high because of influences by plants nearby. This can be estimated by someone who knows plants and their water use. For a traditionally sized family of four, landscape water accounts for about 70% of your water bill.

When recording the water use of plants, they must be separated from other plants such as using this weighing lysimeter and hoisting the individual plants.

How many square feet is your total landscape? Your landscape size is your lot size minus the house, patio, driveway, sidewalks, and any other hardscape that would be difficult to remove. The problem is your monthly water bill comes, at best, in gallons of water. Landscapes vary in size. The size of your landscape is in square feet.

Water bills, like this one from Henderson, NV, lists the water use of an entire home lot. You must multiply this by  approximately 0.7 to get the actual water use of a landscape.

You must convert the gallons of water used by your landscape to the size of your landscape in square feet. The multiplier you needed is to convert a landscape from cubic feet to gallons. The magic number that does that is multiplying the square footage by 7.8. That is, 7.8 gallons fits into a one cubic foot spot. Two cubic feet contains (7.8 gallons x 2 cubic feet) 15.6 gallons for every 2 cubic feet.

The front landscape size is calculate from the total landscaped area, not including the driveway, sidewalk, or any so-called hardscaped area.

Any time your annual gallonage represents less than two feet of water covering your entire landscape, you are doing a very good job! That is less than two feet of water needed to water your landscape each year! When the landscape gallon totals less than 4 feet deep, you are doing an acceptable job. Six feet or more is unacceptable for desert landscapes.

For instance, let’s say your landscape area totals 2000 square feet. 

This size includes every possible spot a plant can be planted. Two feet of water covering this landscape area = 2000 x 2 x 7.8 = 31,200 gallons of irrigation water per year. That is very good.

Four feet of water covering your total landscape area = 2000 x 4 x 7.8 = 62,400 gallons of irrigation water per year. That is acceptable.

Six feet of water or more covering the total landscape area (2000 x 6 x 7.8 = 93,600 gallons of irrigation water per year) is unacceptable.