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.
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| 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).
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!
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!
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| 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.
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.
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| 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.
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.
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| An ideal soil has about 25% water and 25% air between its soil particles. |
Two Ideas About Watering
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!
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.
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.
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.
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| 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”.
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”.
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| 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.
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.
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| 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.
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.
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| 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.
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.
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| 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.
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.
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.
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.
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| 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.
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.
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.
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| 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.
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.
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| 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.
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.
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| 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.
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| Rock mulch conserves moisture too. It just doesn’t add anything back to the soil like woodchips can. |
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