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.

Chickweed is one of the many weed seeds that is killed by high temperatures in compost. Pictured, a chickweed seedling.

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.

140F – no weed seeds are making it out of here alive!

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:

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.

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.

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.

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

Appendix – Research Data – Temperature to kill various weed species

This section, or addendum is presented to summarize various studies I reviewed in researching and writing this article. No two studies are the same, but when taken together it becomes clear that for many (if not most) species, 130F (55C) is where many seeds begin to experience thermal death. The longer this temperature is maintained or exceeded will mean that more and more seeds and species will be killed, thus rendering the compost effectively weed free. All references are cited and sourced at the bottom.

Composting Cattle Manure Study – 1998

A study on composting of cattle manure in windrows found that all the weed species listed below experienced thermal death when composted for six weeks between 55-65C (130F-150F). (Source)

SpeciesCommon name
Amaranthus retroflexusRedroot pigweed
Matricaria perforataScentlesscamomile
Thlapsi arvenseStinkweed
Chenopodium albumLamb’s quarters
Kochia scopariaKochia
Galeopsis tetrahitHempnettle
Malva rotundifoliaRound leaf mallow
Avena fatuaWild Oat
Setaria viridisGreen Foxtail
Polygonum convolvulisWild Buckwheat
Polygonum persicariaLady’s thumb
Galium aparineCleavers
(Tompkins, Darrell K. et al. 1998)

Effects of temperature on cattle manure compost process – 1998

Another study on weed seeds in composting cattle manure found that most seeds died at around 130F. The table below lists the percentage of species that germinated. This applied to the following species:

Max Temp F113 F122 F125.6 F127.4 F131.18 F
Max temp C45 C50 C52 C53 C55.1 C
Duration (hours)97640109142
Species germinated %93%27%73%47%0%
Source (Nishida, Tomoko, et al. 1998)
  • Velvetleaf, Abutilon thophrasti
  • Spiny Amaranth, Amaranthus spinosus
  • Slender Amaranth, Amaranthus viridis
  • Aluma, Amarnthus patulus
  • Ragweed, Ambrosia trifida
  • Beggar-ticks, Bidens frondosa
  • Barnyard grass, Echinochloa crus-galli
  • Bull Thistle, Cirsium vulgare
  • Crabgrass, Digitaria ciliaris
  • Autumn Millet, Panicum dicholtomiflorum
  • Doc-leafed Smartweed, Persicaria laplathifolia
  • Knotweed / lady’s thumb, Persicaria longiseta
  • Pokeweed, Phytolacca americana
  • Small flower nightshade, Solanum americanum
  • Horsenettle, Solanum carolinese

Research study that cooked seeds in moist soil inside an oven

A research study (Egley 1998) examined the percentage of viable seeds in moist soil after being subjected to various temperatures for up to 7 days. Note that note that at 104F, most seeds are still fully viable or nearly so, while at 140F (60C) after 7 days, most seeds have perished. Also of note is that at 158F (70C) basically everything dies after a week.

Percent viable seeds in moist soil exposed to various temperatures for different durations. Summarized data from Egley 1998.

Source (Egley 1990)

There is an interesting caveat in this study. They initially performed their experiment on seeds in dry soil. So, place in a pan layered with soil, then covered, and then cooked with the viability being tested at various time intervals (more than I summarized in my table). What they found was that seeds in dry soil pretty much all survived the very high temperatures.

So, what does that tell us? Well, it shows that the high temperatures without moisture will not kill seeds, at least not to 160F/70C. You need to have at least the presence of moisture.

There could be many mechanisms at play to kill the seed with high temperatures and moisture. Several ideas that come to mind would be does the ‘imbibing’ of water force the seed to begin life, and then allow the heat to kill it? Is there a thermal expansion problem when water is added to the seed? Does high temperature+water break down a seeds natural defenses to decomposition? Or, is it bacteria decomposing the seed?

I don’t have any special opinion as to what mechanism may be causing the seed death. But I do find this research very interesting. And, I would not be surprised if there were different mechanisms acting on different species. However – the main point in regards to gardening and composting is that hot composting will kill the seeds. We should take away the practical conclusion that is useful to us, and theorize about the mechanisms or reasons for seed death as a fun thought experiment.

Direct effect of temperature on viability of weed seeds in compost

The following table is the results of research by (Grundy et al 1998) to investigate the temperature that killed various common weed species. Seeds were incubated between 3-84 days at temperatures listed below, and germination tested. Note that all species died above 130F in their research.

Temperature
Species95F (35C)113F (45C)131F (55C)
Fireweed
(Chamaenerion angustifolium)
29%0%0%
 Wild Chamomile
(Matricaria discoidea)
35%0%0%
Meadowgrass
(Poa annua)
49%1%0%
Black Nightshade
(Solanum nigrum)
100%30%0%
Sow Thistle
(Sonchus asper)
18%0%0%
Chickweed
(Stellaria media)
32%0%0%
White Clover
(Trifolium repens)
0%0%0%
Speedwell
(Veronica persica)
28%0%0%
(Grundy, et al. 1998)

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

Temperature42C
(108F)
46C
(115F)
50C
(122F)
60C
(140F)
Annual Sowthistle
(Sonchus oleraceus)
47 hr14 hr3 hr<10 minutes
Barnyardgrass
(Echinochloa crus-galli)
13 hr6 hr<10 minutes
London Rocket
(Sisymbrium irio)
83 hr22 hr4 hr<10 minutes
Purslane
(Portulaca oleracea)
19 hr1.3 hr
Black Nightshade
(Solanum nigrum)
341 hr200 hr30 hr2.9 hr
Tumble Pigweed
(Amaranthus albus)
270 hr107 hr1.1 hr
Data (Dahlquist et al, 2007)

Time to kill 100% of weed seed at various temperatures

Temperature39C
(102F)
42C
(108F)
46C
(115F)
50C
(122F)
60C
(140F)
70C
(158F)
Annual Sowthistle
(Sonchus oleraceus)
28 days4 days15 hr4 hr15 minutes10 minutes
Barnyardgrass
(Echinochloa crus-galli)
16 hr9 hr15 minutes10 minutes
London Rocket
(Sisymbrium irio)
4 days24 hr6 hr15 minutes10 minutes
Purslane
(Portulaca oleracea)
56 hr3 hr40 minutes
Black Nightshade
(Solanum nigrum)
16 days9 days71 hr2 hr40 minutes
Tumble Pigweed
(Amaranthus albus)
13 days113 hr1 hr40 minutes
(Dahlquist et al, 2007)

Ok! That’s a lot of data, and was a lot of reading to gather all that information. I hope you found it helpful, and have hopefully gained some confidence that you can compost weeds and weed seeds safely.

Find More Composting Tips Here

References:

[1] – Graves, Robert E, Hattemer, Gwendolyn M, Stettler, Donald, Krider, James N. National Engineering Handbook, Part 637 Environmental Engineering, Chapter 2 Composting. pp87.

[2] – Dahlquist, Ruth M., Timothy S. Prather, and James J. Stapleton. “Time and temperature requirements for weed seed thermal death.” Weed Science 55.6 (2007): 619-625. Retrieved 10SEP2022

[3] – Nishida, Tomoko, et al. “Effect of cattle digestion and of composting heat on weed seeds.” Japan Agricultural Research Quarterly 32 (1998): 55-60.

[4] – Egley, Grant H. “High-temperature effects on germination and survival of weed seeds in soil.” Weed Science 38.4-5 (1990): 429-435.

[5] – Grundy, A. C., J. M. Green, and M. Lennartsson. “The effect of temperature on the viability of weed seeds in compost.” Compost science & utilization 6.3 (1998): 26-33.

[6] – Tompkins, Darrell K., Donna Chaw, and Abimbola T. Abiola. “Effect of windrow composting on weed seed germination and viability.” Compost Science & Utilization 6.1 (1998): 30-34.

[7]- Cullen, Mark, Johnson, Lorraine. The Urban/Suburban Composter: The Complete Guide To Backyard, Balcony, And Apartment Composting. New York : St. Martin’s Press, 1994, pp155

Joe Foster

Hi - I grew up outdoors in nature - hiking, fishing, hunting. In high school I got my first job at a garden center where I learned to garden and landscape. I've been growing plants from seed and designing native plant gardens for over six years. I hope to share some of my knowledge with you! Additionally I am a wood worker / DIY enthusiast. I enjoy designing/building projects (with hand tools when I can!). I hope to give you some tips and useful information!

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