Looks good! Just wanted to say, and agree with Axezombie that, plants might not thrive outside of their usual season, but they generally do survive, and can produce regardless. My suggestion would be to add a season compatibility multiplier that can be applied to both growth and health depending on plant type (perennial or annual).
So, let's take... carrots, and strawberries for examples. Carrots are a perennial: the part we eat, the root, doesn't die. It keeps its growth, and continues to live year after year. Strawberries are perennial as well, perfectly capable of surviving winter, as they do in the wild, the part we eat IS annual, and dies off in winter. I would suggest a growth multiplier be given to each season, according to the best time for that plant. Strawberries are typically a late spring crop, and they would grow best then; however, they could still be planted, grow, and produce during the summer, or fall, but it might take longer, or they may produce less. So, 1.5x spring, 1.0x summer, 0.5x fall, 0x winter, or perhaps negative for winter to "reset" the growth. Carrots are a spring and fall crop, so 1.0x spring, 0.5x summer, 1.0x fall, 0x winter.
For health, Let's go with carrots and tomatoes. Carrots, just a neutral 0x multiplier for freezing weather: They can survive just fine. But tomatoes are an annual plant (outside of the tropics), so, let the cold weather kill them off.
For the indoor winter farming... why not? It's a normal, real thing. So long as the conditions are right, plants can grow and survive just fine inside. Reasonable temperatures and light levels. Using the same seasonal growth system, you could do a check if they're indoors and needs are met (also could introduce sunlamps as an added barrier), and instead of a season, the indoors gets a 0.75x growth rate, as a penalty for the convience. They'll grow, but slower than they would outside. Safety and convenience in exchange for resources and speed.
