Sumps, Drains and French Drains

Question from Harry:

I’m glad I found this article. I was thinking of adding french wells: simply using a post hole digger for 3-5 feet and filling with sand in areas where water is accumulating. Would this have the same disastrous effect?

Answer from Pat:

Yes, your plan would have a disastrous effect though not for exactly the same reason. It sounds as if you were planning to build what is technically called a “sump.” (There is no such thing as a “French well.” The correct term is “French drain” and this is totally different from what you describe and it works well. See below for description.)

Basically, a “sump” is a hole in the ground usually filled with gravel (not sand.) Gardeners make sumps under planting holes without knowing that this has long been a failed technology. Out-of-date garden books sometimes recommend this discredited idea and so do some gardeners who don’t know that sumps are a failed technology. Tests have shown sumps never work and actually make drainage far worse since instead of draining water away, they hold water. When you build a sump you are basically creating a buried swimming pool for roots. Plant roots go down to find water. They find the water in the sump and die from root rot. Sumps usually kill plants.

One effective way to increase drainage in a bed that does not have good drainage is to build a raised bed. This method can increase drainage on flat ground. Raised beds do not have to be very tall to be effective. Even 4 inches tall will work if the bed is wide enough. Fill the raised bed with native soil or top soil and dig through the soil at the top into the soil below to plant a tree or shrub. Before filling the raised bed with top soil, always mix some of the fill dirt with the soil below in order to create a “marriage” of soils so you do not create a hard line between two different types of soil.

Another way to create drainage where there is a lack of it is to build a French drain. A French drain is a trench that slopes downhill from the area of poor drainage to a place at a lower level on the property. To build a French drain, first dig the trench making sure the floor of the trench slopes downhill to an exit. Next line the trench with landscape filter cloth, then lay 8 inches to one foot of gravel in the bottom of the trench. Place a drainage pipe with the holes facing downwards or to the sides onto the bottom of the trench. Always place the pipe so that holes are on top or the sides of the pipe or it will eventually clog with earth! Fill the trench with 2 or 3 more feet of gravel. Close up the the landscape cloth over the gravel and then refill the trench with one or two feet of native soil on top. Replace the lawn or other landscaping on top.

When the lack of drainage is caused by the alkalinity clay, then you can increase drainage on clay soil by digging about half a coffee can full of gypsum into the bottom of your planting hole. Gypsum causes a chemical reaction in the soil that breaks up clay particles. However, if you do not live in the West this might not help. Gypsum increases drainage of clay only if the fact that it does not drain is caused by alkalinity. In the eastern USA most soils are acid. In the dry West, most soils are alkaline.

When the lack of drainage is caused by a layer of hardpan buried beneath the topsoil, it is sometimes possible to dig a hole down to the hardpan and then break through the hardpan to a layer of earth lower down that drains. You would then fill the hole through the hardpan with gravel. In this case you are not building a sump but an actual drain that lets irrigation drain out of the planting hole into a lower level of soil that has good drainage.