Jared,

A thermostat isn't going to help. If your mat is only capable of reaching 75 degrees, for whatever reason, then a thermostat set to turn off at 85 degrees will never reach that temperature.

I have two of the smaller HydroFarm 10 X 20 mats and never bothered taking the actual soil temperature measurement.--------I just took a digital thermometer to my mats downstares and placed the probe under the mat so as to get the highest temperature reading. It read 38 degrees C or 100.4 degrees F so it appears that the mats are capable of higher than 75 degrees. I usually have my flats elevated slightly above the mats so I don't overheat them.

One thing you can do is make sure that you have more direct surface contact between the mat and your flat or whatever you are using. If your flat is ribbed or is open mesh, then you are losing heat. If you are using pots they may be too tall or cooling from the sides faster than the mat can heat the soil within.

If you had the soil in direct contact with the mat you wouldn't lose much at all except from the top surface and you would have a better idea of the capabilities of your heating mat. In this case soil moisture would also have an effect on transferred heat because water is a better conductor of heat than dry soil.

I think your problem is one of losing heat from your growing medium faster than the mat can produce it.

Paul

I'm thinking the problem is the mat just doesn't put out enough heat for my application. It would be perfect for vegetables and other plants but I don't think it puts out enough heat for palms because i'm having to use deep pots(4-5"). Maybe i'll have to put them in smaller pots and transplant them at the first sign of germination. I'll give it time and see how well it works out.

So if I do deciede to try something else, what would you recommend for deeper pots? Commercial mat, heat cables, etc.?

I have a heating tray that I purchased at a yardsale last summer. It is a rectangular styrofoam tray with a heat mat in it at the bottom and then you plug it into an outlet. It keep a lot of heat in so not much loss to the surroundings.
May be if you could find a styrofoam box or make one from scratch that will hold your mat you will be able to retain a lot more heat. Just my 2 cents.

What do you have under the mat? You can probably get more heat upward if you place the mat on something that insulates below or reflects heat upward. A couple of reflectors come to mind. One is mylar, I think, and sold at outdoor sports places as a blanket. Another is something that looks like bubble wrap made from aluminum foil. I don't know what it is called but a two foot strip would be quite cheap at a major hardware store. Also try to stop heat loss into the air around your set-up.

I bought some Christmas wrap that was horrid for wrapping, but the back side was mylar so if I throw that over my lights when germinating I can get temps at the 85 degrees no problem, in fact I put some plastic over that then i have recorded temps of 110.
However had someone make me a wood box to put the heat mat on the bottom and large enough to fit 2 regular sized flats in it. i put the shop light right ontop of the box and get excellent germination rates.