If you decide to grow bench plants (planting directly in soil contained in benches, in much the same way as you grow plants in your outdoor garden) select only two or three kinds for your first year. By the time you have planted seeds, potted bulbs, transplanted seedlings or rooted-cuttings into pots, and developed a good watering and fertilizing program, your greenhouse is likely to take up most of your free time. You will find that watering on sunny days is a different business from watering in cloudy or rainy weather, and that ventilating can be time consuming until you work out a fairly definite schedule for it. Soil preparation can become a major project if you let it, but I hope you won't; and only experience will indicate when to fertilize and what amount to give. So try to restrict your first fine enthusiasm just a little. Let me say again: enjoy what you are doing, and let your greenhouse give you the pleasure and relaxation you anticipated.
Unless you have decided definitely on the plants you want to grow, I recommend a cool greenhouse of 45 to 50 F (that is night temperature, you know) for your first venture with snapdragons, stocks, geraniums, and a few hardy bulbs perhaps tulips, hyacinths and daffodils. Plants grown cool are less susceptible to the insects and diseases that develop in a warm, moist atmosphere. Snapdragons and stocks can be grown in flats, but I suggest that you fill one whole bench with soil just for these two. Together they will provide a riot of color, marvelous fragrance, and an abundance of flowers to cut. Grow the ever-favorite geranium in pots. Perhaps you can lift some big plants from your outside garden or window box. Or your local florist may have some in the fall, and house-plant mail-order houses can supply some lovely unusual kinds. Force, say, six pans of bulbs; they will make your first spring excitingly beautiful.
With this seemingly limited objective of two kinds of bench plants, one decorative kind of pot plant, and a few pans of hardy bulbs, you will still be trying your hand at many greenhouse techniques. When the seeds of stocks and snapdragons germinate and the seedlings form their first true leaves (usually when they reach 1 to 1 1/2 inches in height) you will be transplanting them directly into a bench; as the potted geraniums put out new shoots you can increase your supply by taking cuttings. By the time the bench plants are well established, you can bring into the greenhouse (from their cold place outdoors) the potted bulbs for forcing. Even with all these plants, there will be occasions this first year when blooms will be scarce, but you will be so busy with your various projects that you will scarcely miss them.
However, once your snapdragons and stocks begin to bloom, you will have a seemingly endless supply of flowers. Keep cutting them and the striking display will continue. Perhaps you will still yearn for other favorites. But keep it simple the first year while you are learning, and your future greenhouse gardening will be more richly rewarding, for you will have given yourself time to learn by doing, to get the "feel" of growing under glass, and you will be the happier for it.