Oscarsgirl2,
Congratulations! Seed starting can be very rewarding as well as economical. And you can start unusual varieties that you might not find as plants. One way to leverage seed starting is to deliberately start more plants than you will ever need. Then trade them or give them away to neighbors and friends. Or sell them, or hold a charity event where people buy plants at reduced prices.
I start seeds in trays under lights in my bedroom, so I see every stage of the miracle and can fiddle with them to my heart's content.
I think that "greenhouse" and "indoor seed starting" are two very different things, even if you start seeds in a greenhouse.
I suggest that you learn how to control the temperature and humidity in your greenhouse before starting difficult seeds out there. Overwintering plants, or hardening off young plants in a greenhouse would be two ways to "learn" how to "drive" the greenhouse before learning what method of starting seeds you like best. I assume you have either vents or a flap that have to be opened when the sun is strong, and closed at night and on cloudy days.
Really, though, I have no experience with greenhouses, only with indoor seed starting.
I think the key things for seed starting are:
Provide enough drainage and aeration and don't over-water. Don't use a peat-based seedling mix unless it DRAINS WELL and holds more air than water. If you do have much peat in your mix, or a fine-grained mix, or no large air channels in the mix, never over-water AT ALL. Under-water by so much that the roots always have access to air.
The surface of the soil is a special case: you wish it could stay very dry, to prevent damping-off. That's why I scatter some largish bark chunks onto the soil surface after I finish sowing seeds. The bark covers the soil and hold humidity in, even though the surface dries out and prevents damping off. Medium-size seedlings can push through and between small bark chunks, but not tiny seed plants like petunia and lobelia. They need to be surface-sowed. You can add small bark nuggets after they emerge.
Just never let the seeds get all-the-way-dry, below the surface layer, after they have started absorbing water.
Also, don't leave seedlings in small cells too long. They need root space or they'll give up. Pot them up while they are still small and growing vigorously (like before they are 3 inches tall, and before they have 2-4 pairs of true leaves.
Also, as soon as you see any green part emerge from the soil: they need light!. If they don't get strong light,
People worry about seed-starting mixes being very fine, but I don't agree with that at all. Coarse seed-starting mixes have more air, which is crucial if you don't want roots to drown or suffocate. Almost anything holds enough humidity for germination, especially if there is any fine bark or any peat in the mix. Covering with plastic means you don't need to mist or bottom-water or water the soil in the time it takes to germinate.
I'm bad - I tend to spray the surface with water every day, so I need a quick-drying surface material to avoid damping-off (medium size pine bark, or some use grit or coarse Perlite on the surface).
