garden q&a: compost bins targets for invasion
Q: I have a compost bin under a honey locust tree. It's been there three years. I have wire rabbit fence staked to the bottom so rodents can't get in. I harvest the compost in fall and spring and spread it in different gardens.
This fall, when I went to remove the compost, it was filled with small roots throughout the bin. At first I thought some seeds from the vegetables I put in the compost had sprouted, but there were no green tops anywhere. Did the honey locust discover a food source and send roots up? Do I need to move the composter? If so, do you have pointers on where I should place it?
A: It sounds to me like that is precisely what happened. Most trees have not only large primary roots but also many small, fibrous feeder roots. These feeder roots absorb soil moisture and procure nutrients for the tree's growth and maintenance. Nearly all tree roots are found within the top 3 feet of soil, though some species have a larger, deeper tap root. It is also the job of the roots to exchange gasses and help anchor the plant. Since your compost bin is filled with light, nutrient-rich organic matter, it is no surprise that the feeder roots of your honey locust have found their way into the bin.
Unless you want to tangle with those roots every time you harvest compost (they'll manage to re-grow every year), I suggest you relocate the bin. Place it somewhere well beyond the drip line of any large trees and locate it in full to partial sun if possible.
Another option would be to put a sheet of plastic or canvas over the ground below the bin, thereby providing a growth barrier to the tree's roots. You'll need to make sure that the bin is elevated a little off the plastic to allow it to drain (a few wooden shims should do the trick) and plan on replacing the plastic or canvas every time you empty the bin.
If you decide to go this route, also be aware that it means that none of the microorganisms and ground-dwelling insects that help break down your compost will be able to climb up out of the soil and into the bin contents. That means that as you fill your bin, you'll need to toss in the occasional shovelful of finished compost or garden soil to help introduce those organisms. It isn't a big hassle, but a hassle never-the-less.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. She is the author of several gardening books, including “Grow Organic” and “Good Bug, Bad Bug.” Her website is www.jessicawalliser.com.
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