HOW TO WATER
is the most troublesome of all topics for
gardeners. More garden plants are killed
by too little or too much water than by any
other factor. The good news is that a simple
watering strategy is appropriate for almost
everything in our gardens—perennials
and shrubs, trees and even lawns.
WHAT PLANTS
NEED: Plant roots need both water and oxygen,
which can’t be supplied at the same
time. Plants also need to be watered deeply
enough that they can develop good root systems,
not shallow, fragile roots. The best watering
schedule meets these needs by supplying enough
water to soak the soil down to a depth of
eight inches followed by a drying period
which allows air to penetrate the soil, delivering
the oxygen that root cells require. A simple
rule of thumb, ONE INCH OF WATER, ONCE A WEEK, DURING THE GROWING SEASON will typically satisfy these needs.
When we don’t get enough rainfall, we need to water enough to provide the equivalent of an inch of rain to our gardens. Put another way, we need to water until the dry soil has been saturated down to a depth of at least eight inches below the surface. Then we need to let the soil dry down for about a week so that oxygen can get at the plant roots. Remember that when plants don’t have leaves, they don’t have the same water needs. So when plants go dormant, supplying the same amount of water will rot their roots. Conversely, plants that keep their leaves—evergreens—need
water year round, or at least when the ground
is not frozen. That means that we need to
run the hose out to our yews, rhododendrons
and hellebores during February warm-ups and
as late in the fall as freezing temperatures
allow. Also allow for lower water needs during
cool or cloudy weather, and greater needs
on very hot and windy days.
A RAIN GAUGE
is a valuable tool to track rainfall. This
simple plastic measuring unit is available
in well-stocked garden departments. Check
and empty it weekly to monitor how much rain
was actually received; was it just 1/8 of
an inch, barely enough to keep the dust down,
or was it a good drenching rainfall of an
inch or more, sufficient for the plants’ needs for the next week or so? A rain gauge can also be used to calibrate a sprinkler system. It’s
surprising how long an irrigation system
needs to run for that inch: a half hour,
an hour, sometimes even more. Once you know
how long it takes for your sprinkler to deliver
that inch, you can simply set your timer.
Soaker hoses can be calibrated another way:
after the soil has become very dry, run the
soaker hose for a couple of hours, then dig
into the soil and measure how deeply the
water has penetrated. Soaker hoses may require
three, four or more hours to water to a depth
of eight inches.
HOW DO YOU
KNOW if plants are suffering from water stress?
Drought conditions result in wilting and
dry, brown leaf edges. The very perceptive
gardener may notice a “graying” or loss of green to the foliage—a too-dry plant will stop producing chlorophyll. Too much water can also result in wilting—this time because rotting roots no longer do their job of delivering water and nutrients to the leaves. Yellowing foliage is a good indicator that a plant is consistently receiving too much water. Dig into the soil and take a look—is
it consistently dry or always waterlogged?
NEWLY INSTALLED
PLANTS and annuals will need water more frequently
and in somewhat lesser amounts each time
you water. Their limited root systems are
less efficient at obtaining water and don’t
reach far enough. New perennials (and annuals)
may need water two or three times a week,
delivered right to their roots for the first
few months. New trees and shrubs need special
watering for their first two years in their
new locations! Place a hose right over the
root ball and run it at a slow trickle for
20 minutes at a time. Once is enough for
a very small shrub. For larger shrubs or
trees, move the hose to different spots around
the root ball every 20 minutes and water
40 or 60 minutes, about once a week.
© Andrea Green 2005
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