The ideal garden soil both holds water and lets it drain away. Many gardens, though (mine included), are built upon shallow topsoils with an impervious layer of subsoil beneath, or at the base of a hill that collects water after every rain. In cases like this it is worth putting some time and energy into providing a way for water to get out of the garden.
There are really only two ways to dry a soggy soil: raise the garden or lower the water table. Building raised beds may be the simplest solution for a small garden—say, up to a thousand square feet. Use whatever materials you have at hand to build retaining walls for the growing beds, then shovel the topsoil from path areas into them, adding whatever amendment (compost, peat moss, rock phosphate, etc.) are necessary along the way. Put down a weed barrier and a thick mulch in the pathway. Try to get the mulch level deep enough so that even in wet weather the surface of the path will be dry; if possible design the paths so that they act as drainage channels to let surface water run off, away from the garden. The retaining walls will probably need to be eight to twelve inches tall; less than that may not solve a serious soggy soil problem.
We have a plot in our outer garden that is in a slough on the saddle of the hill with a hardpan layer about eighteen inches below the surface. After a heavy rain, the whole slough fills up like a subterranean lake, and though you won’t see standing water on the surface, the soil in that whole section is like soup for weeks afterward. Our first attempt to put a garden in that spot involved waiting three years for a drought that dried it out enough to get in with a tractor-mounted rototiller, chop up the sod, and then form the whole section into a series of high beds.
Unfortunately, the amount of water we found we had in that spot was enough to fill the paths up to the surface of the beds, at which point it cut gullies across them and made a beeline for the next saddle down the hill. To solve the problem we bulldozed a small pond on that lower level, and dug a two-feet-deep drainage ditch the length of the saddle, passing right through the center of the low spot and then spilling over the uphill bank of the pond. After a rain the outlet pipe runs like someone had turned on a faucet.
The first step in drainage work is to figure out where the water is coming from, and at what level. Both surface runoff from rain or snowmelt, and subsurface runoff, where water travels horizontally on top of an impenetrable layer of subsoil, can cause problems for your garden.
Surface water that runs across the garden during rainy weather may look awful but is relatively easy to prevent. Simply put in a diversion ditch on the uphill side of the garden, sloping downhill and off to the side, around the edge of the garden. Make sure the water has someplace good to go: not just off your property and onto someone else’s but into a watercourse, drain, or sewer. A simple diversion ditch can be made with a standard round-bladed shovel. Skim back the sod (if any) along the course to which you want to divert the runoff, and flip it temporarily onto the uphill side. Then deepen the ditch from beginning to end so that it drops at least an inch for every ten feet of length. Mound the soil you remove along the downhill side in a low berm, and, when you’re done, take the sod you removed at the beginning and stored uphill, placing it upright on the top of the berm. Soon the sod will have rooted itself in the berm on the downhill side, firming the berm against erosion and giving you a simple, long-term solution to surface water diversion.
For really serious subsoil drainage problems you should call in a professional. But if you just want to protect a relatively small area, and you’re willing to do a little heavy work, here’s how. You can rent a trenching machine if you need to dig a long trench, but short trenches can be easily dug with a special Dutch spade that has a long, narrow blade—only about six inches wide and eighteen inches long. When digging with a trenching spade you should take very small slices with each spadeful, working your way backwards up from the outlet at the bottom, so that any water present in the ditch can escape instead of interfering with your digging. Just rough in the trench on the first pass, then get the precise slope as you work your way back down to the outlet; it should have a slope of roughly half an inch for every ten linear feet, so that the water can flow freely within the pipe you’ll install. Once you feel this is to measure, lay a twelve- or sixteen- foot 2¥4 on edge in the bottom of the trench to even out the irregularities and set a plumber’s level on it. If the bubble is just touching to outside edge of the second line on the glass, you have the correct pitch.
Dig down to the full depth you can reach with the spade. Once the trench is finished it should be five or six inches wide at the bottom and at least eighteen inches deep. If the total length of all your trenches combined is less than forty feet you might as well use the standard white drain tile available from the lumberyard. It comes in ten-foot sections, measures four inches in diameter, and is fairly rigid, with one end flared so that the pieces fit together. When laying rigid drain tile (as it is known) put the flared end downhill so it doesn’t collect soil. Be sure to buy a cap for the top of each run to keep soil out, and a grate for the outlet to keep animals from entering. For runs longer than about fifty feet, so called “elephant tile” is better. It is much thinner and less expensive than the rigid drain tile and easier to work with, though it doesn’t withstand crushing as well and thus can’t be used where the pipe must run under a driveway or road.
You’ll also need some drain fabric, which is a synthetic soil barrier very similar to landscape fabric or weed barrier cloth. For drainage get the eighteen-inch-wide size; you’ll need enough to wrap up the whole length of drain tile. Lay the soil barrier loosely across the top of the trench and either assemble the rigid pipe or unroll the elephant tile along the trench, on top of the cloth. Then wrap the cloth over the pipe or tile, overlapping the excess, and carefully drop the whole assembly into the bottom of the trench where it should fit snugly. Fill the trench back in and you’re done. At its shallowest point under the garden the drainage tile should be at least a foot beneath the surface so that you won’t hit it during spring preparation of the garden.
You can improve the performance of this kind of drain by adding crushed gravel (1/2-inch is a good size) around the pipe. You’ll need wider drain fabric to completely surround the gravel and the pipe, at least three feet across. Line the trench with the fabric, shovel in about six inches of gravel, lay the pipe, then cover it with another six inches of gravel and cover it by folding the excess fabric carefully over the top. When replacing the soil make sure that no gaps are left in the fabric for soil to work its way down into the gravel and clog it.