I just started my first seeds last night. Onions. I put the heat mat underneath with the controller set at 68F. And set the LED lights for 12 hours daytime.

Typically I run the heat mats on all the time until a goodly percentage are sprouted. Problems mostly occur at the edge of a mat where it is colder, and they don't sprout as fast. The warm center of the mat sprouts sooner and they tend to grow too quickly and can get leggy in the first few days.

So I am thinking now that I should install the heat mats in the same circuit with the light-timer. That would allow everything to cool overnight and perhaps limit the legginess that occurs when the seedlings are unnaturally warm, and search for light in the dark.

So my answers are, Often, and 24 hours a day. But I think I should change to Daytime only.

I'm wondering what everyone else does?

I use a heat mat to germinate most of my seeds (whatever there's room for on it). Like you, I leave it on all the time until a substantial percentage of the seeds have germinated, then I remove it entirely and use it for whatever's next in line. I tend to start my seeds in 4-pk or 6-pk inserts, so I can remove individual inserts that germinate more quickly, and leave the laggards on the mat longer. Sometimes I start in little Dixie cups, which gives me even more flexibility about removing germinated ones from the heat and leaving the others.

One year I used one of those 32(?)-hole starting trays and hated it because of the different germination and growth rates of the different varieties I planted. I couldn't figure out what to do about the heat mat, and then later the early sprouters overshadowed the slower ones and made the problem worse. I guess they would work well if you're growing a whole tray of the same thing. Straying kind of off-topic, sorry!

I never used them. I’ve got nothing against them either. Just haven’t had the necessity of using a heat mat to get good results for germination and overall growth. All twenty four varieties of tomatoes I planted this season sprouted within a couple of days of each other, except two or three types that were much older seed, but they also eventually sprouted a few days after their peers.

My many types of peppers, all but one type, germinated. All the eggplant sprouted.

I’ve got watermelon, Squash, cantaloupe, marigold and cucumbers seeds planted in their 20 ounce plastic containers filled with potting mix outside in the sun today. I’m hoping the sun can provide the necessary energy to make them germinate.

Just to answer a question I had about the sun’s ability to heat the soil I moments ago went out and measured the soil temperature of these melon, cucumbers, squash set containers. Air temperature currently 71° Soil in containers at or slightly below seeds level ran from 77°-85° depending on the container.

Cell phones won’t capture digital displays

The lights keep everything warm enough.
Because if I do start seeds the light goes on day one.

worth1 wrote: ↑Sun Feb 04, 2024 4:28 pm The lights keep everything warm enough.
Because if I do start seeds the light goes on day one.

If mine aren’t out in the sun, they are under the lights with the lights on.

I use an electric cook top to start seeds that need warmth. My house can get rather cool in the off season because I rarely use heat. It's not unusual for room temps to hover around 60°F (15°C). Peppers don't like it that cold. I tried a heat mat but find I have better control with the electric burner.

The image indicates blocks of wood. I now use small blocks of cut clay brick using a tile saw.

If necessary I follow up once planted with heat from an incandescent light.

I do sometimes use the heat mat 24 hrs, and that may be best when room temperature is really too cold. But I had good results using day only heat mat, so I'm doing more of that now. It is much faster for germinating parsley, celery, cilantro. And I think peppers and tomatoes benefit as well, as long as night temp doesn't drop too low.
Caveat - my heat mat has no thermostat and seems to heat unevenly, leaving me a bit worried about seeds actually getting cooked or the damp mix getting anoxic when tamped shut at night in the heat. A certain uneasiness about that got me started on day only.
I always do the cooler tolerant vegs like lettuce and brassicas under the lights, instead of heat mat. And they do get shut off at night, so they get the day/night temperature difference.

I only use a heat mat (and thermostat) for peppers. It's just a little, square 15W one, so having it on for 24 hours doesn't cost much. When they've germinated I move them to an unheated grow tent. Tomatoes, cucumbers, sweetcorn etc. all germinate well on a sunny South facing window. Everything else I leave at room temperature on in the greenhouse.

I've never used a heat mat. My small pots or cell packs are covered with plastic wrap and set under the lights. The lights keep them warm enough that germination is good. Brassicas usually pop up in 2-3 days, tomatoes in about 7.

Out of curiosity, just now I used my digital cooking thermometer and took the soil temp in the cell packs I started two days ago. It's 69 which is what we run the furnace at during the night in winter. I just upped it to 73 for daytime so I'll check the temp again this afternoon.

I switched up my heat mat when I switch up my lights..... I am now using the Spider Farmer heat mat which has a temp probe for the soil. https://www.spider-farmer.com/products/ ... oller-set/ For years I used a heating pad but it was hard to regulate. I'm in New England and keep my house cool. I use a seeding tray with lid and inserts - do either 1 or 2 seeds to a cell and pot up when they sprout.

If the weather was such that I could not run my newly planted seeding containers out in the sun after seeding, then I think I’d be forced to use a heating mat, at least with the peppers.

Once the great majority of the seeds are sprouted and have emerged from the soil, then I’m trying to shed any excess heat around the seedlings environment inside.

In other words, the house tends to run a little cool in the winter for seeds to germinate well and in a timely manner, seeds like tomatoes, eggplant and especially peppers, even in the warmest closet with grow lights over the trays, but I time my seeding date to coincide with a stretch of weather that allows me to place the seed containers in the warmest place outside I can find in direct sun. A couple of days like that really jump starts the germinating and then the warmest closet with the grow light on is warm enough to finish the process.

Once everything is up, then the hope is and it generally works out that the house remains cool enough, 60s, to be pretty good for these seedlings, at least for two or three weeks. Then the garage offers an intermediate zone cooler than the house and warmer than typical nighttime temperatures in the grow out time period in January and February.

I’d rather have two to six week old seedlings closer to 60° than 70°.

I've been using heat mats for six years and get good results. But I also starts my seeds in a basement room where the ambient air temp is 72+ now that my in-laws moved in with us and cranked the heat downstairs. I find now, at 72 to 76f in there, that I only benefit from the heat mat for germinating peppers, tomatoes and eggplants. I do run the heat mays 24/7 during germination. Once they germinate I turn the heat mats off, it's already to hot down there for me.

Our basement air is set at about 60-65F so I do use a heat mat for tomato and pepper seeds started downstairs. Mat has no temperature control and runs hot, making venting tricky. Plastic domes need to be propped up with pencils and pens on a couple of corners, which can be viewed as cat toys and snagged out. Last year I discovered domes which have two adjustable spin vents on top which were helpful. I normally turn the mat off at night, and close the venting to conserve heat. I used to leave heat on day and night, but I since don't have lights on at night, the seeds sprouting at night often came up too leggy if they were kept hot. Turning off heat and light mimics a natural cooling cycle which I think is beneficial for seeds. I had fewer helmet heads on stringy stems.

The heat mats used during germination are under all seeds until germination. That occurs without lights until sprouting begins. Only one set has a temperature control and that is reserved for peppers. The other mats all increase temperatures about ten degrees above room temp which in my basement is between 65 and 68 degrees F. Peppers are kept at 85F for the duration. After tomatoes are in the true leaf stage, the mats are removed under them.

The fluorescent lights used do not add heat to anything and are on for 16 hours per day and off 8. Window grown seedlings are just too leggy compared to the artificially lighted seedlings where the lights can be placed an inch above the tops of the plants. For me, this regulates the growth and makes the plants healthier and sturdier. The basement growing area will not heat up until May which is hardening off time anyway.

My solution to legginess is to start the process for peppers March 1 and tomatoes March 10. I seem to get impatient some years and end up with two foot tomato plants. Oh, yeah, the portable fans go on about halfway though the growing cycle for couple hours per day (lights and fans are on timers) and the trays are rotated every other day when I check how tall the seedlings are getting and whether there are any other problems to take care of.

I use a heat mat for tomatoes, peppers, squash, and tropical flowers. Seeds are in individual pots and come off the mat when each germinates. The mat is on 24 hrs.

I use a heat mat because the temperatures here can be quite cold, even in April when I start most of my seeds. I leave the pots on the mats until the seedlings have sprouted, then take them off so I don't get the leggy growth. My mats are very simple: no probe or temperature control. We've had serious snow, including blizzards, in April so better safe than sorry.

Great responses so far.

The February basement is currently 53F, here. Cold wind outside today, might have got to 40F, down to 30F right now. 71F in Texas, huh...

I like it cool. Just me and the dogs here now, so I have the upstairs set at 57. It will heat up soon enough.

From the first, or very close to it, I have used an inline temp controller for the heat mats. Without it, stuff will cook. Probably not so much with plastic cups/tray inserts with all that surface area, but the soil blocks will really hold the heat. The controllers work well for me, assuming they don't fail.

As soon as possible I want to get this stuff sprouted and out into a walk in plastic fantastic hoop house. Bought it for $60 in November. Something like 13x5. Gonna put it at the comm garden for anyone to use and hope it doesn't blow over. That is always the worry. It will be a while until I start long season stuff like peppers and tomatoes, but I'm hoping to sprout the early spring stuff and get them outside asap. Onions, spinach, collards, kohlrabi so far. I have not had a whole lot of luck with onion seedlings the first couple years...

I use a heat mat with an added digital temperature gauge. I put the probe in the center container and loosely cover them all with plastic wrap. I check they daily. Ok, some twice or three or 5 times a day, lol. I remove the individual cups when the majority of seeds in that cup have sprouted. I keep it on 24 hours but it shuts off when the soil reaches the set temperature and back on when it falls below.

I leave my heat mat, thermostat controlled, on all the time. As I germinate all seeds outside it can be the difference between having to restart seeds or not on our cooler, frostier nights as there is not always the space to move everything inside if I had too.