Most gardeners are a few years in when they learn that just like the crops we grow, garden compost is seasonal. Warm summer temperatures invigorate the life forms large and small that decompose organic matter, so autumn is a natural harvest season for garden compost. It's also prime time for building a new heap or filling a bin with the current year's spent plants, so let the composting fun begin!
Turning a Compost Pile
I like to start by turning a pile that's been getting bigger since spring, which takes a couple of sessions behind the digging fork. The heap is comprised of garden weeds, flower heads, pruned twigs, leftover mulches and withered plants, in no particular order, and it's been on its own all summer. Sound familiar?
As I chop and turn, I pull out woody sticks or stumps that are still intact and toss them onto the new heap I'm building nearby (within easy throwing distance), using my current copious supply of dead veggies and their companion weeds. I pause to add water to both heaps as needed to make all the material uniformly moist.
There are treasures to discover. In addition to looking for un-rotted tidbits to send through a second composting cycle, I look for clumps of stuff that look ashen, as if they had been burned, and put them in the new heap too. The ashen appearance is evidence of work done by super-beneficial bacteria that work wonders in warm temperatures, so adding this material to the new heap is better than any activator you can imagine.
Once the turning and moistening is done, I wait a week or two for the old heap to mellow, a mysterious 'finishing' process in which the texture, or crumb of the garden compost improves radically, probably in response to the incorporation of air during turning. It's a transition worth waiting for.
Using Garden Compost
Then comes the good part. I love to spread a handsome blanket of garden compost under autumn broccoli and cabbage, which looks fabulous, nurtures the growing plants and gives the soil a microbial boost. It also keeps the compost at the surface, where weed seeds can be easily gathered by ground beetles and other hungry seed eaters.
Next I move to beds that will be planted with winter cover crops in a few weeks. Here I like to harness the power of crickets as weed seed consumers, which is easy to do by piling chunky compost over the cleared bed and topping the edges of the long pile with boards. Used this way, the same old boards I use to hold down the edges of row cover or shelter summer seedlings attract large numbers of crickets, consumers of 200-plus weed seeds per day this time of year. When I'm ready to sow seeds of winter grains, hairy vetch or another cold-hardy cover crop, a light raking gets the bed in good condition for seeding.
How to Kill Off Weed Seeds
Do I sound preoccupied with weed seeds? In early summer I pull up two laundry baskets full of weeds each day, minimum, and these go straight onto the compost pile. More recently the pile received thousands of tomato seeds, which are already coming to life following warm summer rains. All of these seeds can be neutralised using hot composting methods, which are worth the trouble if you need weed-free compost to use in potting mixes. But the bulk of my garden compost goes onto beds with no heat treatment, and seed eaters are abundant in the fall, so it just makes sense to take advantage of this natural cycle.
A final note. If you have access to manure from organically-raised farmyard animals, this is the best time of year to add it to a new compost pile. The manure will enrich the pile with nitrogen, and be subjected to decomposition processes for more than six months, which is how long it takes for unwanted microorganisms in manure to disappear or fall to miniscule levels.