“It has been a frustrating spring, with all the rain leaving me with a yard that has been consistently wet and nearly unusable. Water will also pool in a back corner of the yard after heavy or long periods of rain. Is there something I can do to dry my garden out, like adding a dry well to absorb water?”
— Antonio Martinez, Highland Park
A dry well is simply a hole dug in the ground and filled with gravel. This is unlikely to do much to improve drainage, as it will quickly fill with water and continue to pond while the surrounding soil remains too wet.
To correct drainage problems, you need to collect the excess water from the site and then move it out of the area — water flows from the high point to the low point. There are different ways to do this, depending on each site’s conditions.
Installing drain tile can help. The drain should be installed in the lowest area of the bed, and the soil above the drain should not be compacted or heavy clay, which would prevent water from reaching the drain. A very thin layer of clay soil over the drain tile can block water from reaching it.
The pipe must slope evenly from the highest point of the bed to the lowest point, where water can discharge out of the garden to work properly. Use perforated PVC pipe or a flexible black pipe and install No. 8 gravel or ⅝-inch roofing gravel around the pipe to help collect water and help prevent silt from plugging up the pipe.
Perforated PVC pipe will last longer and can be installed with a drain cleanout so a plumber can unclog the piping if it gets filled with silt or tree roots in the future. Pipes that are 4 inches in diameter are typically adequate for most home applications.
A French drain is a trench filled with gravel and will help dry out an area, providing the trench is pitched properly and there is a place for the water to run to. A trench with gravel will not improve the drainage in a garden unless the water runs out of the lowest end of the trench and away from the area. Otherwise, the trench with gravel will fill up with water while the surrounding soil remains too wet.
Another strategy is to modify the grading to improve surface drainage and move water out of the area. This can be very effective when you are able to install a swale or add soil to low areas to direct water out of the garden.
Just adding soil to a low area will not work if the water does not run out of the garden, but you may be able to move water to a garden zone that has less impact on your use of the yard. Grading can be a complicated project, especially if there are shade trees involved.
Grade changes under and around shade trees can have a negative effect on the trees’ health, so consulting with an arborist during the planning process is important.
If the corner of your garden is the lowest part of your garden with grading not an option, then explore adding a drain tile system. Install a catch basin in the lowest spot to collect the surface water.
You would need to pitch the underground drain tile to run water away from the corner and out of the garden. This can be a more complex installation and may require a permit from a local governing body to install.
The best solution for some gardens may be a combination of strategies to improve drainage. If you have catch basins as part of an existing drainage system, be sure to clean debris out of them on a regular basis to keep the drainage system working properly.
Debris will build up in the bottom of the catch basin and eventually plug up the piping. Try installing plants that are adapted to the conditions in that area of your garden if you are unable to improve the drainage.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at firstname.lastname@example.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.