Wow! I thought that heat mats were purely for germination, but I don't use them. I grow my tomatoes and pepper seedlings under lights in the cool basement (50/55 degrees F.) where it keeps the plants short yet sturdy. Peppers do like warmth though, so they might do better with heat mats. I will be interested to hear what others say.

Linda

"I thought that heat mats were purely for germination". You could be absolutely right. I might be overdoing it. Thanks for the reply, let's see what other folks say (this is my first time starting seeds indoors).

Eric, the additional bottom heat just for germination, put it away once the seedlings come up. Leaving it on not helpful and may even be harmful, may spur rapid, lanky, weak growth. Rapid weak growth will be more susceptible to disease and insect damage. Rapid germination is good. Slow and stocky growth after that is better.

Thanks morz8. I will just use the mats for germination.

I remove the bottom heat source as soon as seeds have germinated. I have three large lighted germination carts, though, they don't sit on the ground. Be sure that your light source is excellent.

I only grow tomato and peppers inside, along with several hundred flowering annuals/perennials, and herbs. My other veggies are sown directly in the ground as soon as weather permits.

Thanks Rhizo. It's interesting... my initial plan was having the seedling trays on the heat mats with the lights right above them. Now it seems like I need to have the trays on heat mats unlit, then move them off the mats to under the lights when the seeds sprout.

Would help to be quantitative about heat and height. Are we talking a "cold floor" at 30F, or one at 50F? I'm not sure what the weather is like in Richmond. Are we talking about one inch seedlings, or five inch seedlings? As long as the temps are above freezing, everything will be sorta OK. But growth is going to be a powerful function of temperature. Below 50F, you won't see much growth at all. At 80F, they will grow like crazy. You need to decide when you want to plant out, and how big you want them to be when you plant out, and heat accordingly. If you have five inch seedlings in four inch pots, and you can't plant out for a month, I'd put the chill on them.

Eric - just use the heat mats until you see sprouts and then remove the trays from the mat immediately. You want the newly emerged sprouts to be ~1" under the lights as soon as they break the soil or you risk leggy seedlings.

Lettuce should sprout in about 3 days without a heat mat and the heat may actually be detrimental for lettuce as they prefer cooler soil.

3 to 4 weeks seems way too early to be planting out tender seedlings like tomatoes, peppers, and eggplants. Here in KC I plant out mine no earlier than late April weather permitting (no danger of temps below 40 degrees in the extended forecast).

Thanks to everybody for replying. Here's what I will be doing going forward.

1) Plant tomato, pepper and eggplant seeds in trays on 80-degree heat mats but not under lights.

2) When seeds sprout, remove from heat and place an inch or two under lights. They'll be on a wooden shelf I built onto the basement wall, with grow lights hanging above them on chains so I can regulate the height. I've got the lights on a timer, 13 hours a day. (Basement is a steady 64 - 66 degrees).

3) When seedlings get two sets of leaves, move to a 4" pot and raise the light accordingly. Before I got cow pots I was using red Solo cups with a small hole punched in the bottom.

4) If necessary, move again to larger pots until time to transplant.

5) Harden off by placing outside during the day and bringing back at night. This will be difficult to do consistently because I travel a lot for work.

6) Plant outside once danger of frost is past.

My first efforts have been more of a trial run, so it's great that y'all are helping me with this advice now. I just started a flat of tomatoes and peppers (same varieties as my seedlings already are) before I left for a work trip this week. So I'll have the chance to do it better this time.

Here's the mats and lights I'm using:

https://www.amazon.com/gp/product/B07F821DKQ/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B009GU4RMC/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

I got two 2-foot lights instead of one 4-foot light, so I can raise one independently of the other according to size of seedlings.

Thanks again,

Eric

Not sure why you need heat mats at all. 65F is a fine temperature for germination. For tomatoes, "optimum" is 65-85F, though it isn't as if they won't germinate if the temperature is somewhat lower. They'll grow faster if at 80F though. So the question you need to be asking yourself is (and I still haven't hear the answer to) is - why do I need my seedlings to grow fast? No seedling is going to come out "better" if germinated at 80F than at 65F.

I throw a blanket over my lights, so the temp under the lights is at 75F or so, and I have three-inch high tomatoes in three weeks. No heaters. They'd probably be half that size if I didn't do that. I plant out next week.

Again, it comes down to how far off your plant-out date is. If it's soon, then grow 'em fast. If not, then slow growth is your friend.

OK, I looked it up. Your last frost date is SEVEN WEEKS away. If you plant now, and keep the seedlings at 80F, your plants will be enormous by the time you can plant them out, and it'll force you to pot up. Seedling temperature can be a tool for planning.

The basement floor is colder than the ambient air. I like knowing that the heating mats provide a constant soil temperature. Also, I like the seeds to germinate earlier if possible. Gives me a bit of a head start in case fewer of them germinate, or I make mistakes, and in another way, it makes me happy to see things growing. I get depressed during the cold months so this helps me feel like spring is around the corner.

I live in the city, in a small yard protected from wind. Our last frost dates are considerably earlier than the USDA charts show. I usually plant my tomatoes by the end of March. But if I have to re-pot more than once, and will have huge plants by planting time, then that will be great too.

Well, if you just put your plants on a board, instead of on the floor, or put an air gap between them and the floor, they'll be at the air temperature, not the floor temperature. That's simpler than a heat mat. Hard to understand why a heating mat makes for a constant temperature. It makes for a higher temperature, but doesn't smooth out any changes.

That's fair to go for early germination, in case you have a germination failure. But huge plants by planting time just makes life harder, not easier. Your choice.

The heat mats are thermostat-controlled. I can set them anywhere from 62 degrees to 86, maybe more. I like knowing that the seed trays are at a constant temperature. I already have the mats, so it's not an extra expense at this point. I guess this thread has wandered a bit from my original question, and maybe I posted too many details initially, but I was really wanting to know whether or not seedlings need to be removed from the heat mat after transplating to a larger pot, and I got the answers that they should come off the heat right after sprouting. I think I'm good now.

Thanks to everybody for your help.

Eric

Not a big deal, but I'm not saying the thermostats aren't temperature controlled. Your basement has a HUGE thermal mass, and temperatures won't change much, unless you are actively heating it. Without the heat mats, and with no basement heating, you can be sure the seed trays will stay at a constant temperature. If you ARE heating the basement, and the air temperature fluctuates, the soil temperature will fluctuate too, because that soil temperature is driven by both the heating mat and the air around it.

Your question was whether seedlings needed to be removed from heating mats. My answer was - sure, why not? They didn't need to be on heat mats in the first place if the ambient temp was 65F! But except for plants that don't like heat, like lettuce, adding extra warmth won't hurt larger plants. That's what greenhouses do.

Does this mean that LED lights will allow plants to grow more slowly and strongly than fluorescents which emit heat?

In my case, it's the opposite. My tomato seedlings are in a converted garage, where the temp stays 50-60F in the spring. I use fluorescents, and I capture the small amount of heat they produce by throwing a blanket over the whole setup. Doing that, and adjusting that cover right, I can get 75-80F temps, which are perfect for growing seedlings fast. I WANT the heat. Of course, the lamps are only on for 18 hours, but like outdoors, it gets cooler at night. No prob.

Not sure about "slowly and strongly". Slowly doesn't make strongly. For very low temps, slowly can make weakly.

The spectral emission of "daylight" LEDs is a bit better than "daylight" fluorescents for growing things, but that's not a big deal for seedlings.

Blimey, even here in 'mild' UK Z8, I haven't even started my seeds (14th March is THE DATE). My last frost date (on my bloody windy and low-lying allotment) is mid June (seriously) so absolutely no point in getting my 'maters' off any earlier.

TBF, I have never used lights apart from more nefarious activities involving marijuana.

eta - before white rot arrived on the allotment plot, I did used to grow onions under a light, just to get a start from a December sowing.

I think things happen faster in much of the US, Campanula. One minute it's winter then it's boiling hot. Whereas we have trouble getting sufficient heat for tomatoes to do well outdoors, in some states it actually gets too hot for them, We have a lot of leeway with our long springs and there doesn't seem to be such urgency. I've never used lights either. I've got sweet peas, lettuce, broad beans, parsley and Coriander growing, Runner beans, courgettes and tomatoes will be sown in a few weeks time.

Our tomato season may be actually shorter than yours, in the UK. I plant out at the beginning of March (well, today really, because we just went through a late freeze), and I get product in May and June. By July it is too hot for fertilization, and my tomato season is done. Some try for a fall crop, but putting out seedlings and ensuring their survival when it is fiercely hot is a major pain. The trick here is getting them planted out as early as possible.