Important Things To Consider Before Composting Your Weeds

Initially, it feels great to see a giant pile of weeds in your yard. Those are weeds that would otherwise still be growing in your garden, and each one took work to pull out. But you eventually have to do something with that pile... or the weeds will be growing back in no time. While you could always throw the weeds in the trash, composting is also an option if you want to avoid waste and put the plants' nutrients to good use. However, before you go this route, consider the types of weeds, their condition, and the temperature of your compost pile.

According to House Digest's Garden Editor and in-house Master Gardener, Tiffany Selvey, there are both pros and cons to adding weeds to your compost. One pro is that certain weeds can add a helpful nutrient boost. "Some weeds, like dandelion and dock are really excellent at using their taproots to dig deep into the soil," Selvey told House Digest exclusively. "They can absorb nutrients that aren't available to annual plant roots closer to the soil's surface. By putting these healthy green leaves or the taproots to your compost, you're adding vital nutrients."

The main con, on the other hand, is that weeds might find their way back into your garden by way of the compost pile if you don't compost them properly. That's especially risky if any weeds in your pile are invasive, stubborn, or diseased. "Weeds can be a useful addition to your compost, but there are certainly some you'll need to leave out," Selvey explained.

The rule of thumb is to avoid composting weeds that have already gone to seed, such as those with dry seed heads or remnants of spent flowers. If the compost pile gets sufficiently hot, those seeds will die, but if not, they'll survive to wreak havoc in your garden later. "If you aren't an expert composter — it's best to leave them out," Selvey told House Digest exclusively. 

It's also ideal to know exactly what weeds you're composting, so that you can identify those that are especially stubborn or unlikely to decompose fully. For example, it's best not to compost weeds with woody rhizomes, as these "may not break down and can take root in your garden," Selvey said.

Lastly, avoid composting weeds that show signs of disease or have been treated with herbicides or pesticides. "Powdery mildew and other fungal and bacterial issues affect weeds," Selvey said. "You don't want to add those to your compost bin."

One way to safely compost almost any weed is to make sure that the pile gets hot enough to kill seeds and pathogens. The entire pile needs to reach a temperature of 140 degrees Fahrenheit or more for at least three days. The longer it stays at this temperature, the more seeds will die. You can track the temperature with a compost thermometer.

Your pile is more likely to get adequately hot if you consistently maintain the proper ratio of browns and greens, turn the entire pile regularly, and make sure it stays just wet enough. Properly built compost will reach this temperature within a few days. But if you allow any imbalances, the compost may not get hot at all. "If you aren't 100% sure you can reach the ideal temperature to kill all weed seeds and pathogens, only add healthy green leaves to your compost," Selvey told House Digest exclusively.

As you can probably tell by the name, "cold composting" — which is more hands-off and slower — won't get hot enough for this method of weed management to work. However, some people pre-bake weeds by leaving them in a dark plastic bag in the sun for several days (or even zapping them in the microwave) before adding them to your own DIY compost pile.