Sure, why not? Do you live up in the hills somewhere, where it is likely to get cold? Many strains of cannabis can withstand some "light frost", but a greenhouse in SoCal should prevent that from happening!
What will likely happen is this, as soon as your plants are mature enough to flower - they will (about 4-6 weeks). If you're using clones, they will go straight to flower, so use lots of them because they will be small!
