Sunday, March 7, 2010

Improving drainage

Poor drainage is not an uncommon problem for the home gardener. If, during rainy weather, the water stands on the surface or drains very slowly, you may have to deal with the issue of soil porosity. Wet sites may be caused by poor soil, an unfortunate topography, or compaction. Whatever the cause, they result in an unhealthy soil ecosystem and may encourage fungal root diseases.

Poor drainage sometimes begins when topsoil is removed during construction, leaving only subsoil. At the same time, the soil is compacted by heavy equipment which reduces air space. Poor drainage can also be due to location of the site at the base of a slope; a failure to properly direct runoff water from roofs and downspouts, or improper grading of the site, leading to water standing in low areas.

If drainage is poor it might be worth it to do a percolation test. Check several locations in the landscape. If you decide action is necessary, several options are available to improve drainage.

I have nice soil, but it is a little too rich and moisture retentive for certain choice plants I like to grow. When I plant a Lewisia, for example, I dig the hole twice as deep as I need and fill with a gravel/soil mix. This seems to allow the area around the roots of this particular plant to drain faster. This is sort of a mini-dry well: a procedure that is used on a larger scale to plant trees in wet soils.

To handle a more pervasive drainage Picture 016 problem, you might create raised beds. Elevate the site by adding 10-12 inches of compost and topsoil and work it fully into the bed. A modified version of this will even work on the lawn. Because we had a problem with standing water on areas of our lawn, last summer we bermed the yard up about 8”, tapering off at each side to the previous ground level. This winter, we have had no standing water.

It is not recommended to add an organic amendment to an individual planting hole. If the structure and texture of the soil in the hole is different from that of the soil surrounding it, the roots tend to stay within the confines of the hole and do not penetrate into the native soil. Instead, the compost should be incorporated uniformly throughout the topsoil in the projected root zone. If you add 3 inches of compost to the soil surface and work it in to a 12-inch depth you would have a 25 percent increase in organic matter. Avoid using more than 50 percent organic matter, because plant problems may result from humic acids and other organic compounds (Yeah, we talked about that already…)

If you prefer an engineering approach, you can lay French drains. To totally over simplify: you determine an appropriate place for the water to drain and dig a trench to this area from the garden. The trench should be deep P5070012enough to lay perforated drain pipe below the frost line. Line the trench with several inches of gravel and rake smooth.  Place the pipe on the gravel; add enough gravel to cover the pipe, then fill up to grade with topsoil. You may want to camouflage the end of the pipe, but don’t seal it.

Another approach is to adapt to your site rather than try to change it. Standing water may reveal the perfect area for a pond-or you can opt for a bog garden instead. You can select plants that are not deterred by soggy ground. There are species of trees, shrubs and perennials that will be happy to claim your “wetland” as their home.


  1. Very informative and useful. Will help solve lots of problems. Thank you

  2. I love that you added 8" of dirt for your lawn! I am going to suggest this to other people I know with soil compaction issues. :)