A CPU/ASIC can be seen as an electrical heater with extra steps. So why not heat up your plants with your CPU. The energy flow is: power plant -> CPU -> greenhouse -> the atmosphere
Because plants don't grow from heat, but from light
I remember seeing some talk about a company creating ASIC blocks to heat houses but grows afaik don't generally need more heat the lights and everything creates enough heat for the plants.
Depends on both local climate and lighting method. LEDs output very little heat nowadays