I have thought a lot over the past week about my prospective rain garden, particularly its impact on my trees.
Paradoxically, as noted here by the Groundwater Foundation, a rain garden is actually dry most of the time. Still, the vegetation in the garden must accommodate periods of soggy soil. And there is no way I can build a rain garden anywhere on my property without confronting some trees.
Perplexed about the conflict between my trees and my ambition, I turned to Rain Gardening in the South, by Helen Kraus and Anne Spafford. Kraus is a professor of horticulture at North Carolina State University, and has practical experience with rain gardens as well, having installed one on her own property. Comforted by her advice that rain gardens can be constructed around trees, if the gardener takes proper care of the trees’ root systems, I am plunging ahead.
Here’s the site, looking northeast-ish. (I admit that there’s not much here at present that could qualify as “winter interest.”) You can see the shadows of the two tall oaks, approximately 50 inches in circumference, in the photo.
This site is approximately 495 square feet. To begin with, it will receive drainage from the house gutters of approximately 475 square feet (rounded up very slightly), but may ultimately receive drainage from 945 square feet of impermeable surface. According to Kraus, the size garden required to capture 1 inch of rain from this quantity of impermeable surface (i.e., roof), is 45 square feet at 6 inches deep, or 90 square feet at 3 inches deep. So I think I have a sufficiently-sized garden.
Kraus goes into depth in her book about how soil texture contributes to the health and success of the garden (more on that in another post). Never one to waste an opportunity to over-engineer a project, I plan to reshape the site to look and act more or less like a bowl, with a central area about 10 ft. by 7 ft dug to a slightly greater depth than the rest. I plan to excavate that 70 sq ft area 12 inches deep, with the surrounding parts of the site excavated to 6-9 inches deep. The total site will be recessed 3 inches below grade of the surrounding soil. And by the way, the site slopes downward to the north, so I will add a small berm on the north side of the site to keep the water from draining away too quickly.
Everyone needs a project, and now I have mine! I am prepared, as always, to have to adjust the plan as I go, but I’m confident I have a terrific learning experience (and workout) in front of me.