Composting weeds

Some weeds have roots, seeds or bulbs that can survive traditional home composting and will keep growing even from small fragments. These nasties need some pre-treatment to make sure they don’t spread from your compost heap.

Common weeds

	Creeping Buttercup: In damp/compacted areas. Grows from root fragments.
	Tradescantia/Wandering Willie: Smothers all other seedlings. Grows from root or plant fragments.
	Climbing Dock: Kumara-like tuber. Spreads from seeds and tubers.
	Broad-Leaved Dock: Deep tap root. Grows from root fragments.
	Oxalis: White, purple and yellow flowered versions. Grows from clusters of small bulbs.

Photos: Trevor James and Carolyn Lewis

Weeding technique

Separation

As you weed, keep two containers alongside, and separate the weeds as you go. ‘Harmless’ soft weeds (annual weeds without seeds) can go straight into the compost heap or be used directly as mulch – while the ‘nasties’ go into another bucket, bag or barrow.

Lifting weeds

Lifting weeds out of the soil with a garden fork or by getting your hands underneath the roots works better than pulling hard from the top, which can break the root/stem and leave material underground to resprout.

Hot composting

Hot compost can reach 70°C, killing weed bulbs, seeds and roots. The trick is to make a large heap in one go, with plenty of nitrogens/greens like grass clippings, and keep it aerated.

1. Shred material finely with a mulcher or lawnmower.
2. Fill your compost bin or make a heap at least 1m³ big, all in one go, with well-mixed, finely chopped brown and green material. Moisten if needed. Aerobic (air-loving) microbes break down the material, starting at the centre of the heap.
3. After three days, the heap runs out of air and starts to cool. Lift the plastic bin off to one side and fork the material back into it; or turn piles, swapping the outsides to the centre and vice versa.
4. Repeat turning twice more, on days six and nine.
5. Then leave the pile for three weeks and you should have ready-to-use, weed-free compost

Treatment methods

Use...	for...
Drying: In hot sun, some weeds can be killed by drying out on a sheet of corrugated iron or a concrete slab.	weeds that grow from roots, runners and plant fragments, e.g. Kikuyu, Couch Grass and Tradescantia; avoid weeds with seeds as this method won’t kill them.
Drowning: Put weeds into a rubbish bin or barrel, fill with water and cover with a lid.	weeds with bulbs, tubers and seeds e.g. Dandelion, Oxalis, Dock, Climbing Dock, Blackberry roots and any weeds with seeds.
Weed Bags: ‘Black Hole’ weedbags are available through www.ecomatters.org.nz. Weeds are starved of light and compost inside.	all sorts of weeds, e.g. the grasses or Tradescantia; avoid Blackberry that might rip the bag. Won’t kill seeds.
Tarping: To ‘tarp’ a weedy patch, mow it low, cover with clippings or woodchip, water well, and pin down a tarpaulin/plastic sheet/weedmat. Check after a few weeks – weeds will be severely knocked back and easier to remove.	most weeds, for example Kikuyu, Buttercup, and Convolvulus.

How long will it take?

It can take as little as 1 week for drying in the hot sun, to 2 years for drowning Oxalis bulbs. To check – squeeze a piece of the weed. If there is no resistance or ‘juice’ left, and the material is like straw/sludge, the weed is dead and can be safely composted or used as mulch.

Disposal sites for weeds

All weeds can be dropped off at the Composting NZ sites in Otaihanga, Ōtaki and Waikanae. Composting NZ use a hot composting method that destroys weeds.