With heavy rains still in our recent memories this is probably a good time to cover a few basics of soil hydraulics. Let’s explain a few concepts and talk about how you can improve things for the plants at your place.
Garden soils consist of solid matter and pore space, and the pore space is made up of air and water. If you were ever to sit in on a soil sciences class you’d probably hear that the ideal soil consists of 50 percent solid matter, 25 percent air space and 25 percent water.
The water table refers to the level to which water rises within the soil profile. In simpler terms, if you were to dig a hole for a post and then walk away for a few hours, to what level would water rise in that hole? That would be the water table. From that point down, the soil’s pore spaces are completely filled with water. There is no air.
Building raised beds means elevating your planting beds above the surrounding grade so that the soil in those beds will never be totally saturated – there will always be some amount of air available to the plants’ roots. Excess water will drain away from the roots. That’s a good thing. No, actually, that’s a great and critical thing.
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Other ways to improve drainage:
French drains are subsurface conveyances installed in low spots where you don’t want water to accumulate. You’ll find many video-illustrated examples online, most of them consisting of digging a trench and installing perforated 4-inch pipe surrounded by gravel and encased in landscape fabric to prevent soil from plugging up the small holes in the drain pipe.
It doesn’t take a lot of “fall” (drop in grade) along the perforated pipe for the French drain to do its job properly. Usually just 1/2- to 1 inch per 10 feet of length will be adequate. However, you will have to have a place for the outflow to dump, either into a grassy ditch or a storm drain. You may need to lay a bed of rock at its end to break the flow and power of the water coming out of it so you don’t have erosion.
Berms can be used to guide the flow of water away from places where it might cause damage. Even low mounds of soil like the raised beds described earlier can accomplish that goal, but be sure as you build them that they’re not going to channel water into a neighbor’s garage or across their back patio.
Swales can be cut to perform the same function. Once again, all it takes is an inch or two of dropped elevation to guide water in a different direction. You’ll need turf or a tenaciously rooted groundcover to keep the soil from washing out in heavy rains, but other than that, swales can do a masterful job of getting water off your property and into the storm water system.
On a personal note, when we built our house near the bottom of a hill years ago, I used an inexpensive line level to tailor a wide, shallow swale around the uphill side of our house. We planted sod and watered it to get it started. It was several months before we had a washing rainstorm. With some fear I grabbed an umbrella and walked out to see how my engineering was doing its job. There was the big blanket of water rolling around the house in wonderful order. My little swale was doing its job perfectly.
Surface drains can be extremely useful in clearing out lots of water in a hurry. We have a heavy cast iron grate built into our Pavestone driveway. It empties all the water from our driveway and courtyard, as well as much of what comes off the roof and puts it into a 4-inch PVC pipe where it’s carried below ground toward a creek on our property. I didn’t install the grate in the driveway, but I did the rest of the work and it has functioned perfectly for 30-plus years.
I also installed hard plastic grates in two places in our gardens. They are in areas where I really didn’t want to cut swales through the rest of the landscape. I was able, once again, to lay 4-inch PVC pipe to outflow locations so that water can drain away quickly. The grates are hardly noticeable within the greenery of the lawngrass and landscape.
Your house’s foundation must also be considered as you’re doing this work. Drainage around it is also important. Slabs tend to dam up the flow of water on slopes, so drains may need to be installed. (You already know about having drip irrigation systems put in place to keep the soil hydrated during droughts.)
At this point the discussion has become fairly technical. We may have passed your own personal comfort level to a place where you’d prefer to call in a professional. There are companies that work full-time in residential drainage. They have the training, the experience and the equipment to do the jobs right. Check their credentials and their references and reports on their work. Odds are that they can make a big difference in drainage at your place.