weed seeds in compost

Tips for Composting Weeds

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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.

Composting Issues

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.

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.

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.

How Weeds Survive

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.

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.

Hot Composting

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.

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

  • Turn the pile frequently. All compost heaps have localized cool spots that are slow to break down. By mixing the pile frequently, you ensure that all material is achieving the necessary heat to kill the seeds and roots.
  • Give it time. Practiced correctly, hot composting involves processing a volume of material fully until it is fully decomposed. Don’t continue to add small amounts of additional material to the heap; start another heap while the first one breaks down completely. The compost is ready to spread when turning and mixing the pile no longer causes the compost to heat up.
  • Weed the garden before adding compost. Fresh compost is laden with nutrients, and if there are weeds growing in your garden, adding compost will simply nourish the weeds along with your garden plants. Make sure your garden is well weeded before adding fresh compost to the soil.

Cool Composting

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.

Here are some tips to keep a cool compost pile free of weeds:

Weeds can be safely added to a compost pile if you make sure temperatures are high enough to kill the seeds and roots.

5 Steps to Eliminating Weed Seedlings in Homemade Compost

Maddy is trying to become 100% self-sufficient in compost by composting everything, even tricky weeds that seed easily. Here are the 5 steps she uses to achieve fertile, no dig beds mulched with her own compost.

I have always made compost. Currently, I have a wormery plus a Hotbin chugging away in my greenhouse. These make weed free potting compost and the glorious ‘worm wee’ that fertilises the greenhouse crops. I also have a stack of three tractor tyres that composts large quantities of organic matter plus four black bins that were sold very cheaply by my council a few years ago. These bins get hot and turn garden waste into compost quickly. I often mix them mid cycle to speed up the process. My leaf mould bins have disintegrated. I need to build another one.

My aim is to compost everything I can and become self-sufficient in fertility in my no dig veggie garden area. I grow a lot of comfrey too and make comfrey tea. And I use green manures but sparingly. I find digging them in a chore and they often don’t want to go away.

I am about 60% successful this year and have minimised barrowing loads of composted horse manure with wood shavings from my kind neighbour’s stables. But to achieve just 60% self-generated fertility I have to compost almost everything, even the weeds. Tim and I just don’t generate enough kitchen waste, wood shavings, grass cuttings etc. That means when I mulch the raised beds with homemade compost I get weed seeds.

I have come up with a cunning cold weather plan. I want my weed seedlings to germinate well before the warm weather starts which is also the time when all my veggies (like garlic and broad beans) are starting to appear. This is what I do. Incidently, I first heard this idea from my friend, Paul Wagland.

I spot weed my beds and make sure they are clean. Then I mulch them with a generous layer of compost. Then I cover the bed with clear plastic sheeting (see first photograph below). Anything will do. Mine came as packaging for an item but you can also use cut open clear sacks. The plastic creates a microclimate in late autumn. The weeds gernimate and then I hoe them. A hard frost will then kill them. If the frost doesn’t come I hoe again. Then the bed is ready for planting and the weeds won’t smother or get mixed up with the veg.

I tested this out before posting the idea so you can see it works for yourself.

Here is a bed newly mulched with compost and covered with plastic. No weeds yet.

Here is a mulched bed with germinated weeds below the plastic. You will see an area without plastic and without germinated weeds as well.

Here is the detail.

Here is the uncovered bed with weed seedlings. Lots of them!

So now I am hoeing the seedlings. I only hoe the top mulch. The bed below is undisturbed. I will leave this bed empty until I know the seedlings have been eliminated.

Get started in No Dig gardening? Maddy recommends Charles Dowding and Stephanie Hafferty’s No Dig Organic Home and Garden

Maddy is trying to become 100% self-sufficient in compost by composting everything, even tricky weeds that seed easily. Here are the 5 steps she uses to achieve fertile, no dig beds mulched with her own compost.