Although an unheated greenhouse is too cold to grow heat-loving crops like tomatoes and cucumbers, it is an excellent spot to overwinter flowering plants that thrive or set flowers in cooler temperatures. Even if you live in the far North and the average winter temperature in your greenhouse hovers just above freezing, there is a plant that will grow and thrive there.
Varying in degrees of winter hardiness, tender varieties of potted fuchsias (Fuchsia spp.) can be overwintered in a cool greenhouse, according to Oregon State University Extension. Position them away from the glass walls so their leaves do not touch them; the outside walls can transmit freezing temperatures to their inner surfaces. Put fuchsias in a brightly lit spot in the cool greenhouse. Lightly water as needed to keep them from wilting or going dormant. Increase watering and begin feeding a one-half strength solution of water soluble fertilizer when growth becomes more robust in early spring. Move outdoors after all danger of frost has passed.
A large group of daisy-like plants native to southern Africa and commonly called cape daisy, sunscape daisy or blue-eye daisy, osteospermum (Osteospermum spp.) are commercially cultivated in cool greenhouses for the cut flower industry. Their flowers come in shades of purple, rose, pink, peach, yellow, white and bi-colored. Osteospermum flower during the cool of spring to early summer, rest during the heat of summer and resume blooming in autumn. They need warm temperatures in order for the cuttings to root (the favored method of commercial cultivation). However, once the cuttings are rooted and begin active growth, they require cool temperatures between 45 and 55 degrees Fahrenheit for several weeks in order to develop flowers. Cooler temperatures also keep the plants' growth stocky and prevents them from becoming leggy.
Many tender varieties of rhododendrons (Rhododendrons spp.) will not successfully grow in most of North America. They either suffer winter die-back or lose their flower buds to a late-season spring frost. Rhododendrons can be successfully grown in an unheated greenhouse where nighttime temperatures drop as low as 32 degrees F. Grow potted greenhouse rhododendrons in extra sandy potting soil with large amounts of peat moss and compost. They also require excellent drainage to avoid root rot. Water sparingly during the shortest days of winter, watering only occasionally to keep them from completely drying out. Increase watering amounts and frequency when the days lengthen and more active growth resumes.