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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
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 .  .

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.

The botanical images on this site were produced by The Photon Hunt.

Thank you for visiting!,
Copyright at Common Law by Manuel Flores