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Numerous conditions can cause drainage problems in a garden. One cause of poor drainage that I see frequently in clients’ gardens is soil saturation from the interfacing of two different soils. Interfacing occurs when soils with different structures are layered one on top of the other. Water will permeate the top layer but will stop when it meets the next layer. This can leave roots sitting in anaerobic soil where they may, over time, suffocate and die. I often see this in containers, in small planting holes, and sometimes in large planting beds. There are some precautions you can take to avoid these situations. The concept of interfacing is easy to visualize if you think about drainage in containers. Most gardeners throw gravel in the bottoms of their containers to promote drainage. But when water travels through the planting medium and arrives at the gravel an odd thing happens. The water doesn’t whoosh on through the gravel as expected. Instead, it stops and remains in the potting soil and accumulates in the soil from the bottom up. Eventually, the weight of the water and gravity will force the water out of the soil and into the gravel where it will finally drain from the pot. Depending on the type of soil and the amount of water used, this might possibly take some time. To remedy this problem I don’t use gravel or anything else in the bottom of my containers. If the drainage hole is large, I cover it with a piece of window screening to keep the potting media in the container. Now the water only has to get to the bottom of the container and out the drainage hole. I also place containers on decorative feet and place a slightly smaller non-porous saucer beneath to catch excess water. This minimizes deck stains. I don’t let the pot sit in a saucer full of water. This principle can also be applied to other gardening activities like planting perennials, trees, and shrubs. When we dig holes we disrupt the soil, often smoothing out the sides of a hole with our spade as we dig. Research indicates that adding compost or potting soil to a planting hole creates an interfacing of two different kinds of soil. When it rains or when you water, the water will permeate the newly added, more porus layer of soil first, but when it meets the native soil layer it won’t penetrate it very easily and the hole may fill up like a bathtub. To avoid this situation, I backfill the hole only with the removed soil. To improve the existing soil I apply and maintain a 3 - 6 inch layer of compost or arborist’s chips on top of the beds after planting. I do not pile the soil around the plant’s stem. This can slowly rot the stem, eventually killing the plant. This mulch layer discourages weeds and encourages microorganisms and worms that recycle nutrients into the soil. The most difficult situations to deal with are extreme circumstances like new construction where all the topsoil has been removed. Steep sites are the most difficult becaue if you add a layer of topsoil on top of the hardpan and you live in an area with heavy rains, the newly added soil may slide off the slope once saturated becaue the water can’t penetrate the lower layer of soil. This results in a mudslide. There are two possible solutions for steep slopes. First, if your budget allows, build terraces into the hillside. This will not only stabilize the slope, but will make it easier to plant and maintain. Second, you could plant the hillside with material that prefers the soil and climatic conditions of your region. This could include native plants or plants that grow in similar climatic condtions around the world. Having said all that, what I’m about to say may sound like a total contradiction. In cases where the planting area is large and relatively flat or slightly hilly, I amend the entire area to be planted with topsoil and compost (4 parts premium topsoil (not substrata which some call topsoil) to 1 part organic matter) so that there is a minimum of 18 inches of amended soil, then I mulch with a 4 to 6 inch layer of arborist’s chips. So why do I do this after saying that this will result in poor drainage? Because in my experience it has worked. By amending a large area (not just a limited area like a planting hole) water has much more space to be distributed. It will spread out horizontally once it reaches the interfacing point instead of accumulating like in the bathtub of a hole that you dig for a single plant. Secondly, the organic matter will break down and help the native soil. If you have a clay base it will provide a surface area for clay particles to adhere to, thus opening up the soil and creating air pockets where roots can grow. If you have a sandy soil, the organic matter will slowly increase the water retention capability of your soil and you may even want the soil to hold water if your summers are long, hot, and dry. How often will you want to replace the organic matter and mulch? In sandy soils, replace mulch annually, at least. In clay-based soils, the 4-6 inch layer will need refreshing every two to three years, depending upon rate of decomposition from environmental factors. ~ Nancy Tom is an environmental horticulturist who designs, installs and maintains landscapes for Down-to-Earth Gardens, Inc. This article appeared in Fine Gardening, January 2003. ~ Archived
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