It's that time of year when some of our plants start to struggle a bit as our weather gets hotter and drier. For the Greater Houston area, I still recommend a fairly old-school approach to determine how deeply and frequently to water.
In terms of determining duration, I recommend watering each area once per season (the first year) to the saturation point with whatever form of manual or automatic irrigation you normally use. Log the results in a journal. What you are trying to do is soak the soil in each area of your landscape to the point that water begins to run off to other areas or pool quite a bit. Time how long it takes to reach the supersaturated soil point, then when you water in the future, back the time down by five minutes per section to avoid runoff.
Next, when you are trying to determine how frequently to water there are a couple of considerations.
First, observe your plants and see if they are wilting or not, then you will have a pretty good idea if your individual plants are getting enough water. When plants wilt, at first they are trying to reduce the amount of leaf surface exposed to the sun. But when pronounced wilting occurs, damage has already been done to the plant. If just one or a few plants out of many are wilting continuously in a specific area, it might make sense to move them to other wetter sections of your landscape or to replace them with more drought-resistant alternatives at some point.
Then, get in the habit of periodically checking soil moisture levels a few inches down. The rule of thumb for most soils and plants is to water on average when the soil has about as much moisture content as a well-squeezed sponge. For most plants, if the soil moisture level gets to the bone dry stage, damage has already occurred to the plant.
There are generally two ways to measure moisture:
Low Tech: early in the day, before the sun hits the mulch, stick your finger through the mulch and feel for moisture. If the soil underneath the mulch is damp, you don't need to water; if it is dry, you do. Generally, if it is damp, you should be able to stick your finger in the clay a half-inch or more.
High Tech: The high tech way requires a moisture probe. The probe has a metal spike you stick into the earth and a meter that reads from dry to wet. The advantage of such moisture probes is that appearances are often deceiving. After a long dry spell, our clay may be dry six inches down. A 1-inch rain may wet the top inch, and make the soil look wet, when in reality, the root ball down below is still dry.
Unless you use a probe that can go 4 inches or more down, you are in danger of drowning the plant, wasting water and wasting money; or you are also in danger of not watering enough and losing the plant you worked so hard to cultivate. Moisture probes are often available in nurseries and also online. Probes are essential if you don’t get around to mulching.
Overwatering can be as detrimental to plants as under watering. One major reason not to keep the soil continuously moist is that oxygen in the soil will be displaced by water, creating anaerobic soil conditions that tend to favor disease-causing microbes (pathogens) over disease-fighting beneficial microbes that favor aerobic soils.
A second reason is that overwatering contributes to imbalanced plant growth (usually top growth at the expense of root growth). With most plants if a natural balance between root growth and shoot (top) growth does not occur, then pest, disease and/or inadequate drought resistance problems typically follow at some point.
Over the long haul, it pays to group and zone your plants according to their water requirements. For example if you have sections of your landscape that tend to stay wet, you might want to consider grouping bog-loving plants in that section to create a rain garden. Rain gardens involve grouping plants that enjoy moisture and tolerate poor drainage together in water retentive sections of your landscape. These wet areas could occur naturally, or you could create them to harness rainwater for use in the landscape and to prevent other areas from flooding.
Once you have grouped water needy plants together, moderate water-needy plants together and drought-tolerant plants together (and match them up as well based on light requirements, size, aesthetic considerations and other functional considerations), then it will be easier to water effectively and efficiently.
If you have an existing irrigation system, consider having it reconfigured based on water requirement-oriented plant groupings. This approach can save an enormous amount of money over the long haul, since it will allow you to water each area only as much as needed. By contrast, most people end up determining how much to water based on a few plants per zone (area) that need more water than the other plants. This approach leads to most plants being overwatered and unnecessary water use. It can sound cliché, but with irrigation you will save enormously by watering based on the needs of the many, rather than the needs of a few. Move or replace the few.
Check out the WaterSmart Program's Web site for some excellent information on L.V.I. (low volume irrigation) from Angela Chandler. She will be the instructor at our Low Volume Irrigation class to be held Saturday, Aug. 23, from 9 to 11:30 a.m. at the University of Houston. Register online at Urban Harvest class calendar or call the office at 713-880-5540.
This column is produced by Urban Harvest. Learn about gardening classes, community gardens and orchards, farmers markets and more at the Urban Harvest website. The article was written by Mark Bowen, a former Executive Director of Urban Harvest. Mark is a native Houstonian, a horticulturist and the author of “Habitat Gardening for Houston and Southeast Texas”, “Naturalistic Landscaping for the Gulf Coast and the Bayou Planting Guide”. Tips were also contributed by Bob Randall, Ph.D., who is a cofounder of Urban Harvest and a former Urban Harvest Executive Director. Dr. Randall is the author of "Year Round Vegetables, Fruits and Flowers for Metro-Houston". You may contact him at YearRoundGardening@comcast.net.