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As with nearly every
other factor, landscape plants can vary widely in their
requirements for water. Knowing the needs of your
particular plants is important so that you apply water
appropriately. While most plants come under the standard
one inch of water per week throughout the growing
season, some will be over-watered by this amount while
others will be short of water.
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Types of Plants |
One of the first pieces of knowledge that the
prospective home landscape irrigator needs to know
is the amount of water a plant needs or will
tolerate. This amount can vary quite a bit and the
job is especially challenging in beds and borders
where dozens of different species from all over the
world are thrown together.
The following
describes some broad categories that might help you
to figure out how to irrigate your plants. These are
based on plants growing in the soil. Those grown in
containers may be of similar classifications but the
growing media used may make all the difference so
these are addressed in their
own section.
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Can't Tolerate Drought
- These are often plants that originated from the
margin of lakes or streams or other always moist
areas. They are used to having a ready, plentiful
supply of water available to them throughout the
season.
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"Average" Plants - The
vast majority of landscape plants fall into this
category. They need a slightly moist soil most of
the time. They can withstand short periods of
droughty conditions and even short stretches of wet
soils. These are your "standard" plants that need
about 1 inch of water per week in either rain or
irrigation to do their best.
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Can't Tolerate Wet Soils
- Plants that developed on the plains or in or
around desert areas fall into this group.
Mediterranean plants, succulents, cacti, and others
are from areas that receive very little rain per
year. Too much moisture can lead to root rots.
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Soil Types |
Landscape soils are
made up of three inorganic particles -
clay,
silt
and sand. Clay particles are very small and fit
together very tightly leaving almost no space for
openings (pores) to hold water or air vital to
plants. Sand particles are relatively much larger
and allow for large pores that allow the rapid
movement of water. Silt particles are in between the
size of clay and sand.
The fourth part of
most soils is organic matter or humus. This material
adds to the ability of sandy soils to absorb water
and forms clusters of clay particles to allow more
space for drainage to occur. For more on
soil
composition...
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Soil Water Holding
Capacity |
The soil is often compared to a sponge when it comes
to explaining the process of soil water holding
capacity. Here are the descriptive categories for
soil moisture:
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Saturation - When
the sponge (or soil) are completely full of
water and can hold no more, it is said to be
saturated. Such soils have all of their pores
filled with water and there is no oxygen space
available. The roots of most landscape plants
must have oxygen in order to grow and function
properly. That is why
clay soils that tend to be
saturated after heavy rain are difficult
environments for plants.
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Field
Capacity - If you gently squeeze the
sponge, some but not all of the water will
drain out of it. The water that is left is held
by what is called capillary action. In the soil,
this is the amount of water that is the best for
the plants. There will be about 25% of the pore
space filled with water and 25% of the pores
will be full of air. Field capacity is the goal
of all watering and irrigation of landscape
plants.
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Wilting Point - If
you squeeze the sponge as hard as you can,
almost all the water will be lost. When a soil
reaches this level, only the water that is
closely stuck to the soil particles are left.
Plant roots generally cannot use this water so,
they will begin to wilt.
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Goals of Watering or
Irrigation |
Of course, the primary goal of applying water to our
plants is to allow them to grow and thrive to the
best of their ability by meeting their particular
needs. There are two elements that contribute to
meeting this goal:
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Knowing
How Much to Apply
- As discussed in the Types of
Plants listing above, different plants have
differing requirements for water during the
growing season. Therefore, you need to know the
plants that your are growing and what they need.
Most plants need about an inch of water per week
in either ran or irrigation so you can use that
as a starting point. But, also be sure to find
out which of your plants might be either drought
resistant or drought intolerant.
Remember too that the amount applied in your
landscape will depend on your
Soil Type. Depending on
the relative levels of sand, silt, clay and
organic matter in your soil, the amount and
frequency of irrigation may vary. If the plants
need one inch of water and you have heavy, clay
soils, you may need to make two applications of
1/2 inch per week to avoid
saturation. In
sandy soils, perhaps two applications of 1 inch
each per week will be needed to assure that one
inch is actually available to the plants. In a
good "loam" soil with a lot of organic matter,
an application of one inch at one time during
the week might suffice.
Generally, the goal will be to put on enough
water that it soaks down to the root zone of the
plants and will stay there for enough time to be
taken up by the plant. Grass and most perennials
have roots that are clustered in the 4 to 6 or 8
inch depth. Trees usually have most of their
roots in the top 18 to 24 inches of soil.
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Knowing
When to Apply -
For most plants, the soil needs to be moist but
not water logged or bone dry for the roots to
take up the necessary water. To meet this
demand, you really cannot just water on a
calendar schedule.
We have already discussed how water needs may
vary by the type of plant or the soil type in
your garden. These will partially determine how
fast the water moves out of the root zone of the
plant. However, other factors such as the air
temperature, soil temperature, wind velocity and
humidity in the air will also be factors on
determining when to apply water to your plants.
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Watering Techniques |
There are several techniques that can be used in the
home landscape to deliver water to the root zone of
your plants. In the end, it really doesn't matter
which one you use as long as the water gets where
you want it to go. So the decision boils down to
such factors as personal choice, cost, availability,
ease of use or any other factor.
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Measuring the Amount Applied |
There are several ways to measure how much water has
been applied. One method involves a simple measurement
by placing an open container of some sort under the
sprinklers and measure the amount in it after a
period of irrigation. Don't assume that your system
will deliver the same amount of water in an hour as
a friend's system. Your water pressure and types of
hoses may make a difference in delivery rate.
Another way,
especially for drip irrigations systems, is to run
the irrigation for a certain amount of time and then
dig down in the soil to the typical root depth of
your plants. After all the whole goal is to get
water to the roots so measuring the depth of water
penetration over a period of time will tell the
tale.
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