Watering Tips for Garden and Landscape Plants

One of the most misunderstood tasks that a gardener faces is the problem of watering. God only knows the number of plants that I have killed with too much watering or allowed to die with too little watering. Whether you live in a wet or tropical climate or a very dry one, watering can be a challenge. The key is that you have to support plant growth during times when water is critically needed. Water stress during one part of the year can cause severe problems during the rest of the year due to disease susceptibility and/or increased insects.

Water Loss From Soil
Water can be lost from the soil in different ways. If water from rain, snow, sprinklers or hoses permeates the soil beyond the root zone, it is useless to the plant. Water can also evaporate from the surface soil. This will leave it dry. Water from the lower layers in the soil is then drawn upwards to the surface by capillary action. There it evaporates. As it evaporates, more is drawn upward until the water that was quite deep in the soil is gone.

Then there is transpiration. This is the process by which a plant loses water through it's own leaves. This process is necessary for the plant to grow. Consider your trees. A large tree can lose hundreds of gallons of water a day in the summer because of transpiration (think of it as perspiration). So this water has to be replaced and is most commonly done by precipitation or irrigation.

The Relationships of Soil-Water-Air
It is essential for the best growth of all plants to make sure the correct water-air relationships exist in the soil. First you need oxygen in the soil. If you overwater (too much or too often) you'll drown the plant by removing the necessary osygen from the soil pore spaces. This will cause the plant to suffocate and die. You can recognize symptoms of overwatering by: wilting, yellowing, dry foliage, leaf drop and twig dieback. Overwatering is probably the number one killer of plants.

But then, on the other hand, there is the problem of too little water, which does not allow the roots to replace water lost through transpiration. Roots can dry up and die. The top growth begins to show abnormal symptoms. So you can kill a plant by too much water or not enough.

Heavy clay soils (like I have) are much more likely to be overwatered than light soils. But then light sandy soils are drought susceptible and tend to be underwatered. Light soils allow the water to penetrate deeper and quickler, but they dry out more often. Heavy soils are slower to allow penetration but also dry out more slowly.

So what's a good rule of thumb? Well, you want to fill the entire root zone with water and then allow the soil to dry out partially before the next irrigation occurs. Naturally nothing is easy. The amount of drying time depends on the species and size of the plant. You can allow large trees and shrubs to dry several inches down in the soil before you re-water. If you have a small plant or a newbie, you need to water before very much soil has dried out.

So naturally you need to become familiar with how long it takes the root zones of your plants to become completely moistened. Then you need to knw how deep you can allow the soil to dry before your plants show stress and need rewatering. Quick, light sprinkling will NOT do the job of wetting the entire root zone.

Water penetration
What type of soil you have will determine how much water can be held and how quickly it can be irrigated. As an example, 1 inch of water applied to a sandy soil will penetrate 12 inches deep. It will move anywhere from 6-10 inches into good loam soil. In clay soil, however, it will percolate down only 4-5 inches. If you're dealing with a slope, even less as run off can be faster than penetration.

Time required
So you can see that you have to wet heavy, dense soils for a longer period of time to penetrate and wet the entire root zone. A clay-loam may absorb only 1/10 - 3/5 inches of water in an hour whereas a sandy-loam may accept from 1/2-3 inches of water in an hour. A sandy loam might take 4 hours to penetrate 12 inches deep. A clay loam may take as long as 120 hours to penetrate 12 inches.

Organic matter
Having said that, if you have added organic matter to your soil it will behave differently. Clay soils with organic matter added will accept water more quickly. Organically amended sandy soils also hold water longer and don't need to be irrigated as frequently as sandy soils without amendments. This is why its good to add amendments to the soil at the time of planting as it will help the soil retain the water around the root for a longer period of time.

Compaction and thatch
If your soil is overlaid with a thatch accumulation and you apply water too quickly, the water will not soak into the soil. The same is true of compacted soils. For compacted, thatch-choked areas or under the canopy of trees and shrubs, aerate the soil by removing plugs. Add mulch around trees and shrubs to restructure the surface layer and allow better penetration of water. You can also use wetting agents to help water soak through dry organic layers, like thatch. If you're planning a flower or vegetable garden in compacted soil, you should have organic matter added into the top 6-8 inches. This will allow the water to penetrate easier once the garden is established.

Quick Links to July Newsletter
Lighting for House Plants
How to Select House Plants
How to Decorate with House Plants
How to Water Garden and Landscape Plants
How to Water House Plants
Caring for House Plants
Master List of Decorating Tips
July's Newsletter (2005)
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