I was asked if it was possible to compost weeds as the writer had been told to never compost weeds because it would perpetuate more weeds. The answer, as with so many things, is that it depends. In this case on which weeds and how they’re composted.
Pallet Made Compost Bins
Weeds can be broken down into two groups: the perennials like docks, dandelions and bindweed that have the ability to regrow from a piece of root and annuals that wilt and die when uprooted. I have lots of mares tail – is this suitable for composting?
There is no problem composting annuals but the perennials including mares tail can regrow easily. There is one exception, weeds including annuals that are about to seed or in seed. These should be thought of in the same way as perennials.
Cold Compost Heaps
Nearly all gardeners have a compost heap and most are cold heaps. The green waste is just piled up in a bin or heap and gradually rots down over time, usually about 9 months to a year. Eventually it is used on the patch. Perennial weeds will usually regrow in the heap and you can be adding to your weed problems with weed seeds.
Hot Composting
Hot composting is different. The heap is managed to provide a mixture of carbon rich and nitrogen rich materials. Ideally a carbon nitrogen ratio of 25:1. Bacterial actions will cause the heap to heat up. The aim is to take the heap to 60ºC but not much higher. Once the temperature starts to fall, the heap is mixed so all the materials get equally heated up and rot down.
A hot heap will easily kill perennial weeds along with any weed seeds. But, keep in mind hot heaps take some skill and practice to properly make and manage.
The system I use is to keep a barrel of water and add the perennial weeds to it. The weeds drown and after a few weeks their remains go onto the heap to safely rot down. The water in the barrel will get a little smelly but makes a great plant tonic.
I compost mares tail and bindweed with no problem at all. I cold compost for a 2 to 3 years in a completely enclosed wooden composter. This gets forked over regularly when filling in the first year, then turned annually until ready. I never see any regrowth of the perennial weeds. I have been doing this for many many years and have no bind weed where most of the compost goes (ie the vegetable patch).
Hi Chris – I suspect long composting (2 yrs plus) makes the difference. Bit like covering a patch with tarp for a couple of years.
However, knowing some weed seeds can sit in the soil for years and grow when they hit the right conditions would make me unwilling to put seeding weeds in.
I kill perennial weeds by stuffing them into black builder refuse bags and tying the top tight. The bag expands as the internals rot and produce heat, within a month it’s fit for the heap as it’s just a rotted mess that’s been cooked.
Hi as a newbie to having an allotment i’m learning as i go but have had a few words of advice from fellow allotment owners regarding weeds in compost heap. I was told that round up will kill the weeds and the compost would be absolutely fine,is this the case as i thought it wouldn’t be adviseable?
Thanks in advance
Hi Lee – it’s not really that simple. Roundup (Glyphosate) will kill a weed but if it is seeding then the seeds will be viable, Generally, I wouldn’t worry about it too much but you could hit a problem later in theory.