Can You Compost Weeds With Roots and Seeds? How to Do It Safely

Can You Compost Weeds With Roots?

Yes, you can compost weeds with roots, as long as you sort them first. Annual weeds pulled before they seed can go into any compost pile, roots and all. The roots of spreading perennials such as bindweed, along with rhizomes, taproots, bulbs and seed heads, survive an ordinary home pile and need one of three treatments before they go in: a hot pile held at 140°F, thorough drying in the sun, or rotting in water or a sealed bag.

The Royal Horticultural Society draws the line in the same place. Its advice is to add annual weeds to the compost bin and to keep out the roots of pernicious perennials like ground elder and bindweed, because most home bins do not get hot enough to destroy them. Clemson Cooperative Extension agrees that weeds that have not gone to seed can be added.

What you pulled	Risk in a cool pile	What to do
Annual weeds, no flowers or seed	Low	Compost directly, roots included
Any weed with seed heads	Seeds survive below 140°F	Hot pile turned so all of it heats, or green waste bin
Perennial roots, rhizomes, stolons, taproots, bulbs	Regrow from fragments	Drown, bag or sun-dry first, then compost
Storage roots such as nutsedge	Regrow from tubers	Dry in the sun first, or keep out
Weeds or clippings treated with herbicide	Residue can outlast composting	Keep out of the pile

Why Roots and Seeds Survive a Home Compost Pile

Weed survival comes down to heat. Virginia Cooperative Extension states that the center of a pile should reach 140°F for over 3 to 5 days to kill most weed seeds, and that piles smaller than 27 cubic feet (3 feet by 3 feet by 3 feet) do not hold sufficient heat. A bin that is filled a bucket at a time rarely has that mass, so it never reaches the mark.

Even a hot pile is uneven. The University of Nebraska notes that some seeds survive the most diligent composting, and attributes the escape to cooler pockets that do not hold high temperatures long enough. The outside of a pile is the largest cool pocket, which is why turning matters as much as peak temperature.

For that reason Virginia’s guidance is blunt. Unless you can guarantee the right pile size and temperatures above 140°F for 3 to 5 days throughout the entire pile, leave out the reproductive parts of unwanted weeds: seeds, rhizomes and stolons. Hot composting vs. cold composting explains what separates the two kinds of pile.

Hot Composting: 140°F for 3 to 5 Days

A pile that kills weeds is built to a size, fed a balanced mix and turned on a signal.

1. Build it at least 3 feet wide, deep and tall, and no larger than 5 feet each way.
2. Use brown material equal to, or up to three times, the volume of green material. Fresh weeds count as green.
3. Wet it to the feel of a wrung-out sponge. The seed-kill studies Nebraska cites required at least 35 percent moisture.
4. Bury seedy weeds in the center and check the core with a thermometer. The target is 140°F.
5. Turn the pile when the internal temperature falls below 100°F, moving the top and outside material to the inside.
6. Keep going until every part has been through the hot center. In the Nebraska studies the best results took 21 to 50 days of composting.

The payoff is large. Holding compost at 140°F for three days reduced weed seed viability by 90 to 98 percent in the research Nebraska summarizes, and Clemson notes that most harmful pathogens and weed seeds are destroyed above 140°F. Clemson suggests turning once a week as a simple routine if you are not tracking temperature.

A long-stemmed thermometer is the only way to know whether the core made it. The compost thermometer guide covers probe length and how to read one, and the brown to green ratio guide helps if the pile will not heat.

Perennial Roots: Drown, Bag or Dry Them First

Heat is hard to guarantee, so the safer route for roots is to kill them before they reach the pile. Three methods have institutional backing, and none needs special equipment.

Drown them. The Royal Horticultural Society’s instruction is to leave perennial roots to rot down in a bucket of water before adding them to the compost bin. Use a container with a lid, weigh the weeds down so they stay under the surface, and wait until the roots have rotted to mush. The water turns foul, so keep it away from the house, and pour the whole lot into the middle of the pile when it is done.

Bag them. The same RHS advice allows sealing the roots in an old bin bag until they rot. Texas A&M AgriLife Extension describes a slower version for plants that spread vegetatively: compost them alone, covered with black plastic, for as much as two years.

Dry them. Texas A&M also says such plants can be very thoroughly dried in the sun and then used as a compost ingredient. Clemson gives the same instruction for weeds with large storage roots, naming nutsedge, Florida betony and greenbrier. Spread them in a single layer on pavement or a tarp, off the soil, until they are brittle all the way through.

Whichever method you use, the test is the one in the RHS wording: the roots have to rot down, or dry out completely, before they count as compost ingredients. Anything still firm goes back for more time, even if that means the two years Texas A&M allows.

Weeds That Have Gone to Seed

Seed heads are the hardest case, because drowning and bagging are aimed at roots. Texas A&M’s rule for an ordinary pile is short: do not compost weeds with seeds, or invasive weeds such as morning glory and nut grass. Clemson likewise calls it best to discard weed seeds instead of composting them.

That leaves two honest options. Run them through a pile you are actively managing at 140°F, with turning, and accept that a 90 to 98 percent kill still leaves survivors. Or send them off site. The RHS points out that large composting sites reach temperatures high enough to kill perennial roots and seeds, so municipal green waste collection is the right place for them.

The better fix is timing. Weeds pulled before they flower stay in the first row of the table, so weeding before bloom keeps nearly everything compostable without a 140°F pile.

What to Keep Out of the Compost Entirely

Some material should not go into a home pile under any method.

Diseased plants are the first group. Virginia lists diseased plant material among the things to leave out, and Texas A&M’s general rule is not to compost it because of the chance of reinfecting the garden.

Herbicide-treated material is the second. The RHS warns that clopyralid, an active ingredient in many selective lawn weedkillers, binds to grass and remains active in the clippings. Virginia notes that certain herbicides may not be completely degraded even by composting under optimal conditions. Texas A&M’s figures for ordinary lawn herbicides are 6 to 8 months of persistence in soil, with residue no longer a problem once treated clippings have composted for at least one year. If you do not know what was sprayed, leave the clippings out.

Regulated invasive plants are the third. In the United Kingdom, Japanese knotweed, giant hogweed and Himalayan balsam are classed as controlled waste. Many US states keep their own noxious weed lists, and the county extension office can tell you how to dispose of a listed plant.

If a pile that received weeds starts to smell sour or stops heating, see why a compost pile smells bad before adding more.