The cables are meant to be buried in the potting mix and not laid on top of shelves. I used a heating cable from Stokes several years ago. We built a heat table by making a framework of 2x4's, stapling hardware cloth to the bottom (to allow drainage), laying a sheet of landscape cloth in the bottom (to hold in the potting mix) then filling the "box" with several inches of ProMix, with the cable being buried in the mix. The whole thing was on folding legs that are available at HD etc. The table was setup in front of a large window and worked well for sticking cuttings for a couple months. Then suddenly the cable just quit working.....That was the end of that table. I went to using heat mats and found them much more versatile. I have now been using mats for about 5 years. I have one 4 foot one and 3 single tray size ones and they have been reliable and useful. You only want to put your seeds on heat for germination. Seedlings themselves prefer cooler temperatures and if left on heat will become elongated and weak. In fact, different seeds have different temperature requirements and some aren't put on heat for germination, such as columbines, delphiniums, coneflowers. You will need to look up the requirements for each kind of plant you intend to start.

Actually, I have saved considerably on my heating bills since I started using Speedheat's Rug Buddy. I started with one in the living room and then put them in all of my bedrooms, too. I used to keep my house a toasty 74 - not very green, I know, but I've always ran cold... but now, I only heat the rooms I'm using and keep it at about 68... it's a much different heat than blowing ones... it's like it heats you all the way to your bones or something... It takes about an hour to get the room up about 20 degrees or so if you cover about 50% or so of the room... about half an hour in the rooms that are 85% covered. Of course, your feet are warm within about 5 minutes, so that's helpful. I use the plug-in stat they have now in the baby room because I care more about the control, but the others, I just put them on a timer. They have an electrical cost calculator on their site. Just remember that you will use it less than other heaters because of the comfort level difference... and of course, you don't have to heat rooms you aren't using if you don't want to, which saves, too. - I think there are some that were on the market before that were more expensive to run - and I know hydronic is REALLY expensive. My uncle has it in NY, and it takes a pretty penny... I told him to just do this, but I haven't convinced him yet. lol Here is a link that might be useful: Speedheat's Running Cost Calculator

If you are starting under fluorescents, you really don't need a heat mat. I've done without for 50 years. Especially for a lot of greens - such as lettuce. Lettuce will start at temps just above freezing. You can easily start it in an unheated garage under lights nearly anywhere in the country. I keep the lights on a cycle that has them come on at night, and then they're off for the 6 warmest hours of the daytime. At 41F, it will take 15 days to germinate; at 50F, we're down to a week. At 59F, it takes 4 days to germinate. Good enough. Tomatoes don't really need bottom heat either. In fact, germination drops off as the soil temp rises to over 75F. You'll get 98% germination at 59F, in about 2 weeks. At 68F, you'll still get 98% germination, in about 8 days. By the time you hit a soil temp of 77F, germination is down to 97%, but it only takes 6 days to emergence. But by the time you hit 86F, germination is down to 83%. If you are starting inside in a heated space, tomatoes under fluorescents don't really benefit from bottom heat, and may be harmed - in combination with the heat from the lights, soil temps are likely to rise above the best temps. Peppers do best between 68F and 77F. Easily achievable, in a heated space, with lights alone. Even eggplants - for me at least - do fine under fluorescents without additional bottom heat. The tables I've seen give only a 60% germination rate, tops, at soil temp of 86F - but I swear I've never had germination nearly that bad. Maybe I've just never counted, or maybe the fact that I grow orientals almost exclusively makes a difference. I'll have to pay attention this year. But I've always started them with just the lights, no additional bottom heat, and they've always done fine. I'm guessing the sweet spot for eggplant is probably between 77F (reported 53% germination) and 86F where the 60% germination rate was reported. Also, I've no idea what type of eggplant they planted to get those measurements. I don't know, maybe some varieties really do have germination rates that low - but I'm pretty sure I've never planted such a variety. Spinach is right there in the same range (temperature wise) as lettuce. I imagine many leafy greens are in the same range. I really don't think a heating mat is all that useful. Better to spend the money on lights, a stand, plant trays, something to measure soil temp, and a fan, IMO. I've really not had a lot of use for a heating mat, myself. This post was edited by zensojourner on Wed, Dec 17, 14 at 18:13

"...Is this true?" My guess is that is an attempt by the maker to avoid liability. Seriously, if a horticultural heating mat can't be left unattended - it's useless! I doubt you'd have problems, but you gotta wonder about how much trust you an put into a product which you can't leave unattended. Keep in mind though, that you should be extra careful, whether attended or unattended, using any electrical device around water. In the past I've used heating pads made for sore muscles. I would recommend that before you sow, you put a thermometer in the soil to make sure the soil temp isn't too hot. If it's above what you want (depending on factors such as what seeds you are sowing) place something to provide a layer of insulation. You can usually even use a couple of pencils to lift it up off of direct contact with the pad or metal pan you described. If try a few things, you'll find something that works well. Again, make sure it's safe.