boiling water to kill weed seeds

matthew18(5)January 23, 2010

I just read in the FAQ section of the seed forum that using boiling water kills any weed seeds in compost. I guess you put some compost in a bucket and pour in some boiling water. Has anyone done this? Does it work? Do you let the watrt just evaporate? Do that do anything to the nutrients?

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Kimmsr(4a/5b-MI)

Boiling water kills any "weed" it contacts and may well kill any "weed" seeds, however, that same boiling water will kill off the microbes in that compost. Since I have used the compost I make as my potting soil and since I seldom see "weeds" growing in those pots I don't think the "weed" problem in compost is enough to be greatly concerned about.

    Bookmark   January 24, 2010 at 6:55AM
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matthew18(5)

I'm a biginner with composting and all that goes along with it. That said if its finished compost(by that I mean it looks like black top soil) will killing microbes be a big deal? Will it really hurt the overall value of the compost?

    Bookmark   January 24, 2010 at 10:54AM
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borderbarb

I guess it depends on the area you will treat. If it is just a pot or two, then you may consider it "no big deal" to kill a complex soil life along with the seeds. But I wonder why pulling the weeds isn't a consideration. Or why not mulch to stifle the weeds? And for a larger plot/bed the loss to your soil tilth may be too high a price to control weeds.

May I suggest that you read the thread in this forum: 'QUESTION FOR BPGREEN' for a discussion about compost.
/// http://forums2.gardenweb.com/forums/load/soil/msg010826583514.html?9
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    Bookmark   January 24, 2010 at 1:33PM
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Lloyd

Every once in a while someone posts a question that makes me stop for couple of hours to think about it logically.

Is it really that big of a deal if there are no microbes left alive in the compost? Given that the soil has millions of microbes in a single tablespoon and many of these microbes are probably the same as what's in the compost, if one worked the 'sterile' compost into the soil what difference would it make? The microbes already in the soil would see the 'post as a food and multiply quite quickly.

Now if you were trying to make compost tea, I don't think sterile compost is what you want but for soil, I wonder if it really does make a difference. I suspect it doesn't.

It really is an excellent question.

Lloyd

    Bookmark   January 24, 2010 at 2:12PM
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curt_grow

Lloyd; you got me thinking. If you were to use boiling water on the compost to kill the seeds. Then inoculate the the boiled compost with a small amount of unboiled compost. Then the microbes would recolonize right? Anyone?

Curt~?

    Bookmark   January 24, 2010 at 4:11PM
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lazy_gardens

To kill any weed seeds you would have to pour on enough boiling water to bring the entire container of compost to killing temperatures for long enough to kill the seeds.

Do you know how much water that will be?
Do you know what temperature you need?

    Bookmark   January 24, 2010 at 4:48PM
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Lloyd

Well the OP said "bucket" so you'd need enough water to fill the bucket. Temperature? Hot composting at 140-150 kills most of the seeds so I'd guess that's the minimum temperature you'd want.

Logistically I don't think it's practical on anything but a small scale. I've never done it, it would probably work, if the water is drained some nutrients would be lost, evaporation would not cause loss of nutrients.

If I wanted to try to kill seeds in small batches, I'd track down an old BBQ, put some 'post in it and let it sit in the hot sun for a day.

Lloyd

    Bookmark   January 24, 2010 at 5:16PM
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matthew18(5)

I love reading the follow up posts. This info was under the FAQ's in the growing from seed forum so I just figured this was common knowledge and/or common practice. I went back and quickly looked to see if it referenced a specific trmp...it didnt. Interesting that, in a quick scan, I didnt see anything about killing weed seeds included in the FAQ for the soil forum. I wonder why? I think I'll try it,.

    Bookmark   January 24, 2010 at 8:30PM
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jonas302(central mn 4)

Being that the post is on the seed starting forum it is also to sterilize the compost for seed starting so in this case the microbes are not important and the amount of soil will be pretty small

I have never done either but I hear baking the soil in an oven really smells bad so boiling water would be a good idea

    Bookmark   January 24, 2010 at 10:56PM
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jonas302(central mn 4)

Being that the post is on the seed starting forum it is also to sterilize the compost for seed starting so in this case the microbes are not important and the amount of soil will be pretty small

I have never done either but I hear baking the soil in an oven really smells bad so boiling water would be a good idea

    Bookmark   January 24, 2010 at 11:05PM
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kimpa(z6b PA)

I never have enough compost so I consider my home compost valuable. More valuable than a bag of potting mix. So rather than destroy part of the compost, I prefer to buy seed starting mix. You wouldn't need to worry so much about unwanted seeds after your planted seeds have sprouted and then need to be potted up and fertilized. Your seedlings would be bigger and easier to identify-weed seeds could be easily plucked if you used your compost then.

    Bookmark   January 30, 2010 at 1:33PM
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rott

..

If you feel compelled to pasteurize all your materials before composting, have at it. If you want to pasteurize after the fact then I think you are throwing away something of value.

The folks that grow mushrooms pasteurize everything before grow their mushrooms. If you take all that stuff after they are done with it, you will get all the microbes floating in the air to start the decomposition process over again and it will continue to compost but with a delayed action.

The hot composting process will get you 140 F to 160 F and will kill off most, if not all, the things you may be worried about. Boiling water is 212 F or 100 C and will kill everything. Once everything is killed (pasteurized) it will just take time for all those happy microbes that are ever present to settle in again and digest the result.

Yeah, boiling water will kill the seeds. It will kill everything else too and then you have to wait for life to move back in.

Keep weeding and over time you will have less weeding to do. You will never get rid of all weeding entirely.

to sense

..

    Bookmark   January 31, 2010 at 1:10AM
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tropical_thought(San Francisco)

I would say no, it did not kill my weeds at all. They laughed when I poured boiling water from a kettle on them. I don't think it really kills the weed seeds in compost. I don't think hot composting kills them either. It does not get hot enough. Maybe in labatory settings one could get it hot enough, but pouring boiling water would kill the worms and the micro herd which are the good soil microbes. Microwaving the compost did kill the seeds, but it's not easy to do it on a big scale. Weed seeds of poa annua are very tough. I think they can live in the soil for like 20 years, and nothing short of nuclear blast can kill them.

    Bookmark   January 31, 2010 at 10:03PM
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rott

..

I used to kill the tap root of dandelions pouring boiling water from a kettle into the hole after I dug up what I could with the fork. Because the soil absorbed the heat so quickly, it would not kill off the grass surrounding the hole and sometimes it didn't reach the whole tap root. I don't do that anymore because I don't get established dandelions anymore.

I imagine poa annua is tough to overcome based on all the websites devoted to controlling it. Maybe the problem is less that the seeds are so tough but that there are so many seeds.

..

    Bookmark   January 31, 2010 at 10:38PM
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