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weeds growing in compost heap

For hot composting to fully kill all weed seeds and roots, follow these tips:

In an ideal compost heap, the temperatures generated by the breakdown of plant material can get quite warm, and if temperatures exceed 145 degrees Fahrenheit, pretty much all seeds and roots will be killed. However, if the temperatures do not get warm enough—or if a portion of the compost heap does not experience sufficiently high temperatures—seeds or perennial roots can survive the composting process. When these seeds or bits of root later reach your garden inside the compost, they can—and usually do—quickly germinate or take root again.

This can actually be rather charming when the volunteers are tiny impatiens seedlings, tomato plants, or even pumpkins that volunteer because last Halloween/s jack o’ lanterns were added to the compost heap. It’s far less charming when the volunteer plants are hundreds of dandelions or tiny sprigs of bindweed or crabgrass that get into the garden via the compost you spread.

Compost is a great way to recycle organic material in your garden. All those spent flower blossoms, fall leaves, dead plants, grass clippings—even non-meat kitchen scraps—can be transformed into a great soil amendment and nutritious mulch, simply by throwing them into a heap and allowing the refuse to decompose naturally.

Hot Composting

How do you know if your compost is getting hot enough to kill all weeds? A variety of compost thermometers are available that can gauge the temperature of your pile. Experienced gardeners may simply thrust a hand into the pile. If it feels uncomfortably warm to the touch, it likely is warm enough to kill all seeds and roots in the pile.

The classic method of composting—the method purists would call the “right” way—is known as hot composting. This simply means that you turn the pile regularly and allowing it to really heat up to 145 degrees Fahrenheit or more. A properly maintained hot compost pile will kill weed seeds, as well as many other pathogens, so you can compost weeds without having to worry about them popping up in your garden beds.

Done correctly, composting creates a sterile organic material that does nothing but good things for your garden and the plants in it. However, nearly every gardener who practices composting has occasionally experienced “volunteer” plants sprouting up in the garden where the compost has been spread.

How Weeds Survive

A gardener who experiences such an explosion of volunteer weeds may well swear off composting altogether, or at least stop adding weed material to the compost pile. To be clear, there is no reason to stop composting weeds. With a slight adjustment to the composting process, you can ensure that weeds and their seeds will be killed completely and won't be resurrected where you least want them.

So-called "cool composting" is a more informal style of composting. It is a passive method that doesn't involve constant temperature monitoring and mixing. In cool composting, fresh material is constantly added to the top of the heap as the lower levels are breaking down into compost. In cool compost bins, gardeners periodically remove the prepared compost from the bottom of the pile as fresh material is constantly added to the top. Cool composting is an easier style, though it can take somewhat longer.

I think you are being over cautious and slightly neurotic -there are weed seeds in you garden-if the compost is rotted use it -you are never going to wipe out weeds any way-the use of the compost far outweighs the odd weed

about a year ago, some weeds were accidently put in our compost bins – I didn’t realise this had happened until a few months ago when my husband told me. Anyway, obviously I dont want to now use the compost on our garden, but is there anything I can do with it, rather than just digging it all out of the bins and taking it to the recycling centre? e.g. could I put it in black bags and leave it in the sun for a few weeks (if we have that much sun!) – would the build up of heat kill any remaining weed seeds?


Ahh, ok, good to know I am just being paranoid! Thanks everyone, I shall use the compost. and a very good point that we will never eradicate weeds – a look at my garden tells me as much!

LavanderLois, Everything goes in my compost apart from the odd nasty weed, they go in the bin.
As long as your compost gets plenty of heat, is it steaming warm to the touch? then you can use the compost after six to eight months.
We will never erradicate weeds, they blow about, birds drop them and they can sleep in the soil for years then soil disturbance will set them off.
My compost bins are home made and large filling one whilst emptying the other, they do get up to heat, smaller ones may not although in time what is in there will rot down, it just takes longer.
Lift a hand full and feel it, then smell it if it feels and smells sweet then use it as normal, if not then give it a bit more time but do not watse any of it as compost is pure gold.
If you get the odd weed and you will then a Dutch hoe is the ideal tool, I carry mine everywhere and splat any weed I see,, it also comes in handy to lean on and contemplate, well that is what I call it.

Hi my compost heap never gets warm though i admit i am a bit lazy in turning it.Verdun how big is your heap? mine is four small palets fastened together should it get warm if i turnit more often? Thanks