Keeping Roots Out of Your Compost-to-Be (and a Crash Course on Layering)
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Q. Hello Mike: I have a compost bin made out of four wooden skids. I placed two layers of cardboard in the bottom last fall, and filled it with leaves that I had shredded the best I could (three times with a bagging mower). But when I went to empty the compost onto the garden this fall, I had Forsythia roots coming up from the bottom of the pile. These roots are going to interfere with turning the bottom of the pile in spring and summer. What can be done to prevent this? Would placing a mesh weed guard under the cardboard help? It would be difficult to change the location; it's placed behind the garage, out of the wife's sight as much as possible. Anywhere else it would be in plain sight or off the property.
---Bob in Portsmouth NH
A. Forsythia is a funny plant; it can become invasive, but not the in any of the 'normal' ways. Mostly, this beautiful tall rangy plant that produces a giant flush of yellow flowers in the Spring is a 'clumper'; the crown of the plant gets wider over time as it matures, but it doesn't vine and it doesn't send up adventurous roots in the areas around it. Instead it spreads by what's called 'layering' or 'pegging'—any branches that hang low enough to touch the ground will begin to root and form a new plant, much the way that strawberries do with their runners.
Gardeners will often use a variation on this adaption to propagate certain plants, like spirea, climbing roses and, of course, forsythia. They'll clear an area down to bare soil, bend a branch down to the ground and use a hoop-shaped metal pin, a brick, a rock or something else to hold the branch down so it stays in contact with the soil. Water that area a little bit every day and the branch will send out a root. After enough time has gone by, you sever the connection to the mother plant, dig up the new one and plant it in its permanent home. (Note: This specific technique is called 'simple layering' because there are four or five other kinds.)
So one answer is to dig up the area under the bin, get rid of any existing roots and then keep an eye on the 'mother' forsythia that must be very close to the bin. Keep any low-hanging branches well pruned and there shouldn't be any issues. (I keep the bottom foot or so of my forsythia pruned to prevent them from layering their way into the house.) You can also drive some edging into the ground next to the bin just to be safe.
Now, "roots in my compost" is a fairly common complaint; and if the plants in question can survive several feet of material being piled on top of them, cardboard won't stop them. Cardboard works best to prevent 'regular' weeds from sprouting up in freshly tilled soil, like when you build a raised bed. Till it all up, lay cardboard in the bottom, frame it out, fill the frames with compost and/or screened topsoil and the cardboard will smother the baby weeds that are guaranteed to sprout from the seeds you exposed while tilling. I wouldn't expect cardboard to be able to stop something big and woody.
But wood would; a big thick hunk of wood in the bottom should do the trick. But let's not cover the bottom with it entirely. I always like my compost bins to have ground contact, so that earthworms can come and go and help the process along. So I wouldn't use a hunk of wood that covers everything side to side, weed fabric, a tarp or anything else that might exclude them.
I'm thinking about a perfectly-sized wooden board—maybe a couple of boards that you'll join together and cut to size—that you can drop on the ground to cover almost all of the bottom of the bin, but that will allow an inch or two of clearance all around on the sides. (Think of these spaces as bike paths for your earth worms.) A truly solid bottom will also be a bonus if you have a really tough plant trying to get in, like wisteria.
And let's heat this pile up a bit as well. Composting shredded leaves alone makes very good compost; and is, in fact, the perfect imitation of how Nature does this. But it takes a lot longer for those leaves to become Black Gold than if you had mixed in a high Nitrogen source like coffee grounds especially up in New Hampshire, where winter is long and cold and composting slows down, often to a screeching halt. (And where people probably drink a lot of coffee, if only to keep their hands warm.) Mixing in lots of spent coffee grounds as you build the pile will help it heat up faster, and you won't have to wait a year to get finished material.
And finally, I will—ahem—suggest that your wife's visual objection is not to the compost itself but to the sight of four pallets nailed together. Perhaps you'd have more wiggle room with placement if you purchased one or two of those sealed bins made of recycled black plastic. They look pretty spiffy, have a locking lid on the top and a 'coal chute'-like door down at the bottom that makes getting at the finished compost a lot easier. And they're a lot easier on the eyes as well.
Plus the holidays are coming; and who wouldn't want to find a bright shiny new composter under the tree?