Light Levels Are More Important than Temperature for Plant Growth
For growing food, the length of day is important and greenhouses with less than 10 hours a day of sunlight means growth stops or at least slows down even if temperatures are kept artificially high. Yes, you may get a new leaf or two under low light conditions, but if you plan to eat spinach or kale or any other winter crop, the plants need to be at edible size before the sun levels are lower than 10 hours a day. In most northern gardens, the period between early to mid-October and early to mid-February are no-grow days for spinach even though it thrives with outside temperatures dipping as low as -11 F (-24 C).
