How to Water Plants in the Desert

The most critical plant management issue when growing plants in the desert is how much water is enough, when to apply it and where to apply it.

Drip emitters are watering but there are no plants! Water where plants are located and if they are removed, don't forget to stop the water.

Secondary to this issue is how well the soil surrounding the roots hold onto the water or release it when the plants need it. In short, how much water should the soil hold and how much should it let go? (drainage).

How Plants Use Water

Plant roots take water from the soil and send it to the leaves. As plants get bigger, they have more leaves so their need for water increases. Plant roots are always trying to find more water. They may not go looking for water but they can find out where it is! 

Roots don't respect property lines. If trees find water, they grow in the direction.

Plants always try to become bigger. This is why plant roots grow into new areas of the soil where there is water. These new sources of water are tapped into by the growing plant roots. These new sources of water become very important to the plant as it gets bigger. 

When soil fills with water, it drains. The water that drains from the soil leaves voids in the soil that fill with air.

The ideal landscape soil holds on about 50% of the applied water but if there's too much, the soil releases it so the roots can "breathe". This is called drainage. In the ideal landscape soil, about 50% of the applied water escapes the soil as drainage. The end result after drainage is complete is about half of the soil holds on to water while the other half has released it and these places are then occupied by air.

One gallon of applied water to dry soil = 1/2 gallon remains in the soil and 1/2 gallon drains.

An ideal soil has about 25% water and 25% air between its soil particles.

Two Ideas About Watering

 There are two methods floating around about how to water. 

Gas Tank Method. Some advocate filling the soil surrounding the roots with water and letting the plant "drink" from this pool of water until it needs more. Think of it like filling a gas tank on a car. The gas tank is filled and you drive around as much as you want but you refill the tank when it's about half empty. In irrigation lingo, many plants are watered again when about half of the water held by the soil is gone. It's different in cars. Gas tanks are filled with fuel when its about three fourths empty or more. Allowing plants to use three quarters or more of the water in the soil, like a fuel tank, would kill most plants except cacti!

One gallon of applied water to dry soil = 1/2 gallon remains in the soil and 1/2 gallon drains. One quart of this water is used while one quart remains in the soil.

The Daily Sip Method. The second idea about how to water gives "sips" of water to plants every day. People who follow this idea give plants a drink of water every day. These drinks of water are more often in the summertime and less often in the wintertime. Also, these drinks of water are taken more often in the summer months than the winter months. This isn't like filling a gas tank at all. This is more like drinking a glass of water. In the summertime you drink water more often and in the wintertime you drink water less often.

Every day give plants a "sip" until the plant uses one quart.

Pluses and Minuses of Both Ideas

Gas Tank Method. The first idea, "filling the gas tank" lets the plants "drink" from this supply of water any time it wants or needs to. The easiest water for the plant to get is the water closest to the surface of the soil. When water is plentiful in the soil, plants drink from it luxuriously and greedily. Unfortunately, the plant must compete with evaporation of this shallow water into the air. Both the plant and the air are "tugging" at the same shallow source of water.  

 

Water was applied to fast for the soil to hold all of it or the soil couldn't hold anymore.

When the gas tank drops to maybe three quarters full, the plant drinks from it a little bit more, cautiously. It taps into water that is a little deeper in the soil. It's harder for the plant to take water from the soil when the soil holds on to it more tightly. As the available water in the gas tank drops to 50%, the plant finds it even more difficult to get water and tries to find this water in still deeper soil. But the plant is still "happy".

This mesquite tree is growing about 50 feet from a river. Its roots are growing deep because it of soil moisture from the river.

When this water supply in the soil, or gas tank, sinks to about 40% or less of the original amount, then that's when we begin seeing many plants starting to wilt during the hottest part of the day. This is our visual indicator that the plant can't get enough water for its leaves and newest stems. This can lead to permanent wilting; there just isn't enough water in the soil that the plants can find.

Mid day wilting of vegetables on a hot day. They will recover when it gets cooler but growth may be lost today.

A side note. Sometimes plant wilting is not because there isn't enough water available to the roots. It can happen when the air is so hot and dry that it's pulling the water from the leaves faster than the roots can find it. This can happen to vegetable transplants with tops growing too fast for their roots to send water to the leaves. It can also happen to newly planted fruit trees, shrubs and landscape trees. When this happens, it is called temporary wilting. As plant roots become established in the soil, temporary wilting may disappear.

Lettuce growing in plastic and straw mulch in Kosovo.

Another side note. Can you see the value of a surface mulch? Mulch is something applied to the surface of the soil that slows down the loss of water from the surface to the air, i.e., evaporation. When surface mulch is applied such as woodchips, rock or even plastic, evaporation slows down tremendously, improving the odds that plant roots will outcompete for the same water.

 

Plants growing in desert soils may grow better if woodchip mulch is used instead of bare soil or rock mulch.

The Daily Sip Method. The second idea, where daily sips of water are given to plants, requires regular, and sometimes daily inspections of plants. To do a good job using this idea, you should be very familiar with how much water different plants need. Otherwise plants receiving the smallest sips of water will dictate when to apply water to other plants. When these underwatered plants show signs they are not receiving enough water, they force us to apply water to all the plants receiving water at the same time as they do. 

This type of watering can be very wasteful and result in those plants, already receiving enough water, to get more. Hand watering plants not receiving enough water, instead of using the irrigation system, can reduce landscape water use and your water bill. But its time consuming.

A side note. Another drawback to this idea of watering is the encouragement in the growth of shallow roots on plants. Not terribly important with small plants, but when irrigating large trees this type of watering encourages plant roots to grow near the surface of the soil. A major reason for deep roots of trees is to prevent them from blowing over during strong winds. This type of watering encourages shallow roots to form on large trees. The formation of  "sip watering" and the formation of shallow roots leads to the blowing over of large trees.

This mesquite tree, (roots can grow to 200 feet deep!) blew over because it was irrigated with flowers and a lawn which makes shallow roots.

What's the solution?

1. Use the "gas tank method" when watering plants. The size of "gas tanks" depend on the size of plants irrigated. Vegetables, lawns and annual flowers require watering to a depth of 8 to 10 inches. Small and medium-sized shrubs and perennial vegetables should be watered 12 – 18 inches deep. Small and medium-sized trees should be watered 18 – 24 inches deep. Very large trees should have water applied 36 inches deep.

2. Use a soil moisture meter for knowing when to water again. Inexpensive houseplant moisture meters can be used for annual and perennial vegetables, lawns, annual flowers, small shrubs and perhaps even small trees and fruit trees. For larger plants, purchase and use a more expensive soil moisture meter. Measuring the moisture in the soil near the plant roots can give you an idea when the "gas tank" is approaching 50% full and it's time to water.

Moisture meters can be under $10 and others less than $100. For a few uses and shallow depths, choose the $10 version and be preparted for it to be thrown away after a time. They all have a gauge reading from 0 to 10, Dry to Wet.

3. Use a thin metal rod, like a 4 foot long piece of rebar, to determine how many minutes are required for an irrigation to deliver enough water to fill the gas tank. This metal rod can also be used to determine the size of the drip emitters (gallons per hour) needed to deliver that amount of water.

 A stick of rebar can be used for judging how deep water is draining.

4. Water the area under the plant to at least half the area under its canopy. For small plants this might mean using two emitters, medium shrubs four to six emitters, small trees and large shrubs 10 to 12 emitters and so on.

Water the soil under the canopy to AT LEAST half the area under it. This is not enough.

5. Apply mulch to the surface of the soil. For desert plants, rock mulch can be used. For non desert plants consider using woodchips. Vegetables will benefit from plastic mulch to warm the soil and keep it moist.

Rock mulch conserves moisture too. It just doesn't add anything back to the soil like woodchips can.

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