"Would that cause any problems with their day/night cycle being
backwards for when I want to transplant them outdoors in the spring?"

Nope, shouldn't be an issue at all. Whether it's a good idea (whether it will accomplish your goals), I don't know.

P.S. Is your garage dark during the day without lights?

Yep! These are in a grow tent also, windows closed. Dark during the day for sure! The goal behind turning the lights on at night is twofold, I suppose. 1) the light can heat up the tent when it's colder at night and I won't have to provide as much supplemental heat because daytime is a little warmer in the garage. And 2) electricity is cheaper at night.

I grow my seedlings on a reversed day/night schedule in my cellar every year and I've never had a problem with it. My cellar isn't completely dark during the day (we have a wood furnace down there), but that doesn't seem to have a negative impact, either. During hardening off the seedlings get their day/night cycle switched around to "normal".

I do it for space and cost reasons.

Thanks; that is good to know! It sounds like we have some of the same motivations for our setups. I'm experimenting with balancing the heat from the lights with heat from a dog house heater mounted to the wall, the soil is getting down into the 50s at night and I think that'll be too cold for some of them. I hope it works... Next would be to try a 250 watt ceramic heat emitter like they use for reptiles. Cherry tomatoes just starting to peek out as of today!

I grow hosta seedlings in my 60 dgrs basement under 24 hrs light. They are now in 9 oz cups. They will get used to the day/night cycle when I bring them outside in May. I use standard 4ft fluorescent lights for light and heat. I have made growing chambers on my 2 shelves with 4 10x20 seed trays, right now have 72 dgrs there all by heating with lights. Perhaps you need some old towels over your tent to contain the heat. By the way, when I kept the lights 1 inch above domes of the seed trays for germination, I had 81 on the surface.

Insulation is a good idea - I have insulation underneath it to insulate from the cold concrete, but obviously heat rises...

Mine are HO fluorescents and keep the soil at about 85 when they're on at the height I have them now (for germination - about a foot above a 4x4 flood table). You're right that the lights provide pretty good heat when they're on. They'll be raised up in about a week once true leaves start showing up - don't want to fry the little buggers.

How do you keep it warm when the lights are out? My soil drops to about 60-65 degrees overnight currently with no light on from 11 pm until 4 am.

It is important to keep in mind that ambient air temps, the warmth created by lights, isn't the same as soil temps. And it is soil temps that are vital when germinating.

Soil temps will always run 8-10+degrees cooler than air temps. That's why soil heating mats are used rather than the lights. Heating mats maintain a constant soil temp regardless of if the lights are on or off.

Dave

Thanks Dave - you're absolutely correct - that's why I go by the temperature of the probe I have placed in the soil, not by the ambient temperature. That really gets at the heart of my question, too - I'm looking for something to keep my seedlings warm because they sit in a flood table and I can't keep seed mats directly beneath them.

The reason I have them in a flood table is so I can timer-water them, as I travel for work all week, every week. I also find more benefit and convenience to bottom-watering for sensitive herbs like oregano.

In my mind, that leaves me with two options - heat the air enough to heat the soil, or find some waterproof sort of heat mat that can keep the soil heated in a flood table. My tent contains both a thermostat for measuring the ambient air temperature and a soil probe with a thermostat on it, so I can always tell the temperature of each.

I'm hoping that the creative minds of this forum may have some ideas that might help.

I have (aquarium) thermometers at soil surface. Seeds are only lightly covered with soil. I keep lights on all 24 hrs. During germination I kept domes on the seed trays which were standing on a 1 inch thick plywood shelf, one shelf above the other with the lower light warming the shelf above. It worked great over the last 4 years.