I have been using a heat mat to germinate my tomatoes for several years. I use a temperature regulator set at 80 degrees. It keep the soil within a couple of degrees of the setting. Today it was reading 81.6 degrees. Then I place a piece of plastic wrap over the seeds to keep the moisture and heat in. One thing I am always ambivalent about is when to remove them from the heat mat and when to take them out from under the plastic wrap. I plant 3 seeds per cup, assuming I won't get 100% germination. Would you remove them from the mat and from under the plastic as soon as you see the first seed starting to germinate in a cup or wait for 2/3 or all 3? The mat clearly helps because I have some germinating early on day 3 after planting, which I don't believe happened before I started using it. It was probably more like 5 days. Also, would you remove the plastic as soon as one germinated or also wait a bit? I am leaning towards taking them off the heat mat as soon as I see a little curling green stem peaking out just because it isn't technically necessary and it could encourage fungal growth. I have already removed a few small areas of fluffy white growth on the top of the medium. The plastic wrap keeping the moisture in can both help and hurt. Its keeps the ungerminated seeds moist, and I wonder if it also keeps the seed coat softer and might decrease helmet heads, but like the heat mat it also increased the chance of fungal issues. I don't have a cool spot to grow my seedlings in so they are grown at room temps of about 70 degrees so they tend to grow fast enough as it is and if left on the heat mat too long they also shoot up and get leggy very fast. I guess I am leaning towards taking them off the mat, but not sure about the plastic wrap. If the top 1/8 inch of soil dried out the seeds would probably not germinate, although for the first time I am home during the day to check on them and give them a spritz if necessary, but I don't want it to become a full time job babysitting them, lol.

Fortunately, knock on wood, I have never had a problem with damping off or fungal issues damaging the seedlings themselves, but I am always mindful of the possibility.

I only use a heat mat for my peppers. I turn the heat down to 75 from 85 after about half the tray has germinated. However, I turn the heat down instead of removing the mat because I start them in January and it is generally a bit colder in the room where they are located. If I was to use a mat for tomatoes, I would just remove the mat completely after half germinated since the location should be warmer in March/April.

I generally do not use domes or plastic wrap. If I did, I would probably remove it around the same time as the heat mat.

That being said, I did use a dome for my first set of pepper seeds this year. I think it created a problem where my cotyledons yellowed/dried up sooner than I had expected after I removed the dome. Not sure if it was due to the increased light intensity or heat/low humidity in the room. Plants are all totally fine, but that was the first time that happened to me.

No dome on my second set of peppers. They had no problems and are all looking great so far.

@Sue_CT I tried the method linked below for the first time this year and so far have had decent success after two days on some jalapeño seeds.

Yes lots of people do that. My seeds are already planted and I have not wanted to add an additional step to germinating and then planting. If they germinated in baggies or paper towels at the start of my schedule of 3 days in a row working or worse, my 3 days on, one day off, two days on, they would not get planted for 3-6 days after germinating. Plus it seems like more work to label all the baggies with variety, I would have so many different bags because I only grow 1-2 plants of each variety, labeling them all and then labeling all the cups again when I plant the sprouts, it just seems like more work. I need to be able to stick the seeds in the soil and let them go other than some quick checks or spritz each day. In the past I have had at least 80% and usually 90% germination in seed starting soil. I am just trying to perfect the method. I don't really understand the need to germinate them before planting instead of just letting them germinate in the medium.

I use a clamshell instead of plastic wrap while things are on the heat mat. You can leave it on if there are helmets, and the extra humidity helps.

For your existing setup, if the plastic wrap is in contact with the soil, then I would say remove it if you're seeing any sign of molds. Some wierd stuff can grow if there's moisture, dirt and no oxygen flow. Give it a good spritz and some air.
With regards the heat, I tend to keep them on the mat until I see most of the seedlings are up. But when you get in between stages ie some are up but half are not, it gets tenuous like you said, and becomes a somewhat anxious babysitting job. Not only the fast ones can get leggy but the roots can get cooked and nasty in the higher moisture environment next to the heat as I managed to do at some time in the past. Sometimes I'll hook em out and pot them up as they shuck their seeds. But for keeping them all together, turning off the mat at night at least seems a good idea. If you don't see them coming up at the desired rate, put them all back on the mat for a morning or afternoon.

No, I never put plastic or anything else in contact with the soil. I leave at least 1/2 inch between the soil and the top of the cup. These are small little dixie type cups like they used have dispensers for for your bathroom. I think I am dating myself now, lol. I think I weighted down the edges of the plastic wrap too much in an attempt to protect them from the 2 cats, but in truth they haven't seemed too interested in them so it was probably unnecessary. I took the ones sprouting out from the mat and plastic, but I might put all but one back. The one cup has all 3 seeds starting to sprout already. So they really don't need the heat anymore. They might need the moisture, I will have to watch for helmets.

This is a picture of my setup. It is not from this year but is identical to what I have right now, right down to the heat mat. Every cup has the name of the variety on it so I don't mix them up.

That setup looks fine to me, @Sue_CT . Shouldn't be a problem to keep them under plastic on the mat until they're all sprouted. I would just remove the plastic top once or twice a day to make sure the volume of oxygen isn't used up in the top of the cup, that is if the plastic had contact all around the rim. Which you're probably doing anyway if any of them needs a spritz.

One thing I've noticed is that the volume of soil seems to affect how hot it gets - or maybe it's the depth that matters. Anyway I think if it is too hot, the seeds respiration may deplete oxygen in a small enclosed space. I make sure to open my clamshell too, just to be sure there is air exchanged, just in case.

@Sue_CT are you using a soil mix with micorrhyzae? That's something that I noticed causes the white "fluff", but doesn't seem to hurt anything.

I always sprout my pepper seeds in some vermiculite, sitting on a griddle on my large range - has 4 pilot lights under it, so it is a very warm spot! I put some spacers under the tray, adjusting them a few days early, to get the temp at 88-90°. After sprouting, I dig them out with an olive fork, and plant them in the pots on a heat mat in the seed room, but not too warm - 78-80°, which is what I'll be starting tomatoes on. I keep a dome on the seeds when starting, and moisten the vermiculite with a dilute H2O2 solution - about a tsp to a pint of water.
Here's the tray I sprout the pepper seeds in, on heat.
Strip tray I use for starting peppers in vermiculite , on heat. by pepperhead212, on Flickr

I have an electric range so that won't work for me. But the heat mat with a temperature controller works fine, and I have already invested in it, so I might as well use it.

Using Miracle Grow Seed Starting mix. It does not list it on the package, but I have some Espoma organic that I used last year that I had some left and it does list it. I think most of the ones I have used in the past do. I would have assumed the MG would too, but it doesn't specifically list it.

I pre germinate my seeds in water. I tried with a paper towel and plastic bag but accessing the sprouts was too much work and the paper fell apart. Instead I use a tiny bowl with about 1/4 inch of water. This year for the first time I placed the tiny bowls in a Pyrex glass rectangular storage container with some water in it. The storage container is placed on a counter top electric burner set to a very low setting. I first placed a ceramic floor tile to help distribute the heat. The temp stays between 75 and 80 degrees F depending on room temp which is rather cold this time of year. I place a small piece of tape on top of the storage container lid with the name of the tomato.

I pre germinate because I don't like wasting my time planting seeds that won't germinate. For example I have some seeds from 2005 which I guessed might not germinate. I placed about 30 in a bowl and several days later only one has germinated.

@pepperhead212 That strip tray looks pretty interesting...where did you get it?

Since all of my seeds are less than 10 years old, most less than 5 yrs old and I get 80-90% germination, I don't worry about wasting my time planting them. But I can see how it would be helpful for very old seeds.

@pondgardener I probably wouldn't even have remembered it, it was so long ago I got them, if it weren't for the fact that there are still 3 in the original box they were in! (Only one started cracking after many years of use, and I'm on the second one) And on the side it says JOHNNY'S SEEDS! I checked, and they still have them.
They are shallow - only 1" or so - and I remember them recommending it for starting onion seeds, though that's not why I got it.
https://www.johnnyseeds.com/tools-suppl ... t=19&sz=18

@pepperhead212 Thanks for the link...1" depth would seem to be perfect for starting something like lettuce or onions early.

@Sue_CT I didn't intend to imply you were wasting time with your method. I was referring to myself and one reason I pregerminate. I have a mix of old and new seeds. Your method is certainly efficient. Is it one and done?

Why are you using small pots? Do you plan on up-potting or are those the final pots? I use 2 1/4 x 3 1/4 inch rose pots. They make efficient use of space because of the rectangular shape. The pots are reusable, I have been using the same ones for about ten years. I plant the germinated seeds just one time. If I guess wrong for plant-out date I may have to up pot.



Also are you watering from the bottom? You should be able to keep the soil damp without the need for the plastic cover if they are kept in a tray.

I do pot up to solo cups later. So yes, I plant twice. The only reason is that it saves a lot of growing medium. Otherwise I would have to plant each seed in its own solo cup and those that never germinated would be wasted. Once I pot them in the solo cups it is one plant per cup. Plus if the plants are getting a little leggy by then I have the chance to plant them deeper in the solo cup. My plants are larger by the time I plant them out than would be good in the size pots you are using. This is typical although an old photo. This is on May first, and my plant out is May 15-30th, usually later than earlier in May. This is before starting to harden them off and they are already getting a little leggy inside. I would not want 3 plants this size in your sized pots. So they would end up being potted up again no matter how they were started. But then I would be planting 3 times.


2012-05-01 Tomato seedlings

I planted 81 seeds on St Patrick's day and only 17 have not yet started to germinate on day 6, so that is 80%. I am hoping for some slow starters, but this is why I like to start my seeds earlier than would normally be recommended. I have time to reseed and even order fresh seed if I need to for a couple and still have them ready in plenty of time to put good sized plants into the garden. The great majority germinate by day 4 but I am still getting a few each day. Now for the helmets. I have had quite a few come up with helmets on, and although many probably would have shed them on their own, every one slid off pretty easily after a good spritz with water with just my fingers. So I think the plastic wrap helped, and it was good to keep it on until they were all germinated in a cup. I think it keeps the seed coats soft and pliable. But since they were easy to remove I decided to do so rather than keep them under the plastic after full germination so I didn't run into problems from them being too wet. Once I loosened the plastic and started lifting it off a couple of times a day to move out those that had germinated, I didn't see any more white fluffy growth. As pepperhead said, that has never actually harmed any seedlings in the past. I usually do scrape it off though. So maybe if you find you are getting helmet heads you can try this loose plastic wrap and water spritzing to keep them in a humid environment for maybe a day before trying to remove them and end up damaging a lot less seedlings.

The problem I have with heat mats is what I came back to tonight - the temp probe on the floor. Probably since last night. And the 4 trays on the big mat at 110F. As long as that doesn't happen, its pretty much foolproof.

edit:
I even have a low temp/ high temp alarm with my newest thermostat, but I had the low temp alarm set too low. I had it set a few degrees above ambient temp. The probe was on the table, I guess, so caught heat from lights to keep it at 73F. I should have had it set at:
80 target
1degree offset heat
1 degree offset cool
90 hi alarm
77 low alarm
for basement ambient temp in lows 60s yet.
4 trays of pepper seeds, been at least a week, many were up, then at least 24 hours of 110F. We'll see.

The white fluffy stuff on top is not a problem, it happens, doesn't hurt anything. I keep plastic humidity domes on until I start seeing most of the blocks have some emerging loops showing.

How does it end up on the floor? That said, I have cats, sooo....