Is composting weed seeds possible?

Hi I just recently obtained 4 large black plastic bags of dry weeds and weed seeds (couch grass, wild oat, a prickly one forgot the name). I heard you can apparently make compost from it by putting the plastic bags in the sun.

Eager to do this right I hopped on google but then all the advice I read was like "avoid doing this when the weeds have gone to seed" and "you can compost MOST weeds" and "avoid the seed heads"

My bags are FULL of weed seeds. Also can't solarize them on concrete because it's windy as hell here and they'll just fly away. Is it actually possible to compost weed seeds or am I playing with fire? Will composting with worms work because they will hopefully eat the seeds or? (I'm in Australia)

Hi Alex,

Personally, I would be wary of leaving black plastic in the sun. That sounds like a recipe for microplastics that will end up in your compost. Even "UV stable" plastics will break down in direct sun, over time.

As for your question about composting the weed seeds, a big enough, hot enough compost heap will have no problem composting and sterilising the seeds. This link suggests 120-150 F will be sufficient to kill off the weed seeds and plant diseases.

If you have access to fresh manure (chicken or duck works particularly well), you could heap that over the top of the weeds to create a hot compost heap. The fresh manure gives off a lot of heat as it decomposes. Cow or pig manure, or horse bedding, work well too. Alternatively, a plant-based source would be grass cuttings, hay, or straw soaked in urine or comfrey tea.

You could even exacerbate the heat if you can salvage an old window or a piece of glass to place over the top - just be careful of the possible fire risk.

If the compost starts to cool and you are worried that it hasn't reached the requisite temperatures, you could always use it in an area of the garden that you are less precious about as a sort of quarantine. If it grows well, and doesn't give rise to a large number of weeds, you could move it. Otherwise, cover it up to kill off the weeds and use it as a base for a lasagna/hugel style bed.

Actually, as I write this it occurs to me that you could do that from the offset. Weed seeds will need light to germinate and, if you can cover the compost (and keep it covered) with a thick layer of mulch or topsoil, it will enrich the ground without the weeds growing.

Alex Laker wrote:Hi I just recently obtained 4 large black plastic bags of dry weeds and weed seeds (couch grass, wild oat, a prickly one forgot the name). I heard you can apparently make compost from it by putting the plastic bags in the sun.

If I was given those bags I would use the trash can method of composting.

It is really windy where I live so I don't compost as it would just blow away.  I would not want to take a chance on weed seeds blowing all over my property.

Here are some threads about the trash can method that you or others might find interesting:

https://permies.com/t/139253/composting/Compost-trash

https://permies.com/t/139110/composting/faster-composting-idea#1090632

https://permies.com/t/9414/composting/trash-compost-bin-work

I also like Luke's suggestion to use manure to get the compose hot.

Another thing I have done is cold composted them, then pre-germinated the weeds before using the compost by watering and giving sunlight.. I followed that by removing all sprouted weeds after two days and returning all those back to the compost. Probably left a few that had different germination conditions, but it certainly made it easier to use the compost quickly.

You don't hear much about it on permies, but you can also use a solar box oven to "sterile" compost. Heat it enough to kill any seeds that are lingering. Basically your idea about leaving them in the black bags in the sun, but without the possibility of the bag breaking down. You'll either need a really big box oven or many days to get all your material through the oven. As you said it's windy, I'd do this before composting rather than after, though most people sterilize their finished compost for creating a planting medium. You can use an oven thermometer to check to see that the material is reaching 140°F inside the oven.

I never trust myself to get a given compost pile really hot, and then mix it so every part gets into the inside and gets that hot. And some seeds survive in soil for years.

One thing you could do that would prevent them from growing into problem weeds would be to bury them at least 6 inches and preferably more than a foot below the surface of the soil. Under perennials or bushes or trees so you know they won't get dug up again within a few years.

That bucket method is worth a try, though when I did it I didn't trust that no seeds survived, so I treated the solids that came out of it as weed seeds. Just soak the weeds and deadheaded flowers that could be full of seeds in a bucket of water for a couple of weeks. The liquid... well, yeah, I diluted it with water and used it as fertilizer. It smelled so bad... I can't quite express how bad. Like vomit. And the rat-tailed maggots that showed up in it freaked me right the hell out though they turn out just to be harmless hoverfly larvae. But I think it probably did get a lot of the nutrients from the weeds back into the soil.

Rebecca Norman wrote:I never trust myself to get a given compost pile really hot, and then mix it so every part gets into the inside and gets that hot. And some seeds survive in soil for years.
... That bucket method is worth a try, though when I did it I didn't trust that no seeds survived, so I treated the solids that came out of it as weed seeds. Just soak the weeds and deadheaded flowers that could be full of seeds in a bucket of water for a couple of weeks. The liquid... well, yeah, I diluted it with water and used it as fertilizer. It smelled so bad... I can't quite express how bad. Like vomit. And the rat-tailed maggots that showed up in it freaked me right the hell out though they turn out just to be harmless hoverfly larvae. But I think it probably did get a lot of the nutrients from the weeds back into the soil.

The "rot barrel" is a reliable method. At my place, I put weed seeds (dry) on the biochar burn pile or (green weeds) into the water-filled rot barrel for as many weeks/months as I think is necessary. It's a nasty stinky putrid anaerobic mess, but when I use that vile liquor to soak biochar and wood chips I receive a magic mix that does wonders when thrown down around heavy feeding perennials like raspberries.

Alex Laker wrote:Will composting with worms work because they will hopefully eat the seeds or?

Alex, unfortunately worms do not kill weed seeds. Not sure I have any better suggestions than what has already been discussed.

Another vote for the nasty barrel of weed water (or, as David the Good says, putrid swamp water). I always have a barrel going for comfrey tea, sometimes I throw manure in there, and sometimes it's these horrible weeds I have that are super invasive and will take root wherever they get thrown, even on a compost pile. My bucket is small, one of those maybe 20L/5 gallon ones. it's enough, and you water down the tea anyway.

A lot of great suggestions.

I thought of another option after reading all the suggestions.

I feel that putting weed seeds into a pot of boiling water and cooking them would kill them.

From the looks of it, anaerobic compost and no light is the way to go. I like the boiling water idea as well, I'll have to give it a try too. Thank you everyone for the suggestions, I learnt about many methods I have never heard of before

Anne Miller wrote:
I feel that putting weed seeds into a pot of boiling water and cooking them would kill them.

Yeah, the first year I moved to my own place here in the desert, lots of Tribulis terrestris came up in July. Also known as puncturevine and goat-head thorns, it's a really obnoxious thing to have around. The seeds are spiky, hard, and large enough that they seem like maybe very long lasting.

Since I had to pull a lot after they'd set those horrible seeds, I pressure cooked them and dumped them down the composting toilet, haha. That'll teach 'em!

The information given in the replies is solid and I've considered all of it prior to composting weeds, which I have done for many years.  So all I have to offer is my experience:

1)  Make sure your pile gets hot and toss the pile for heat distribution;
2)  As one reply noted, hard seeds such as goat head seeds matter, don't use them;
3)  If my pile contains a large proportion of weeds, I bury that particular load especially deep;
4)  I have found letting chickens graze my pile to be an effective way of eliminating seeds, they're mini tractors.


Hopefully my years of doing this will be helpful.

Good luck.

If you can get the seeds to 140 degree or above and hold them for a bit is all that is needed  Oven, black car in the middle of summer on a summer day and many others.  Then compost once they are sterilized.

al aric wrote:4)  I have found letting chickens graze my pile to be an effective way of eliminating seeds, they're mini tractors.

Good comments, and #4 is a fine idea. If chooks see tasty greens that are not toxic, those little dinosaurs will peck down the nasty shoots until they die. Excellent!