Hey Mary.
I am gonna be starting Plumeria seeds (~50) in my basement. Prob start in January or so. I need to get a later start than Ken cause of Zone difference and I do not have a real greenhouse - just a pretend one. :-)
I will definitely be using a heat mat(s) cause my basement temp will only be ~ 65. I will also use a plastic lid on the seed trays until I see germination. I do remove them whenever used for a few hours a day. I will remove the heat mats when the seedlings are a couple inches high or I need them for something else. (I start from 500 - 2,000 seeds in my basement over winter.) A good heat mat that fits under a standard seed tray is $ 25 - 30. You can also order thermostats that allow for more control over the temps for ~ $20. I do not think they are worth the expense for most hobbyists. (I mean is really critical to have the temp at 81 rather than 78? ;-) )
I have several banks of flourescents and use natural spectrum light bulbs and I have 2 - 500 watt metal halide warehouse lights. The overall combo keeps the area pretty warm.
I have both humidifier and dehumidifier in my basement so I can control humidity levels. Mostly run the humidifier as from Jan first thru Feb 28 my gas furnace is running a lot and humidity drops big time.
I also run fans over my seedlings.
Good luck. Keep us posted.
