Green and Growing: Heat up the compost pile to put a freeze on next year’s weeds

It's been a while since the summer days when I watched my grandma pull weeds from the neat rows of her veggie gardens, but I well remember carrying little buckets of upended plants towards a steaming compost pile. The summer air felt slightly warmer when I got close. The compost area had its own scent. And when she once put my hand on the dark stuff, I thought it was on fire.

Grandma knew something about weeds that too many have forgotten: Hot compost is one of the best ways to make good soil amendments from unwanted plants and kill the seeds of next year's weeds besides. Better yet, hot composting does this without making ash, without a trip to the town dump, and without adding methane emissions at a landfill.

According to information published by the Weed Science Society of America, some weed seeds become non-viable after only a few days at 130 degrees. Others require as much as 30 days at 145 degrees. Some species even require a few days at temperatures as high as 160 degrees. While we home composters may not always achieve the ultimate hot operation, we can all create piles that get hot enough to make a difference. Grandma had chickens, and their manure was key to heat generation. Cow and rabbit manure are great, too. But animal manure is not a requirement.

Dawn Pettinelli, an extension educator at the UConn Soil Nutrient Analysis Lab in Storrs, runs the Master Composter program each fall. She said the formula begins with the size of the pile.

"It should be at least four feet by four feet by four feet to maintain a high inner temperature," she said.

Second, it needs to follow a recipe that will cause microbes to get to work.

"We need a mixture of materials with a carbon-to-nitrogen ratio of about 25:1," Petinelli said, referring to what many call the brown-to-green ratio. Unfortunately, many of us stumble at this point—because that ratio refers to the actual content of the materials, which can occur in all sorts of shapes, sizes, and weights.

"Usually we would talk about three or four times as much brown material as green," she said. "But it really depends on the materials' moisture content and C:N ratio. For instance, you could do 1:1 volume with brown leaves to grass clippings. But you would need about one-part leaves to three parts vegetable wastes, if that's what you have available. Or maybe it would be 1:4 if all you were using was leaves with heavy, wet watermelon rinds."

"It often takes some experimentation," she said, adding that compost heat is also affected by moisture, oxygen, and pH. Pettinelli likes the compost calculator at

As for compost vessels, the designs are many.

"The choice of a bin is subject to individuals' needs and means," she said. She refers to a UConn fact sheet that shows ten possible styles.

Compost tumblers have caught my eye recently. Fully enclosed, these are great for batch-style composting. Fill the vessel on Day 1 and close the door. The material is enclosed and out of sight. Batches finish in about six weeks if the owner remembers to turn the tumbler a few times every other day, and ambient temperatures are above 40 degrees. Most compost tumblers sit on frames above groundlevel on a turning spit, so they are not susceptible to squirrels, skunks, raccoons, or mice.

According to Kay Kehler, spokesperson for Mantis, a company that has sold tumblers for 25 years, "The internal dimensions are designed to optimize the heat core and create finished compost in six weeks." Other features include internal fins to increase mixing, plus drainage holes and strategically-placed aeration vents.

Mantis offers one model on wheels. "You can bring it into a shed or garage for the winter," Kehler said, "and keep making clean compost."

Putting aside considerations for style and speed, Dawn Pettinelli offers this wisdom. "The location may be the most important decision. Put it somewhere where you'll remember to use the composter."

I think my grandma would agree.

Here are a few more helpful references:

Kathy Connolly is a writer and speaker from Old Saybrook who specializes in land care and horticulture. View her speaking schedule or contact her through her website.


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