Start temp is the minimum temp required for the light to operate correctly i.e. the CFLs wont operate in sub freezing temperatures. Some even post max operating temps too. This should not be a consideration for your purchase however as you will have the bulbs operating in an indoor grow area, on concern about that aspect. Just make sure you get the most wattage for lumens and the right color (6,500 and 2,700). Works for me. btw I would go with the cheaper one if everything else is the same.