Alternatives to seed starting mat?

Hi all! I'm trying to grow some passionflower vines from seed this year, and am having trouble getting them the requisite 8 hrs at 86 degrees a day. I tried setting up by the woodstove (not hot enough), making a mini greenhouse in a roasting pan with hot rocks (doesn't hold temp), and putting them near an incandescent light (barely broke 75). Next I'm thinking maybe to try warming an old cooler? Would love any tips or wacky ideas, though. Even if you think I should just buy the mat (or seedlings haha)

Things I have done/do

Use the underfloor heating in the bathroom. (Obviously only works if you have underfloor heating!)
Use the top of the hot water tank/furnace
A heat lamp (you need to cover them with something as well or it will dry them out)
Florescent tubes. (I start seedlings under 4x fluorescent tubes, that heats the area up from 16C to about 25C (77)as a byproduct. if you boxed it in it would get a lot hotter)

I used to run batches of seeds in germination tests on gourd seeds.
Even with a heat mat I had to unfold a box about the size of the mat and tape the corners to stay open. It still didn't get up to the right temp until I laid a flat piece of cardboard across the top. I just had a cheap plastic stick-on thermometer on the container to adjust the temp.

I do have a tall cabinet near an overhead vent for heat and something like that might work for you. But, again, you would have to surround it and keep as much heat in as possible. That would be hard to maintain that high of a temp in your house though as the thermostat would kick on and off repeatedly.

Maybe a small countertop type oven with a highly adjustable variac, basically an adjustable transformer, in place to control the temp would work.

Actually had another thought, a crockpot running on an external thermostat should do it, I would test with some other seeds first though to make sure it never decided to cook them.

We use a heating pad like is used for sore muscles with a light over the plants.

Something I've done to keep my orchids happy in winter is put them in a 10 gallon aquarium along with a candle warmer. Basically a mini hot plate. I put a mason jar full of water on it and covered the top of the aquarium. That set up kept them around 80 degrees even though the house temperature was 60-65 degrees, sometimes a little lower. It also really helped with the humidity for them, which was otherwise quite low.
I bet the cooler idea would work well with that or something similar.

When I made my potting bench, I had an extra under-tile heating grid from building the house. I put that on my bench and tiled it. The under-tile heater has a thermostat and I can set it to optimum temp. Waterproof, can't damage it and the cat likes it. The mass of the tiles holds the heat and is an easy cleanup.

I used some rigid foam insulation scraps from my daughter's cabin build to insulate a 4x2x2 foot box and put a coffee can full of water with an aquarium heater in it. The idea was that it would warm the space and keep it nice and humid. The build worked ok but unless, like I did, you have all this stuff just sitting around it's probably cheaper and more efficient to just get a mat. I just thought I'd put it out there.

The seedling heating mats do have the benefit of being waterproof (purpose-built), and standard cell-tray sized. (you can get clear domes to fit the cell trays, to hold heat and humidity) There are versions that are thermostatically controlled (rather than just a power setting). If you are likely to do more seed starting in the future, it could be a worthwhile investment.

Heating pads and electric blankets are less expensive, and some plastic sheeting solves the water issue (with some care). There is nowadays a timer safety feature built-in to prevent burns and fires from overuse/forgetfulness, so check what that is... The pads might only be 2 hours? The blankets we got were 8 hours? So for us, that meant a resetting at midnight to heat until morning (in a cool greenhouse), it's also just a power setting, some tinkering to translate to a measured temperature, and no compensation for changes in greenhouse temps. (for indoors, in a home, this might be less troublesome)
They are also useful as heating pads or blankets...

Incandescent lights under a shelf or in a box ought to work, you'll have to tinker with wattage, and the enclosure of lights/plants to get what you need... but likely able to cobble together from things at hand? Could be controlled with a vacation timer. You still have to be careful with combustibles near the bulbs...

So far having very good luck with a crock pot on “keep warm” and a candy thermometer to monitor temp. Comfortably in the 80-90 degrees range. Needs a fair amount of babysitting, but I’m home all day anyway these days.

Skandi Rogers wrote:Actually had another thought, a crockpot running on an external thermostat should do it, I would test with some other seeds first though to make sure it never decided to cook them.

I wonder what mine runs at on low? Would there be risk of the pot cracking, or are you thinking to remove it?

Lot's of great ideas here because I am also trying to germinate some seeds that would prefer a warmer house than I live in. In the short term, I'm using the top of the fridge, but I know it's marginal - it's way more efficient than the first fridge I bought!

We've got spare coolers and hubby used to have a fish tank, but I think he relied on the light to do the heating. My friend's trying to get peppers to germinate, so I'll pass the fish tank idea along to her.

I love the idea of upcycling because both the heat pads I borrowed, didn't work. They seem to have a very short lifespan, so I'd prefer something I can fix.