Regional Gardening Reports :: National Gardening Association

In the Garden:
Pacific Northwest
February, 2006
Regional Report

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I love my crocuses! Neither rain, nor sleet, nor snow will stop these robust beauties from poking up their heads in late winter.

Dealing With Soggy Soils

This winter has been the soggiest I can remember in decades. Although we haven't broken any official records, we've come close, and I think we deserve some kind of recognition for enduring endless downpours and gray, cloudy skies for the past four months.

My gardening friends and I share a concern about the long-term effects super-saturated soils will have on our plants. Native plants can be quite resilient, but I suspect we'll be seeing lots of damage to other trees and shrubs in the months to come.

My colleague, Craig Cogger, a soil specialist with Washington State University, explained the dynamics to me. He said the amount of damage will depend upon how deeply rooted our plants are, how sensitive they are to temporary saturation, and how long the soil in the root zone remains soggy.

Soggy Soils Defined
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. Water fills up air pockets in the soil, shutting off the oxygen supply to plant roots and to the microorganisms that live in the soil. The soil eventually becomes anaerobic, a term which means oxygen starvation. This doesn't happen immediately, but after a few days of saturation, most of the oxygen is gone. At this point, plants that need plenty of oxygen for root respiration are going to be stressed, and it's likely they will eventually die.

When the soil becomes anaerobic, the microorganisms that require oxygen will also begin to die, and a different population of anaerobic microorganisms takes over. The processes of decomposition are less efficient though, so organic carbon levels tend to accumulate in soils that are often wet for long periods of time, and this can also affect plants.

What Damage Looks Like
Since damaged plants lack the ability to pick up water because of damaged root systems, one of the first symptoms of damage from soggy soils is wilting. Affected trees and shrubs may also show symptoms of nitrogen deficiency (leaf yellowing). Trees such as dogwood and Bradford pear tend to develop marginal leaf burn or the leaves can turn red or purple. Cessation of growth, twig dieback, and leaf drop are also common symptoms of damaged root systems.

After the soil drains, plants with severely damaged roots may suffer drought stress. For many of these plants, the only functioning roots are near the soil surface, and when dry weather follows a wet period, surface roots quickly dry out. Plants exposed to prior flooding become more susceptible to Phytophthora root rot or collar rot because prolonged exposure to wet conditions promotes susceptibility to this disease.

Alleviating Damage
What can be done to improve the health status of water-damaged trees or shrubs? Unfortunately, not much. Whatever you can do to improve surface drainage will help soggy soils to aerate faster, thus improving chances of survival. Shallow ditching may help, but take care to keep it shallow! Remember, too much digging will lead to further root injury and intensified Phytophthora root rot problems. "Spiking" the root zone (making many small holes with a punch device) to improve aeration is also of questionable benefit for the same reason. Drenching roots with fungicides probably won't help much either. And hold off on fertilizing until the soil dries. Otherwise, this could lead to further root damage.

Preventing Future Damage
Sometimes simple actions can help improve chronically wet soil. First, supply your garden with plenty of organic matter, which opens pores in the soil and allows water to penetrate deeply rather than puddle on top.

For persistently wet areas, plant landscape plants that can tolerate wet conditions. Don't choose plants that need quick drainage for a spot that tends to have standing water in the winter. Douglas fir, for example, is particularly sensitive to wet soil conditions, whereas Ponderosa pine is much more tolerant. Some plants, such as Oregon ash, have special mechanisms to provide oxygen to roots during wet times so they can thrive in soggy soil.

For plants such as fruit trees which must have dry feet, plant in berms, raised beds, or planters. Build the beds high enough to settle and still be above the flood.

If your soil stays wet in the spring, you should delay tilling and planting. Working wet soil can create hard, impermeable clods, and seeds are more likely to rot in cold, soggy soil.

If you've done none of these things and your soil is on a broad, level terrace and contains naturally restrictive clay layers, about all you can do is wait for the rain to stop and the sun to come out.

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