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Missouri Environment & Garden


David Trinklein
University of Missouri
Division of Plant Sciences
(573) 882-9631

Wet Weather Woes

David Trinklein
University of Missouri
(573) 882-9631

Published: June 19, 2015

"When you're up to your neck in alligators, it's hard to think about draining the swamp." So goes the wording of an old saying aimed at reminding people that certain dilemmas in life could have been prevented, had we thought about their cause earlier. It bodes well for those of us having plants that are suffering from the very wet spring we have experienced this year.

Photo courtesy of wikipedia.

The first question to address is, “How does too much rainfall damage plants?” The answers lies in the fact that the cells of plants (including their roots) respire just as do the cells of animals. Since oxygen is needed for respiration, plant cells die without adequate oxygen. The oxygen content of the atmosphere is about 21 percent. The oxygen concentration in the soil atmosphere is significantly lower. Should it drop to less than 10 to 12 percent, plant roots suffocate and die.

When excessive rainfall occurs, soil pores that had been filled with air suddenly become filled with water. The latter forces oxygen-laden air from the soil fairly rapidly. The result is a sub-oxidized, or oxygen-deficient, soil atmosphere.

Plants in standing water or sub-oxidized soil first lose their lower roots where the oxygen concentration initially is lower. If the water persists in the soil for long periods, the roots will gradually die upward until only surface roots remain. The greater the number of roots that die, the less likely the plant will survive after the soil finally dries. Fortunately, not many gardens experience standing water for extended periods of time.

When the soil oxygen concentration drops below the above-mentioned critical level, water uptake decreases with an hour’s time. The result is a phenomenon known as “water wilt” where plants show symptoms of wilting even though there is abundant water in the soil. Water wilt is more common among herbaceous plants (e.g. tomato) than on woody plants.

Another problem that tends to develop in wet soils is compaction. Driving equipment on or, to a lesser extent, walking on wet, saturated soil tends to reduce larger inter-particle pore spaces into smaller ones, resulting in soil compaction. The problem is more severe in clay soils than in sandy soils. Compaction not only reduces the amount of air in the soil while it is wet, it will continue (to a lesser extent) the problem after the soil dries.

When the soil warms during the early spring, woody ornamentals begin to develop new roots. When the soil is saturated, these roots become oxygen-starved and die nearly as quickly as they are formed. These “feeder roots” are very important for the well-being of the plant. Thus, trying to establish new trees and shrubs in the landscape when the soil continually is wet becomes a challenge.

Additionally, plants that have lost roots during an extended wet period are ill-prepared to handle the rigors of a typical Missouri summer. If the summer remains cool and moist, few root-related problems would be expected. However, if the weather transitions from cool and wet to hot and dry, the reduced root area cannot keep up with the loss of water via transpiration. Leaf scorch, twig dieback, wilting or even death of the plant may result.

Because of heavy spring rains this year, garden plants should be monitored carefully during the next months. Water them regularly if we should experience dry periods. As a rule, most garden plants require about an inch of water per week during the summer. If this is not received as rainfall, supplemental irrigation should be practiced. When doing the latter, water well but avoid frequent, light watering. The latter tends to discourage plants from developing a deep, penetrating root system.

What about “draining the swamp?”

Other than not locating a garden in a flood plain, little can be done to prevent damage to plants growing in standing water. However, there are things that can be done to minimize the damage to garden plants during extended wet periods of weather. All involve working with the soil.

Plants growing in permeable, well-drained soil are less subject to wet-weather damage than those growing in “tight” soils. Permeable soils retain less of the rainfall they receive because of their large inter-particle pores. Thus, the pore spaces that held air before a rain event soon re-establish their air content, thanks largely to gravity.

Permeability is a function of soil structure. The latter describes the arrangement of soil particles (solids) and the pore spaces between them. When soil particles aggregate, they form “clumps” and have larger pores form between the solids. This promotes rapid infiltration of water and good drainage. Conversely, when soil particles are dispersed, pores become small and are more likely to retain water rather than air. Dispersed soils are notorious for being poorly drained.

Few things are better at building good soil structure than organic matter. As organic matter is broken down by soil microbes, the mineral components of soil (sand, silt and clay) tend to become coated. The latter facilitates the afore-mentioned aggregation, which provide large pores and that allow rainfall to pass through the soil

Soil organic matter is liable, meaning it is constantly undergoing chemical, physical and biological change. For this reason the incorporation of about four inches of well-decomposed organic matter on an annual basis is considered to be a “best management practice” for garden soils.

Gypsum (calcium sulfate) is a naturally-occurring substance that has been used for centuries to improve soil. Gypsum causes soil structure to be more sponge-like, causing water to infiltrate faster through the soil rather than “pond” or run-off. The end result is an increase in the amount of oxygen available to plants roots.

Another technique to cope with wet weather or poorly-drained soils is to plant on berms. A berm simply is a mound of soil with sloping sides. Since berms are sloped, rain is more likely to run off than to be absorbed by the soil. Properly designed, berms also tend to control erosion in gardens that are not level.

In closing, Charles Dudley Warner is credited with the saying, “Everybody talks about the weather, but no one does anything about it.” Evidently, Mr. Warner was not a gardener.

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REVISED: September 28, 2015