The tenderest perennials are good candidates for this, as they'll die if you don't give them shelter over a cold winter. You will always have the cuttings. Don't keep them soggy, watch for fungus problems, remove any bad leaves, and don't let the greenhouse temperature go below 55 degrees Fahrenheit. They will bloom in late winter, and you can repot them to enjoy on your terrace next summer. Generally, blooms in winter are the result of forcing spring flowers or delaying summer and autumn blooming plants.
Seeds are extremely fine.
In August, sprinkle seeds on surface of moist vermiculite and cover with plastic to keep moist. As seedlings grow, keep potting up and pinching. They'll bloom from midwinter to spring. Keep greenhouse temperature above 50 degrees Fahrenheit to keep them flowering. Bulbs, for forcing.