A few weeks ago the local lawnmowing guy put a call out, looking for places to drop grass clippings, to save trips to the landfill. It’s that time of year – a bit of rain, a bit of sun and the lawns are quickly as high as an elephant’s eye.
Yes please, I said. To a gardener, especially on sandy soil, this sort of biomass is pure treasure. Nitrogen-rich grass clippings are great for kick-starting compost heaps, heating them up for a fast breakdown.
The first big pile, about two trailer loads, arrived just before I went away for a four-day trip. By the time I got back it was putrefying, slimy green and smelling of manure. Once grass has got to this stage it can be hard to bring back to usefulness, but aerated well and layered with lime, twiggier hedge trimmings and stalky borage, the clumps should break down into good compost.
Now another trailerload has just been dumped and I’ll be barrowing into the evening to get it off the berm. Left to sit more than a day the clippings go anaerobic, especially in the centre of the pile. (Anaerobic bacteria are the ones that create disgusting smells – a good airy compost heap smells like the forest floor.)
To turn the grass into black gold, rather than slime, add lime and layer it with stalkier material. This creates air pockets and adds the carbon needed to sustain the microbes after the quick-burning nitrogen has been used up. Diversity is the key to making good compost; a bit of grass, a bit of weeds, a bit of seaweed, some hedge clippings and old broccoli plants, sawdust, ponga fronds, manure, leaves…
In three days, the heap is 70 degrees, hot enough to cook an egg. At this point you have two choices. If you turn the heap, aerating it, the heating process begins again. This is fantastic for dealing with bad weeds like wandering jew or kikuyu, and kills most weed seeds, resulting in a sterile compost. Turn compost each week for three weeks and it will be just about ready.
Being a busy person, I usually go the other route and leave heaps unturned. Thermophilic (heat-loving) bacteria die off and others take their place. Left this way, compost is ready in about six months. The disadvantage of `cold composting’ is that weed seeds will remain, but so to will seeds of lettuce, calendula, borage and other self-sowers.
One concern with grass clippings: make sure lawns have not been treated with broadleaf-killing herbicides, as these can persist through the composting process and end up damaging your crops.
- Compost is alive. It’s more like making bread than mixing concrete. Inoculate new heaps with sprinklings from an old compost heap or worm farm to introduce the necessary microbes.
- Compost microbes need air and water to work. Too dry, and the pile will just sit there unchanged so water each layer as you build, and cover the heap to stop it drying out. Not enough air and you get slimy, smelly sludge – so add stalky material and fluff up with a fork if needed.
- Layer fresh green materials (grass, weeds, manure) with drier stuff (stalks, hedge clippings, leaves).
- For hot, quick compost turn it weekly. For cold, slow compost, dampen, cover and ignore for six months.