Most plants will not survive long-term in water-saturated soil. Roots of plants not intended for wet areas can rot from excessive moisture. Rotted roots mean the plants receive insufficient moisture and minerals, leading to death of the plant. Before setting out landscaping plants like trees, shrubs or flowers, or before planting a vegetable garden, we gardeners need to follow common planting directions. Those planting instructions often include “plant in a well-drained location.” How do you know if the soil drains well in the desired planting location?
What is “well drained”?
To support vegetation, rainfall needs to percolate from the surface, through the soil. Think about a drip coffee maker. The water drips on top of the ground coffee, runs through the grounds and drips into the coffee decanter. If the decanter is not present, the valve on the coffee grounds basket is closed. Water backs up and can overflow the basket. The grounds are completely saturated with no means for additional water to seep through the grounds. For anyone like me who has forgotten to place the decanter under the coffee grounds, we know what the results will be-overflowing coffee because the water is blocked from reaching the decanter.
When rainfall seeps deep into the soil without pooling on the surface, then the soil is well drained. On the other hand, rocks or hard clay may prevent the rainwater from seeping (or percolating) through the soil leading to soil that does not drain well.
Look for the obvious
Water may pool in the yard during a rainstorm. During a heavy down pour, rainwater runs to the lowest location in the landscape. If the water continues to run off after the rain stops, perhaps to a storm drain or lower area of the yard, then the heavy rainstorm may not create a water-saturated location. However, planting shallow-rooted plants, like annual flowers, in an area that pools during heavy rainfall is not advisable. The overly moist soil could cause root rot.
When it is not raining, look for areas in the landscape where plants are currently growing successfully. Odds are that those locations drain well. If in doubt, conduct a drainage test.
Drainage test
Grab a shovel and ruler and head outdoors on a sunny day when it has not rained in at least two days. You will dig a hole to measure how long it takes water to percolate through the soil. Start by digging a hole 12 inches deep and 12 inches across if planting shallow-root plants like flowers or vegetables. Dig the hole 24 inches deep and 12 inches across to test water drainage if planting deep-rooting plants like shrubs or trees.
Fill the hole with water and give it time to drain, moistening the soil in the surrounding area. Fill the hole with water again and check the time on the clock. Return to the hole in 15 minutes, measure how much water has perked through the soil, and then multiply that number by 4. The results tell you the hourly rate of drainage. For example, if the water dropped by 1 inch after 15 minutes, then the water drains at the rate of 4 inches per hour (1 times 4). Cornell University Department of Horticulture’s article on Soil Basics tells us that 1 to 6 inches of drainage per hour is desirable.
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