A small greenhouse allows you to raise vegetables and fresh flowers all year and protect them from the elements and insects. For maximum production and diversity, choose plants that produce the most flowers or vegetables in relation to their mature size.
Lettuce is a popular green that is commonly used fresh in salads. Lettuce can be harvested by cutting the individual leaves from the plants as soon as the plants reach 3 to 4 inches in height. Also, you can wait until the plant matures and harvest the entire plant by cutting at the base with a sharp knife. A mature lettuce plant raised in a greenhouse will take up about 6 square inches of space.
Snapdragons grow 2 to 3 feet tall, depending on the variety, and produce a beautiful cut flower. They grow in cooler temperatures, so they are a good choice for winter production in the greenhouse. For summer production, shade needs to be provided from the hot afternoon sun.
Green onions or bunch onions are immature onion plants that are harvested before they make a large onion bulb. Green onions are popular for cooking and fresh in salads. Plant green onion seeds in the fall, and they can be cut fresh for several months after they begin to grow.
The variety of tomato known as a patio tomato grows well in the limited space of the small greenhouse. According to the Ohio State University Fact Sheet: Growing Tomatoes in the Home Garden, tomato plants produce best if daytime temperatures are less than 90 degrees F and nighttime temperatures are above 55 degrees F. Patio tomatoes do not need support, and they produce their crop over a period of six to eight weeks. They can be removed after production ceases and replaced with another type of plant.
Peppers grow best if provided some protection from the hottest afternoon sun. Most pepper varieties grow to a maximum of 36 inches tall and produce as long as the plants don't freeze. According to the Ohio State University Extension Fact Sheet: Growing Peppers in the Home Garden: Peppers should be planted 18 inches apart in rows 24 inches apart.
Daffodils are a lovely cut flower for a small greenhouse space. They grow to a maximum of 36 inches tall. According to the University of Missouri Extension Office: Spring flowering Bulbs: Daffodils, bulbs should be planted 12 inches apart if you would like for them to naturalize, but "for quick display, closer spacing should be used." They should be cut for cut flowers close to the ground when the flower bud shows color. After the green leaves turn brown, the bulbs can be dug and replaced.
Carrots take up less space than many other vegetables because they produce their edible portion below the ground. They need to be spaced only 2 to 3 inches apart but need deep, loose soil to produce a good carrot crop.