How Hot Composting Will Kill Weed Seeds – Guide With Data

When making compost piles, frequently we will find ourselves in a situation where we have the option of adding an ingredient that may contain seeds. Whether it is seeds from a bell pepper in kitchen scraps, dandelion seeds from grass clippings, or other random weed seeds in yard waste – how do we know these seeds aren’t going to germinate in the future and require us to pull them? Nobody wants to have to pull a bunch of extra weeds that they inadvertently sowed themselves in the garden! So, what can we do?

Answer – to keep weed seeds from germinating in compost, you must build a large balanced pile that achieves Active Composting and generates enough heat to kill the weed seeds. I will show you this, and more in this article where I document studies that found out how hot a compost pile needs to get to kill weed seeds.

So, the good news is that if a seed reaches certain temperatures for a period of time, the seed will experience a thermal death. [1] Yup – that’s right. A hot compost pile can actually kill the weed seeds, rendering them harmless organic matter for our pile. The key is that you need to make sure you make a hot pile.

An active hot compost pile can kill weed seeds. All species of weeds have different requirements in terms of temperature or duration at temperature for a ‘thermal death’. But as a general rule, a compost pile that can maintain a temperature of 130-140F (54-60C) for seven days will kill most weed seed exposed to those conditions.

Each species is different, but most weed seeds will die after being exposed to 130F for a matter or hours. [2] The moisture level of the seed matters as well, as dry seeds will have higher thermal death temperatures than ones exposed to moisture. [3] For a compost pile, this shouldn’t be an issue, as moisture is required for composting to take place.

So this article will be an in-depth dive into how to stop weed seeds from germinating in compost. None of us want to have to pull more weeds than necessary.

In this article:

• Should you compost weed seeds?
• How can a compost pile kill weed seeds
• What temperature will kill weed seeds?
• Guidelines for making a hot pile
• Final thoughts
• Appendix – Research Data – Temperature to kill various weed species

I have received thousands of comments on my YouTube video on composting. And one of the frequent questions or comments I receive is regarding weed seeds in compost piles. Some commenters are concerned, while others have told me that previous compost piles they have made sprouted lots of weeds. The reason for their troubles is most likely their piles didn’t get hot enough, and thus they inadvertently spread compost + weed seeds when top-dressing compost in their gardens.

In fact, many backyard compost piles don’t get hot enough to kill weed seeds. And we should do everything we can to avoid spreading weeds in our garden as they will compete with our desirable plants for nutrients, space and water.

Should you compost weeds or weed seeds?

We can avoid having weed seeds sprout in our compost pile by two ways. The first method is to not compost anything that has a seed head or seeds on it. Now, if you can remove the seed heads with sheers or pruners without too much effort, then you should always do so. This is simply common sense. If there are no seeds in a pile, then there will be nothing to germinate.

The second method to prevent weed seeds from sprouting is to make an active, hot compost pile that generates sufficient heat to kill the weed seeds.

If you are able to make a hot compost pile that can reach 130F-140F for a week or more, then it is perfectly safe to include most weeds or weed seeds as an ingredient to your compost pile. The high temperatures will cause thermal death to the seeds. [1] [2]

If you are unsure if you can make a hot compost pile, then it is probably best to avoid weed seeds or anything with a seed head as an ingredient. All compost piles turn into compost given enough time, but without high temperatures weed seeds will survive, and could germinate in the future when the compost is used in the garden.

So, if you are going to compost in winter you need to either follow my principals to keep your pile hot, or remove seed heads.

How can a compost pile kill weed seeds?

When a compost pile is initially constructed, if it is balanced properly with green and brown material it will enter the active composting phase. Active composting is when microbial activity is quite intense, generating much heat that consumes and breaks down material that is easy to decompose. It also deconstructs plant cellulose at this phase, which is not an easy material to decompose at lower temperatures. [handbook]

The large amount of heat generated during the active phase raise the temperature of the pile. This high temperature is what will kill our weed seeds, and so we want to be sure we can achieve high temperatures! How long we need to maintain that temperature will depend on how hot the temperature.

For example if we are able to achieve 160F for a few hours, that would be enough to kill almost all weed seeds. Conversely a temperature of 100F will not kill many seeds at all no matter how long we maintain it. The relationship between thermal death of a seed and temperature is not linear. It is an exponential curve, and each species will have unique characteristics / temperature limits.

A well turned pile is necessary

At some point, the temperature of our compost pile will plateau and stop increasing. The overall pile size will also be shrinking at this time. This usually indicates that the material is decomposing, and thus compressing on itself. As it compresses, air is squeezed out of the pile, and the microbial activity lessens, thus lowering the temperature. When this occurs, a mixing or aerating of the pile will usually stimulate microbial activity by adding air (oxygen). And the temperature will rise again.

When we turn our pile to get the temperature rise, we need to move the material on the outside of the pile to the inside. That way any weed seeds that were on the outer layer of the pile get moved to the hot center, and will hopefully be killed by the high temperature.

What temperature will kill weed seeds?

So, how hot does our pile need to get to kill weed seeds? Well, it depends. Each species of weed seed will have a different temperature that will kill it. Also, it will need to be at that temperature for a certain amount of time. As the temperature increases, the time required to kill the seed will decrease. And most seeds will have a minimum temperature threshold for thermal death being possible.

As a general rule, most weed seeds will die if subjected to 130F (55C) for 7 days. Furthermore most weed seeds will die within several hours when subjected to 140F (60C). And of all seeds researched, they all should die within 1 hour when subjected to 160F (70C). [2][3][4][5][6]

There has been much research into this topic, but the literature is not comprehensive. It primarily covers species that germinate in farm fields (which would make sense due to economics). Lower temperatures (110-120F) can kill many or most weed seeds, but the time required will increase substantially. I can tell you that I’ve never had a problem with weed seeds in my compost, and I’ve composted plenty of things with seeds. You just have to make a hot pile!

Guidelines for making a hot pile / recipe

Ok, so we have established that most, if not all seeds will die a thermal death. Now what can we do to ensure our compost pile reaches the necessary temperature? Below are my brief guidelines to ensure a hot pile is reached. *Note – if you are somewhat new to composting, you may want to start with my article on building a pile from scratch.

The basic guidelines to ensuring a hot pile

Composting can be a complicated topic, and many sources out there seem to enjoy explaining compost in either the most generic or complicated manner possible. You just need to remember that you need four ingredient categories (greens/browns/moisture/air) and that the pile must be large, 3-4′ diameter and tall, and you can have a hot pile. Trust me, I’ve been doing this for years and make several cubic yards of compost each year. [7]

• In order to heat up, a compost pile should be 3′-4′ diameter x 3′ tall, or have a volume of 3 cubic feet.
  • This volume will allow the pile to sufficiently insulate itself and trap heat, allowing the temperature to rise to higher levels.
• The materials used to balance the pile should be 50% green and 50% brown (roughly)
  • Stockpile the materials if you can, that way you can build the whole pile at one time.
  • See our list of 101 compostable materials for ideas
• Thoroughly the moisten the materials when first building the pile. After the pile has been constructed, you will want to maintain a moisture level similar to a wrung out rag
• Make sure the pile stays aerated. Turn the pile once per week at minimum.
  • If using grass clippings, consider turning more frequently to prevent grass clippings from matting up
• Purchase a compost thermometer to check the temperature
  • If you notice the temperature dropping, it is time to turn the pile!
  • This isn’t absolutely necessary, as I composted for my first 5 years without one. But they are very helpful to check the temperature during the composting process. We picked up this one for $25, which is a littler ‘beefier’ than the cheap ones. I’ve found that sometimes you meet resistance when pushing it into the pile, so I think the thicker diameter probe is a good idea to prevent damage or buckling.

Troubleshooting a cold compost pile

If your pile isn’t heating up, it is usually for one of the following reasons. Try to diagnose what the reason is and correct it.

• The pile isn’t large enough. It should be 3-4′ diameter minimum. In winter, it should be larger to provide more insulation. Go for 5-6′ diameter and as tall as it will mound up.
• Not enough green material.
  • A compost pile should initially be comprised of 50% green material. If you have a large pile and it isn’t getting hot after 2 days, add green material! If you need a quick source of green material, go to your local Starbucks, as they give out their used coffee grounds. 3-4 bags should do the trick in most situations.
• Too dry
  • If the pile doesn’t have enough moisture, the composting processes will slow or cease making the temperature drop
• Not enough air
  • As material decomposes the pile will compress on itself. When this occurs air can get squeezed out, resulting in anaerobic bacteria taking over. Anaerobic bacteria does not generate much heat, and is slower at decomposition. [1]

If your pile has gone through several temperature rises (after turning), and the temperature just doesn’t seem to rise much, it most likely means that you have consumed most of the nitrogen material. That is ok, and is part of the composting process.

Your pile is now entering the curing phase, in which it should still be turned weekly and kept moist. But in the curing phase, your compost will start fixing nitrogen from the air, turning into hummus material, and generally becoming more stable and decay resistant. Once mixing no longer causes the internal temperature to rise, the curing is complete. This generally takes 3-6 weeks. [1]

During the curing phase it is possible for weed seeds to enter the pile via wind, etc. This is a risk, so covering a pile or transferring to large containers may be necessary. For what it is worth, I leave my piles totally exposed during the curing phase, and I almost never have weed seeds germinate in my compost after top-dressing. So, for my suburban backyard that borders a forest, this risk is quite low. But, your situation may be different.

Video Guide

Below is a video I made discussing this topic as well as fire risks with compost piles. You may find it interesting.

Final thoughts

Active or hot composting can kill weed seeds by sterilizing them at high temperatures. And a properly constructed pile will likely get hot enough to kill all weed seeds that are present. Thus when you apply your compost, it will be nothing but beneficial.

Every species of weed seed will have a different temperature at which they are killed.[1] It is a time temperature balance, in that certain temperatures may only need to be achieved briefly to kill a seed, while at a lower temperature it may need to be exposed for several days.

If you are new to composting, or aren’t confident in your ability to build a hot pile, then it would be good practice not compost weed seeds until you gain experience. If plant material contains seeds, then it is possible they survive a cold composting process and germinate after the compost.

Find More Composting Tips Here

Time to kill weed seeds at various temperatures

A 2007 study made a very controlled experiment testing the temperature and duration to effectively kill various common agriculture weed species. Their research (Dahlquist et al. 2007) found a non-linear response curve of thermal death of weed seeds. Using regression, they even fit a model for each species studied.

Overall, like we see at other studies, temperature is the key factor to killing various weed species. What set their study apart from others is the time durations measured and then modeled. This confirms our intuition and other studies that the longer a seed is subjected to high temperatures, the more likely it will experience thermal death.

Time to kill 90% of weed seeds at various temperatures

Temperature	42C (108F)	46C (115F)	50C (122F)	60C (140F)
Annual Sowthistle (Sonchus oleraceus)	47 hr	14 hr	3 hr	<10 minutes
Barnyardgrass (Echinochloa crus-galli)		13 hr	6 hr	<10 minutes
London Rocket (Sisymbrium irio)	83 hr	22 hr	4 hr	<10 minutes
Purslane (Portulaca oleracea)			19 hr	1.3 hr
Black Nightshade (Solanum nigrum)	341 hr	200 hr	30 hr	2.9 hr
Tumble Pigweed (Amaranthus albus)		270 hr	107 hr	1.1 hr

Time to kill 100% of weed seed at various temperatures