Fire in the Hole! Don't try this at home :-)

Frederick, MD(Zone 6b)

I've read in a few places that it's a good idea to sterilize your soil-less mix before starting seeds, and it seemed like a good idea.

The directions I saw said 6 minutes in the microwave would do it, but of course I had to improvise. I thought, well, I sterilize canning jars and so forth for 10 minutes, so wouldn't that be better? Well, the soil is in the microwave, and I'm on the phone with the mechanic, when I smell smoke.... I dash around the kitchen, sniffing, phone to my ear, and I track the source to a little tendril of smoke emerging from the microwave. Egads! I turn off the microwave and peer inside, and I see tiny coals among the soil particles. Egads!! Fortunately, the tupperware container isn't melting or anything, and I slap the lid on to deprive my little fire of oxygen. I quickly finish my business with the mechanic, who is wondering why I seem suddenly distracted, and I take the container of soil outside to cool down. Egads!!! Who knew??!?!! Well, no harm done, and I'm pretty sure my mix is sterile now!

I though I would pass this story along as a word of warning not to exceed that recommended 6 minutes in the microwave!!

BTW, the Pro-mix plus perlite mixture in question has been dampened for use, but still has a smokey smell (imagine that). Do you suppose there's any reason not to use it for winter-sowing?

UPDATE I've learned a few things since this incident, although it still cracks me up, especially the mechanic's reaction when I said, "Aaagh! 'Scuse me, smoke... fire... " and then set the phone down ("Hey, lady! Lady??!? Are you OK? Lady!!").

You MUST add enough water to dampen the potting mix; I'd say 1 cup water per quart of mix. I use a big plastic container that I get from the commercial kitchen shelf at Costco (I think it's for dough raising; it has measurement marks and a yellow lid). I add water, put the lid on loosely, and microwave for 12 minutes or until I see good steam. Then I carefully tamp the lid down into place for another 15 minutes so the steam can do its work.

In addition to killing pests/pathogens, this gives me nice fluffy moistened potting mix to work with. I do add extra perlite and/or polymer moisture crystals to my mix before nuking (just for convenience), but I don't add time-release fertilizer (like Osmocote), because overheating can cause it to dump its fertilizer all at once (not good for seedlings).

This message was edited Jan 19, 2013 4:56 PM

Dansville, NY(Zone 6a)

i was just reading a few blurbs i found on a google search on micowaving potting mix.
they all said to not go over 200F temp on the soil or it will relese toxins into the soil.
i cant remember off hand but it was from some of the salts in the soil.

I just took a small cardboard box that would fit into the micro and jabed the temp prob
into the side of the box so it was in the middle of the soil ..

I 'll go off and google again and post that microwave info if i can find it.

edit ...

This message was edited Feb 16, 2005 4:54 PM

Frederick, MD(Zone 6b)

ahhh. Thanks. I will toss the mix and start again. No temp. probe on my microwave, so maybe this time I'll bake it in the oven so I can control the temp. Our old microwave is a pretty powerful one, so if I try that again I will do it at a much lower setting!

Frederick, MD(Zone 6b)

I have soil heating in the oven now, but upon further googling I am going to take it out and just rely on the use of hydrogen peroxide water for wetting. I'm not trying to prevent damping off, which has never been a problem for me, and this isn't previously used soil. I'm just trying to prevent fungus gnats, which are really just a nuissance rather than an actual problem, and maybe the hydrogen peroxide will discourage them. I may try again sterilizing soil to use for indoor starts, but I think I'm likely to do more harm than good with the winter sowing I'm trying at the moment...

Thanks, Sparky, for warning me before I sowed my seeds into toxic soil!

Dansville, NY(Zone 6a)

no problem, i know how the bugs are. i just got over (i hope) a case of the
spider mites and whiteflys. i had a bag of some pro mix soil, dont recall the
brand and the stuff was stored outside at a farmer market and ther were holes
in the bag to start with. Where i think i got all my bugs from. i was growing
starwberries in the spare room i have, had like 30 plants fruting in november
when i got attacked by the mites.. lost the battle .. hopefully this microwaving
will kill the bug in it if there was any ..

Waxhaw (Charlotte), NC(Zone 7b)

Remember to moisten the soil before trying to sterilize it in the microwave.

Frederick, MD(Zone 6b)

you know, I think that was what caused the fire..... I should've looked for more information before popping in a big bowl of dry soil mix! Anyway, I baked it in the oven for a while.... Next time, I may try the microwave again.... but very cautiously.... :-)

Fort Wayne, IN(Zone 5a)

I have managed to start a fire in a microwave before but never with potting soil. To pasturize the soil in a microwave you need to add lots of water. I add a minimum of 1 cup of water to 1/2 gallon of soil and heat for 5 minutes or 2 cups of water to 1 gallon of soil for 10 minutes. Then when it is finished the container is closed tightly so no steam escapes. It is the heat of the steam that kills the insect eggs and weed seeds as well as bacteria, etc. The release of toxic fumes when heating the soil in the oven(mentioned in the Colorado State publication) does not apply to the microwave method. In fact it may not even apply to the soil in the oven heating method if you read the publication. The moisture in the soil heated in the oven simply goes away as you bake it out of the soil and never gets to the temperatures the water gets to in the microwave. In the oven the soil will become surface hardened and water resistant. In the microwave the steam penetrates the soil particles and any clumps break up. Steam sterilization was used in commercial planting operations for many years before chemical sterilization became common. Jesamine

This message was edited Feb 20, 2005 1:59 AM

Frederick, MD(Zone 6b)

Thanks, Jess! I may try your method in the microwave for the next batch. Your explanation about steam sterilization in the microwave makes good sense. I haven't noticed the soil mix becomming hardened or water resistant from baking in the oven, but I tried to cover it pretty well to keep moisture in. It did take several hours at 225' for the soil to reach 180' in the oven, but I stuck a temperature probe into it to keep track.

Palmyra, IL(Zone 5b)

Doesn't it stink?

Frederick, MD(Zone 6b)

I didn't think it was too bad, but I kept the lid on the roasting pan while in the oven, and I put the pan outside on the deck to cool down. I lifted the lid a bit and put it back quickly, because there definitely was an odor, but I think the lid kept it contained.

I cross-posted this little incident on the trading forum, and Weezingreens posted to say she always does hers in the oven. She uses turkey bags with cookie sheets underneath and does 2 bags at a time with the oven set to 200' or so. She uses a thermometer to track the temperature and keeps it at 180 - 200' for 30 minutes before turning off the oven. Since she's been using the same starter mix for several years, sterilizing it each season, her method must be OK.

Crossville, TN(Zone 6b)

I am bumping this up as it answered my question. Thanks, Linda

SE Houston (Hobby), TX(Zone 9a)


I've been sterilizing coco coir in the microwave for two seasons now, with no adverse effects to the growing medium.

I didn't know about the 200 degree maximum heat, though. I put enough water in my container to keep the coir from bursting into flames, and nuke it for 20 minutes on high, or until it starts to steam. Never covered it to capture the steam, although I will from now on.

Thanks for the damping off tips, too!


Santa Cruz, CA(Zone 9b)

wow i should try this for my more difficult seeds. Sounds interesting.

(Sheryl) Gainesboro, TN(Zone 6b)

I wonder if I just have really tough weeds - they are still sprouting in the soil, even after microwaving for 10 minutes.

Frederick, MD(Zone 6b)

If I don't sterilize, I often get some sort of little weed seed sprouting in commercial mix (both Miracle Gro & ProMix)... I'm not sure what they are (I don't let them grow to adulthood, LOL), but I don't see them in mix that I've nuked.

I guess your weeds are extra hardy, pagancat!

(Clint) Medina, TN(Zone 7b)

I always use Jiffy Seed Starting Mix. I don't cook mine at all. I mix a little hydrogen peroxide in with the water I use to wet it before sowing. I never had a problem even without the peroxide though.

Mackinaw, IL(Zone 5a)

Turkey bags. . .that is a stroke of genius! I was trying to figure out what containers I could use, and cringing at the idea of filling my baking pans or plastic microwaving containers with potting mix. LOL at how often you said "Egads!" in the first post, Jill!

Frederick, MD(Zone 6b)

I picked up a pair of containers from Costco, big ones with yellow lids that I found near the professional kitchen stuff (pots, pans, chaffing dishes). I think they're meant for dough raising or general dry good storage, but I use them only for potting mix. (They're also a good size to measure an extra scoop of perlite, sand, or other add-ins.) They fit into the cavity of my fairly big microwave oven... with the one we had previously, I used the really big tupperware "bread bowl," probably a similar capacity but shorter and wider.

Thanks for the bump -- I updated my first post!

Chevy Chase, MD(Zone 7a)

I heated up a soil mix in the mike years ago -- and it destroyed the mike. I had it very very full -- I'm sure that is what I did wrong. But as a result, I'm wary about doing it again. So I recommend a wee bit of caution.

Everett, WA(Zone 8a)

Maybe spraying the surface with "insecticidal soap" would knock out the fungus gnats, if hydrogen peroxide doesn't help.

Or sprinkle coarse chunks of pine bark on the surface, so that the exposed surface stays dry, and the soil mix under the coarse mulch stays moist.

I keep thinking about vermiculite (expanded mica) and Turface (expanded shale). And Perlite (expanded something).

I wonder if I could expand or "pop" bark nuggets in a microwave, to make them more porous?

Frederick, MD(Zone 6b)

Put the fire department on "speed dial" first...

Seriously, sounds like it could be a fun experiment. Maybe soak the bark bits first, so as the water turns to steam the small water-filled cavities will explode outward. You probably want to experiment on a very small scale... LOL

Everett, WA(Zone 8a)

YES to all of the above suggestions!

Recently I've been thinking about the fact that CHEAP bark mulch tends to be scooped from logyard trash and may have weed seeds and soil microbes ("dirt").

If I was making it commercially, how about high pressure steam sterilization after screening? That would get the nuggets wet and pre-heat them. Then blow them through a high-power microwave, or just really hot air, and see if they pop.

Before that, a pilot plant.
Before that, a home-scale test.

Before that, I should blow up just a few ounces at a time, so the ER visit and newspaper articles aren't TOO embarrassing.

Frederick, MD(Zone 6b)

I'll keep an eye on the AP headlines

Ottawa, KS(Zone 5b)

Hi Critterologist,

"If I don't sterilize, I often get some sort of little weed seed sprouting in commercial mix (both Miracle Gro & ProMix)."

I have used Premier ProMix BX for years, with no weeds apparent, and I don't heat the product or anything to kill weed seeds. I do pick out a number of small sticks and such, but only because I am crushing and kneading the compressed product with my bare hands to loosen it up, and the little sticks just kind of annoy me. After loosening it, I mix in some extra Perlite. I buy the compressed bales because it is a little cheaper that way.


Frederick, MD(Zone 6b)

I haven't noticed that little weed for the past several years, so maybe it wasn't in my mix. I only sterilize the mix for seed starting trays or special cuttings... no way am I going to run a bale or two of Pro Mix through the microwave 2 gallons at a time!

I'm with you on liking Pro Mix BX.

Ottawa, KS(Zone 5b)

" I only sterilize the mix for seed starting trays or special cuttings..."

I think that is a good idea. There are seed borne diseases. I am even trying to think of a good way (easy to do) of sterilizing my seeds themselves. I know that at least some commercially available seeds aren't sterilized.

There is a cheap way to sterilize zinnia seeds by a technique similar to pasteurizing, but it requires precise temperature control to cook the bad guys while not killing the embryo. Some, but not all, seed producers use it.

All the chemical methods I can think of involve wetting the seed with a solution containing the chemical (hydrogen peroxide, chlorine dioxide, Physan 20, etc). That wouldn't be a problem for indoor hand sowing, but my outdoor zinnia sowing involves dropping the seeds from a plastic funnel/bowl down an aluminum tube that is aimed to place the seed. Wet, or even damp, seeds would simply not slide down the tube well. I like to plant my seeds from a seated position on a rolling tractor scoot to avoid standing or stooping while doing it. (As a "senior" citizen, I like to baby my back as much as possible.) Ironically, Powdery Mildew isn't a problem for me with my zinnias, but several other diseases, primarily Alternaria, which is seed borne, are somewhat of a problem.


Frederick, MD(Zone 6b)

After I harvest zinnia seeds, I dry them for a while on paper plates... I wonder if you could soak them briefly in a hydrogen peroxide solution and then spread them out to dry? Or would they all start germinating? You might try it with just a few and see how it affects germination after they're dry again.

I routinely soak pepper & tomato seeds before sowing them, not sure if my splash of hydrogen peroxide is enough to kill pathogens or not, but it's probably better than nothing. For tomato seeds, I just try to be sure any non-commercial seeds have been fermented, as that makes a huge difference.

Ottawa, KS(Zone 5b)

" I wonder if you could soak them briefly in a hydrogen peroxide solution and then spread them out to dry? Or would they all start germinating?"

The suggestion of experimentation is a good one. Also, after I place the unsterilized seeds in a shallow furrow in the garden, I could "water them in" with a sterilizing solution like hydrogen peroxide before and/or after covering them. I'm going to try that, and label the rows that have been treated with what, to see if there is any significance in foliage diseases like Alternaria.


Lake Stevens, WA(Zone 8a)

Hey ZenMan- could you give us a photo of your seed sowing setup? I want to make one as a gift. Maybe you could start a thread about it.

Ottawa, KS(Zone 5b)

Hi mlmlakestevens,

I don't have current photos handy, but these was taken a few years ago. I am still using the same setup. I use chrome wire steel shelving to support everything, and I hang 4-foot 2-bulb fluorescent shoplights from the shelves above, using wire chain and S-hooks. The individual shelves are 2 feet deep by 4 feet wide. That makes room for four standard 11x22 trays per shelf. I use PermaNest trays because they are rigid, and last many years. The shelves are 6-feet tall. I usually have just three shelves per unit, with a fourth top shelf. That gives my plants some growing room.

I start my seeds individually in 3.25-inch square pots, to avoid an initial transplanting. Premier ProMix BX is my favorite indoor growing medium. I add some extra Perlite to it for better aeration and drainage. When my plants start to get a little root-bound in the 3.25-inch pots, I transplant them to the garden or I re-pot them to 5-inch square pots. Those usually get transplanted to the garden, but occasionally I will re-pot to a larger pot for a more mature indoor plant. Mostly I just use my shelves to start seedlings, though. Zinnias are my hobby, so most of what I start is zinnias, although I also start a few other things, like tomatoes. I label each pot individually, and that label stays with the plant in the garden for the life of the plant. I save seeds from my best specimens.


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Frederick, MD(Zone 6b)

Z-man, that's so much prettier -- and neater! -- than my setup! I do use perma-nest trays, though... lucked into a sale on them a few years back, and they hold up so much better than those black $1 ones (which are probably $2 by now LOL).

We should start another thread for showing seed starting setups, because everybody seems to do it just a little differently, and there are such great ideas out there.

There are a couple of photos of my setup in my seed starting 101 article about lighting:

Milton, MA(Zone 6a)


Did I read right? You can get a mature, flowering zinnia indoors? We have lots of sun at our new little house...

Lake Stevens, WA(Zone 8a)

ZenMan-I was very interested to see your impressive setup, and I think I will get some of the trays for my sister, who is seed starting this year. I like critterologist's idea. However, I was not specific enough about what I hoped for photos of: the funnel/aluminum tube thing you use for sowing seeds in the ground! It sounds very clever.

Columbus, OH

I sterilize soil in the oven, I never even thought of the microwave! I've been tempted to wash seeds that tend to rot with some chlorhexidine solution before planting, betadine...something.

Ottawa, KS(Zone 5b)


"Did I read right? You can get a mature, flowering zinnia indoors?"

Yes, you can get mature flowering zinnias indoors. In fact, I have on many occasions grown them through the seed saving stage, to complete the indoor generation and get seed to start the next generation. However, I don't recommend zinnias as house plants, and I don't think you will see them listed in any book about house plants. I am a zinnia hobbyist, and I have motivation to grow zinnias indoors, to extend my hobby through the Winter. It is encouraging to me that I never have to stop growing zinnias.

But as house plants, they are high maintenance and, for most people, would be more trouble than they would be worth. They grow very fast, quickly outgrow almost any pot you put them into, have a high intake of nutrients and water, and are susceptible to indoor pests that aren't normally a problem outdoors. Aphids and thrips are potentially devastating insect problems, and spider mites are another potential disaster that are immune to most insecticides (because they aren't insects). I have developed methods of dealing with those problems, but they go far beyond what most people would put up with. In my opinion, only a devout zinnia hobbyist should consider growing zinnias indoors, except for a few weeks to get an early start on a Spring planting.


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Ottawa, KS(Zone 5b)

Hi mlmlakestevens,

" I was not specific enough about what I hoped for photos of: the funnel/aluminum tube thing you use for sowing seeds in the ground! It sounds very clever."

I don't have any photos of it handy. It seems to be a discontinued commercial product. When I get a little spare time I will take pictures of it and post them. I plan to make an improved version of it myself, as a backup for the commercial product I now use. Basically it is just an improvement over the old funnel and tube homemade rig. I will be making a modified version of the seed bowl and funnel piece (customized for zinnia seeds), and attaching it to a length of aluminum tubing from a home improvement store. I will also experiment with a length of clear plastic tube, but I suspect that static electricity will be a problem with it, causing the zinnia seeds to stick to the clear tube instead of falling down it.


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