Keeping
Plants Alive -
2010 to 2014
Texas Drought
|
Deep watering
can save your trees.
February, 2012
Update - By now, many of you have received enough rainfall
to ignore
these recommendations. If you are not sure, dig a hole 10" to 12" deep,
but not in a low spot, in your landscape. If the soil is dry at the bottom
of that hole, please read this article and irrigate as suggested.
© Manuel
Flores
|
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Our typical sprinkler systems
are designed to supplement normal rainfall, not replace it. They provide
water for shallow-rooted plants like sod, groundcovers, bedding plants
and certain desert-adapted shrubs. Established trees and shrubs with deeper
root systems survive primarily by tapping deep, subsurface moisture replenished
by our average annual rainfall of 32 inches.
What happens during periods
of insufficient rainfall?
In the case of summer dry
spells lasting a few weeks, mature trees and shrubs withdraw moisture stored
in the subsoil and shed some leaves to compensate for the aridity. During
longer growing-season droughts, the absorptive roots (and their associated
mycorrhizae) first go deeper. However, once they deplete those subterranean
sources, those roots change course and go towards the surface to compete
with the shallow rooted plants for water provided by the weekly "fix" of
sprinkler irrigation.
In our heavy clay soils,
it is estimated that one-inch of rainfall or irrigation will saturate the
top 3-inches of soil. That is sufficient for the naturally shallow-rooted
plants as well as those trained to be shallow-rooted by incorrect irrigation
techniques. It is not sufficient to maintain trees and shrubs, especially
when droughts are prolonged.
A Strong La Niña
The current severe drought
in Texas is one of the results of the strong La Niña event that
began in the latter half of 2010. Climate scientists have recently determined
La Niña conditions have come back to the equatorial Pacific. Their
latest synopsis states, "La Niña conditions have returned and are
expected to gradually strengthen and continue into the Northern Hemisphere
winter 2011-12." This catastrophic news is part of the 8 September 2011
El
Niño/Southern Oscillation (ENSO) Diagnostic Discussion .
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Without intensive intervention,
our stressed landscapes will soon be dead landscapes. Those who follow
the strategies outlined below should have few, if any, woody plants die.
Those ignoring this advice or waiting too long to implement these tasks
will have much replanting to undertake when the rains resume.
At this point, the eventual
severity of this new La Niña is not known. Instead of receiving
20% of normal rainfall, like this year, we could get a bit more if this
La Niña is not as intense. Whichever it is, we can expect less-than-average
rainfall from now until late spring of 2012. The
Three-Month Outlooks, show the best estimates of temperature and rainfall
trends. They indicate our natural and man-made landscapes will be under
moisture stress at least throughout that period.
At best, we can pray for
beneficial rains to occur this fall before the La Niña turns off
the tap. If we do not receive at least 8 to 10-inches this fall, our horticultural
emergency will have dire consequences.
The Root of the Matter
We classify the roots of
perennial woody plants as either providing anchorage or absorption. Though
in most cases, actual absorption of water (and nutrients in solution) occurs
through the agency of mycorrhizal organisms. These beneficial fungi are
intimately attached to the roots they supply and are compensated with the
products of photosynthesis. Thus the soil environment must not just be
appropriate for plants, it must be suitable for the mycorrhizae that forage
on behalf of those plants.
All rain is acidic. The alarmists
who railed against "acid rain" were actually opposed to precipitation that
was more acid than normal. The acidity of rainfall allows soil nutrients
to be available. In our area, with limestone aquifers and lakes created
by damming canyons eroded from limestone plateaus, our irrigation water
is alkaline to very alkaline. From the perspective of chemistry, irrigation
cannot function as surrogate rainfall.
If alkaline irrigation cannot
function as surrogate rainfall, can it be modified chemically? Yes, wholesale
growers of nursery stock routinely inject fertilizers and acids to create
"artificial rain." The injecting equipment (along with the required back-flow
preventer) can be installed for about $1000 to $1500, plus the cost of
the water-soluble nutrients and acids.
Correct Irrigation Method
and Frequency
Once treated, can irrigation
function as surrogate rainfall? No, not unless larger volumes of water
are applied at greater frequencies to break the surface tension of soils,
as well as to exceed the saturation capacity of the clay components of
soils so water can percolate to deeper strata. From the perspective of
soils and their structure, typical irrigation cannot function as surrogate
rainfall.
The best strategy, though
usually prohibited by municipal water monopolies, is to irrigate on two
consecutive mornings, just as shallow-rooted plants are about to show signs
of drought-stress. In other words, irrigation is most beneficial if applied
on the day before a plant would have wilted, not based on some arbitrary
schedule imposed by fiat. Also, each of those two waterings should provide
one-half to three-quarters-inch of water. Use less in deep soils and more
in shallow soils. Following this frequency throughout the growing season
permits us to irrigate about every ten to twelve days. Though with moronic
watering rules in place, we have no choice but to irrigate once weekly
(morning and evening on the designated day) and have been using more water
than necessary. When will bureaucrats and politicians realize their expertise
is limited and unintended consequences are the rule, not the exception?
Under a regime of legislated scarcity and central planning, irrigation
cannot function as surrogate rainfall.
Deep Watering
Whether you out-source your
water needs to municipal water monopolies, or have your own well, you will
have to implement tree survival watering techniques if our fall rains do
not provide at least two-thirds of the current deficit. During mid-October
to mid-November, you will have to replace the subsoil moisture this drought
has depleted. If your water purveyor penalizes you for winter landscape
irrigation, do this before that period starts. If our fall rains are meager
and such irrigation is not completed before even more stringent watering
restrictions are implemented, many native and exotic woody plants will
disappear from your landscape.
Deep watering for trees and
large shrubs is accomplished quickly with a root-feeder having a long probe,
or slowly, by letting a trickle of water flow overnight at the drip line.
The drip line is the extent of a tree’s canopy mapped on the ground. It
is best to apply deep watering at points about 120o apart, around
the tree or shrub. With a root-feeder, irrigate as deeply as the probe
can be inserted until water bubbles up at the surface. Then, turn off the
flow at the handle, withdraw the probe and re-insert at the next spot along
the drip line. If you prefer to let the hose drip all night, ensure about
100 gallons are applied overnight and move the hose to the next location
for the following night. Each sequence of deep watering at three spots
along the drip line constitutes one deep irrigation. Each deep irrigation
episode should be done at three equidistant points that are offset from
those used the previous week. If these deep watering sequences are repeated
weekly for four consecutive weeks this fall, the trees should leaf out
successfully next spring. If the drought continues, repeat the deep watering
regimen during April and May of 2012.
If you are not penalized
for outdoor watering in winter, feel free to deep water your trees more
often, especially broad-leafed evergreens that can suffer dessication injury.
This deep watering is in
addition to normal irrigation. It is done to recharge the subsoil for the
benefit of the large woody elements in your landscape.
Additional Drought Strategies
Bare soil in non-turf areas
is not just the equivalent of issuing an engraved invitation for weeds
to germinate, it also permits rapid loss of soil moisture. Apply a thick
mulch of ground hardwood bark wherever sod does not grow. Since I have
a large and very effective external olfactory apparatus, I never use a
mulch containing sewage sludge.
Thinning trees and shrubs
also helps to reduce evapotranspiration (fancy term for water loss) but
it does stress a plant. But, better a stressed plant than a dead plant.
Before thinning, please observe what is being shaded by said branch between
1:00 PM and 5:00 PM. Often, such observations will keep us from thinning
a drought-stressed shrub and instead, giving it more water.
The following technique will
enrage the yard nazis (Homeowner’s Association) in most gated communities.
It consists of shading young or small plants with a rectangular piece of
horizontally-oriented shade cloth suspended several feet over the foliage.
This is very effective in reducing heat-stress as well as water loss. I
have begonias and ferns alive today because they were shaded while becoming
established.
Another strategy employed
by very few is to replace irrigation requiring plants with drought-tolerant
ones. Alas, most replacements after a severe freeze or killer drought resemble
what had succumbed. We are optimists with blinders – we can’t imagine such
calamities returning soon, if at all.
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