You need to always supply heat for any snake. But depending on where you live will tell me whether or not you need one for a garter as they live in regions that get cold. Nonetheless you will need ambient temperatures of 75 to 85 Degrees. 90-95 in the hotspot. Give them lots of room. Under Tank Heaters (a heat mat) can be good but you will need to purchase a rheostat or thermostat to keep the temperatures in check and lower your risk of a fire.