Does a compost pile destroy weed seeds? Or more specifically, does YOUR compost pile destroy weed seeds?
We are regularly assured by composting experts that hot composting destroys seeds … yet I have some pumpkins that beg to differ.
Those pumpkins grew as volunteers from a compost pile a few years ago. Granted, it wasn’t a regularly turned compost pile, meaning they probably missed the hottest part of the heap, but how many of you turn your compost regularly? And I’m going to bet that you still had little tomatoes or weeds pop up in it. Come on, admit it!
My bet is ALL of you.
Here’s an example of “hot composting kills weed seeds” advice from Aggie Horticulture:
“The composting process also naturally kills weed seeds. Properly managed, a compost pile should easily reach 140°F, which breaks down all organic matter, including weed seeds.”
They recognize the difficulty, though, as the next line reads:
“The keyword is properly.”
My bet is that few gardeners reach that lofty, “proper” status.
Why Our Backyard Compost Pile Doesn’t Kill Weed Seeds
A typical backyard compost pile isn’t insulated or turned often enough to maintain heat. Those viable seeds in the compost don’t get rotated through the hot center of the pile.
Yes, the heat generated by thermophilic bacteria (an organism living at hot temperatures) is high enough to destroy weed seeds, but getting every bit of your compostable materials hot enough to kill the seeds takes very good compost management.
My old compost pile didn’t do it. It was built from reclaimed landscape logs with too many gaps to get everything hot. Plus, turning it was a pain.
I imagine if you owned a cement truck and packed the barrel of it with a proper mix of carboniferous and nitrogenous materials. Then you rotated it every day or so, and perhaps insulated the inside with foam. You could get that compost to heat up perfectly.
I’m joking. A bit.
My composting methods have gone from complicated to simple over the years. I’ve realized creating perfect compost doesn’t really matter.
Nature doesn’t create perfectly sifted, totally rotted, brown humus. No! She throws logs and leaves on the ground. There’s always some finished material and some fresh material, fungi eating at this, and some insect boring away at that.
But Let Me Back Up. What Prompted Today’s Post?
This Viewer Asked a Question
There was a comment that prompted today’s great big post on weed seeds in a compost pile. Four words that led to 1,145 words (give or take):
Martha asked this question on this anaerobic compost tea video I posted:
My answer was:
“Good question. I try to avoid throwing plants with mature seeds into the tea. They never seem to get completely die in a hot compost pile, either. Even though we hear all the time that “hot composting kills weed seeds!” It’s probably true for the ones in the middle of the pile, but I’m always getting volunteer tomatoes, wheat from straw, weeds, and pumpkins popping up even from a hot compost pile. My guess is that this tea method will rot down most of the seeds, if it sits long enough … but not all of them.”
It takes a lot of faith in your compost to deliberately throw in weedy materials, no matter how you’re composting.
If you have spiny pigweed going to seed in your food forest, do you really think you’ll be able to throw that in your compost bin and use the resulting compost in your spring gardens without spiny pigweed popping up?
Do you want to take that risk?
I Hear You, “But I Compost the Right Way!”
That’s fine—I appreciate the “thermometer and sifter” brigade.
To those about to compost, I salute you!
I am totally sure that I could destroy weed seeds by hot composting if I thought it out properly. However, my interest is more in gardening than in the processes that lead up to it. Making a “perfect” looking compost pile, or compost for that matter, isn’t as important to me as growing corn, pumpkins, beans, yams, and fruit trees. I also don’t like spending money to make perfect systems.
If you enjoy it, that’s fantastic. I love the smell, look and taste (well, maybe not taste) of finished compost. I made some nice-looking stuff myself this year and just sifted it the other day.
I made that compost with almost no work, though. No thermometers, no turning, no measuring ratios of carbon/nitrogen to get that 25/1 mix. No, I just threw it all on the ground in one of my garden beds.
And—oh YES—LOTS of seeds came up in it! Enough to start my new fruit tree nursery.
I view this as a feature, not a bug. Sometimes I just let compost piles turn into garden beds since there are so many volunteer edibles coming up.
But What About Killing Weed Seeds???
Right—that’s what you all want to know, right? How CAN you compost those pesky weedy plants?
My favorite method is to keep them out of the compost pile and gardens altogether.
In my former food forest, I would chop down weeds and throw them on the ground around my fruit trees and other shrubs. If they self-seeded and came back, I’d chop them down again.
Unlike delicate annual garden plants such as lettuce and cabbage, trees and shrubs don’t need to be perfectly weeded in order to produce. I just knocked down the weeds again and again. Every time I did, guess what?
Those fallen weeds rotted into humus.
Nature does this all the time.
The winter freezes come once-a-year and kill all the weeds. They fall to the ground and rot into the soil, which improves it.
If you want to use weeds to feed your gardens, you’ll have much better luck in a no-till system where you throw a pile of seedy weeds on the ground. Then, cover them up with mulch … and then, DON’T TILL!
If you till, you’ll bring those seeds up to the light and warmth. They’ll go crazy in your eggplants. However, beneath a layer of mulch, they’ll eventually rot away safely.
That’s my two cents on composting and destroying weed seeds. Yes, a compost pile can destroy weed seeds … BUT … and it’s a big but … most of us aren’t doing it “properly.”
Don’t trust too much in the magic of compost to pile-drive your pesky pigweed problems.
Personally, I prefer cold composting anyhow! I believe it keeps more of the good stuff in the pile instead of steaming it away into the air. Nature almost always cold composts! While that process takes longer, I think it’s a simpler and gentler method. I wrote an entire book on composting (Compost Everything: The Good Guide to Extreme Composting) and many of the methods in that book are cold compost approaches.
You might also like these composting articles from David the Good:
How to Build a Super Simple Compost Pile From Local Materials
Back to Eden Chicken Run Composting: Easy and Productive!
Nature Is An Extreme Composter—You Can Be, Too!
So, tell us … have you had success hot-composting seedy weeds? The comments below are waiting for yours!
(This post was originally published on July 31, 2017.)
David The Good is a Grow Network Change Maker, a gardening expert, and the author of five books you can find on Amazon: Compost Everything: The Good Guide to Extreme Composting, Grow or Die: The Good Guide to Survival Gardening, Totally Crazy Easy Florida Gardening, Create Your Own Florida Food Forest, and Push the Zone: The Good Guide to Growing Tropical Plants Beyond the Tropics. Find fresh gardening inspiration at his website TheSurvivalGardener.com and be sure to follow his popular YouTube channel.
Hi I grew up in a country that didnt actually built compost. We just threw everything back in the soil. We never used manure either. But everything just grew so well. The question of weeds growing in the compost, well I dont think you will ever manage to get rid of weeds doesnt matter what and where. We used to prepare big areas for planting by burning it. After a while weeds and trees come out again. Even if you kill them with weed killer (which we didnt use) they still grow again. Seeds stay in the soil for a long time they will always and always grow again. The only way to get rid of them is to keep pulling them. Sorry! thats the reality!
I have always found that the plants I thought were weeds have purpose. Noxious poke weed? Poultice for mastitis. Lambs quarter and chickweed in every garden bed? Edible. We don’t have to list the numerous ways that dandelions and plantains are useful…edible and medicinal. And the list goes on! I believe in the saying “there is no such thing as a weed, only a plant out of place.”
You’re right. Isn’t it amazing how some of the worst “weeds” still have wonderful uses?
Definitely believe that that you stated. Your favorite reason seemed
to be at the web the simplest factor to have in mind of. I say to you, I definitely get irked whilst folks consider issues that
they plainly don’t realize about. You controlled to hit the nail upon the top and also defined out the whole thing
without having side effect , other people could
take a signal. Will likely be back to get more.
I live in ecuador and we are in the rainy season – hot, humid, our summer. I dont know what i would do without out the green “things” growing everywhere. they are the basic fodder for my compost.along with kitchen scraps, trimmings and peanut shells 2 $ for HUGE bag. and i agree about purslane and a little dandelion, plantain (that i actually cultivate for skin care and back pain), etc.some of th eplants considered weeds even bu ecuadorans produce tiny flowers that the bees love. As for what isnt killed in the compost? endless tomatoes and the occasional mango – which I hafta to crush and dispose of back in the compost cause I have no room! Pulling greens is endless at this time of year but like I said builds great compost. Once the dry season hits i never have enough of anything to build, just store in a heap out of the way and the eventual compost heap.
here is a question. what about dog and cat poop? I have a link to an alaskan site that offers great info on the poop from all of those sled dogs. It gets stored somehow during the winter and then they build a HOt compost pile once the weather is warm enough. Once the compost is finished cooking they let it sit for awhile then use on non-root/non- vegetable gardens where the vegies/fruits are not touching the ground and of course bushes and flower beds with non-edibles.. I have two large garbage pails full of dog poop and peanut shells right now, plus another two that have been filling up with cat poop and litter – the requisite minimum of 18 months once the pail is full. I will be picking up rice hulls and peanut shells to act as the carbon base plus cardboard boxes that are piling up right now.
I would compost dog manure in a regular pile. Cat, however, can contain toxoplasmosis. If you want to compost it, I would build melon pits like this: https://www.youtube.com/watch?v=qCPRClS6-tY
When you have lots of manure, I suggest:
In the 40 years here on my homestead creating tons of compost, I have never turned it nor bought into that being a good idea .I want a living compost- not a dead one and when it gets hot the nitrogen is going off in the ammonia . I want a stable humus .I make large windrows using my horses manures bedded in pine sawdust .I stopped using straw because it is full of herbicides and chemicals .For the last 25 years,I have used the biodynamic preparations. It looks and smells like humusy soil with little to no materials that have not broken down. I never sift it. I cover it with a tarp after I build the pile which takes all winter .The worms digest it for another 6 months (I do not need to buy worms -they just show up) After about 6 months the worms leave and I have beautiful living compost ready to use . I eat and use weeds medicinally .They are my allies . I use them as mulch too and livestock feed .I usually make enough to have 1 and 2 year old piles to spread in my gardens
I never really had a problem with weeds, they have their place too.
I never turn my compost pile. I don’t mind the stray plant that grows. I take it as a sign that my compost has the nutrients it needs
Great article. Thanks. I’m reading your zone pushing book right now, but I’ve got Compost Everything on my kindle for later.
I don’t worry much about weeds in my compost, except for grasses. I toss grass into the chicken run and let them eat anything that grows. Then, eventually, I’ll dig out the manure and other composted materials. If I remember, you had a video where you did something like that too.