Should You Cover Your Compost

Q. I'm using leaves, grass clippings, coffee grounds, etc. to make compost. Would it be helpful to cover the compost pile with a plastic tarp for the winter? Would that help break down the leaves and grass clippings? (I could water the pile if the cover would keep it too dry.)

----Henry in Ambler, PA

A. OK, listener/readers—you've probably been trapped on this ship long enough to know what I'm going to say before I let loose with an actual answer…

First, I'm hoping that Henry shredded those leaves, because whole leaves can take years to break down. And whole leaves mat together so fiercely that water isn't going to be able to get to the center of the pile from any source—unless maybe you jam a hose down into the middle.

Second would be my double-barreled grass clipping warning:

1) Never include clippings from a lawn that's been treated with chemical herbicides or the resulting compost could kill any plants that aren't lawn grass. (Natural products like pre-emergent corn gluten meal and iron-based broadleaf herbicides are fine; they won't cause compost problems.)

And 2) removing the clippings from your lawn prevents it from receiving a gentle natural feeding every time you mow; grass clippings are the perfect fertilizer for the times in between actual feedings.

I also note that Henry includes the dreaded 'etcetera' in there. I always praise the power of coffee grounds to help shredded leaves cook down quickly, but many people make the mistake of including too much "etcetera' like basic kitchen waste, which can slow things down; or in the worst case scenario, attract dangerous creatures like rats and racoons.

Food waste is best recycled in a worm bin, which is a doubly great idea in areas where winter snow and ice can make it difficult to reach outdoor compost areas. The other good choice is a sealed outdoor bin; there are several lids that keep out really vexing vermin like racoons.

I personally have four; two round and two rectangular. They work equally well, but the round ones are really bulky and difficult to transport. The rectangular ones are a much better choice if you're having them shipped to you (say to give or get as a holiday gift; hint, hint) or have a small car; they're made up of four flat panels and a lid that are really easy to transport and assemble.

And again, these sealed bins work surprisingly well. I just have to make sure I have a big supply of shredded fall leaves nearby to mix in when I add fresh kitchen waste.

Now—did I just answer the question? I said that I personally have four lids—so it's obvious that I cover at least some of my compost…

Ah, but this is a different kettle of fish….eh, shredded leaves. Henry wants to put a soft plastic tarp over his pile, which would smother it in addition to preventing rain from getting through. There's always open space inside the tops of my sealed bins; the lids are never laying right on top of the material.

I did pull a tarp overtop of a big outdoor pile once when I was just a compost beginner, thinking that it would trap heat and help 'cook' the compost—but it just made it a nasty moldy mess. And whatever wasn't moldy in the Spring was bone dry.

So no soft covers that would lay right on top of the material; but you can use a "hard" supported lid if you like. I remember seeing a photo of a winter composting experiment by the great Four Season Farmer Eliot Coleman up in Maine, where he made a giant enclosure out of some salvaged bales of hay and put a big piece of wood overtop to act as a lid and theoretically trap heat. He just lifted the lid to mix in new material; and once or twice a winter, he'd leave the lid off during a snowstorm to keep things moist. That's very similar to how I 'water' my plastic bins; I just leave the lids off to catch a heavy rain once in a while.

So if Henry is composting inside something like a big wire fence, he could put some kind of 'hard lid' on top. And people in areas with excessive rainfall could well benefit from covering their pile during really pounding thunderstorms. But being open to normal amounts of rain and snow helps keep the compost nice and moist, which is key to keeping it cooking along.

…As is good airflow; so whatever you choose to do, don't let a soft cover sit right on top of your compost.