



Rain, rain and more rain. Will prolonged, saturated soils hurt your landscape plants?
"Well, yes, no, maybe," said Oregon State University soil scientist Herb Huddleston. "It depends on how long the soil in the root zone stays saturated, how deeply rooted the plants are, how sensitive they are to temporary saturation."
In general, if the upper foot of soil is saturated for no more than a few days at a time, there will be little or no damage to most plants, explained Huddleston, who studies wetland soils in OSU's Department of Crop and Soil Science.
When soil is as saturated as it is on the west side of the Cascades in the winter, Huddleston said there can be two major effects to your garden.
"In the short run, water-filled soil pores shut off the oxygen supply to plant roots and the microflora and fauna that live in the soil," said Huddleston. "In technical terms, we say the soil becomes 'anaerobic.' This doesn't happen immediately. After a few days, however, we can assume that most of the oxygen is gone, and as long as the soil stays saturated, it is replaced very, very slowly by diffusion from the air above the soil. Then plants that need lots of oxygen for root respiration are going to feel the stress, and eventually will die and begin to rot in the soil."
The second major effect of saturated conditions in soil is both chemical and biological.
"When the soil becomes anaerobic, those microorganisms that need oxygen to live begin to die off, and the anaerobic microflora take over," he said. "Anaerobic processes of decomposition are less efficient, however, so organic carbon levels tend to accumulate in soils that are often wet for long periods of time. That's why wet soils often have blacker colors to greater depths than in better drained soils."
What can a home gardener do to minimize the adverse effects of chronically saturated soils?
- Keep your garden soil well supplied with organic matter, which maximizes both the total porosity and the ability of water to flow through the soil, said Huddleston. Organic material promotes the development and maintenance of good soil structure, which helps the soil to drain readily under natural conditions after the rain stops. -Provide some sort of surface drainage with shallow ditches if your garden area has a natural slope. "But if your soil is on a broad, level terrace and contains naturally restrictive clay layers, or is in a slightly concave, depressional area, about all you can do is wait for it to quit raining and the sun to come out," said Huddleston. - Plant landscape plants that tolerate wet conditions in the wetter places on your property. Don't plant things that need really good drainage in spots that tend to have standing water in the winter. "Douglas fir is particularly sensitive to wet soil conditions, whereas Ponderosa pine is much more tolerant. Some plants actually thrive in wet soil conditions, and they often have special mechanisms to provide oxygen to their roots during the time the soil is saturated. Oregon ash is such a plant, as are the multitude of sedges and rushes that occupy true wetlands." - Plant wet-intolerant species in berms, raised beds or planters. For instance, fruit trees don't like "wet feet." If you must plant them in a wet area, plant them in a bed of soil piled above the winter-saturated zone. Build the beds higher than you think they need to be; they will settle. - Use structures and ground covers to prevent erosion from rainfall. Railroad ties, stone walls, and ground covers such as green manures or ivy all help slow runoff and topsoil loss.
By: Carol Savonen
Source: Herb Huddleston