In addition to selling greenhouse plants, your own plugs can be planted in the floor of the greenhouse, in large pots and hanging baskets, and outside under temporary covers, to grow early vegetables, herbs, and flowers. You can grow vegetable garden varieties with the same homegrown flavor and nutrition as those you grow during the regular garden season.
Planting and growing early vegetables is similar to growing them outside in an intensive garden. You will need to provide fertile soil, plan for the efficient use of space, and choose varieties that grow well and produce early. The only additional care they need is heat, water, and protection from early insects and disease.
If time and money are a premium, a greenhouse full of vegetables is less expensive and time-consuming to produce than a greenhouse full of bedding plants. Rows of vegetables can also be grown along side rows of bedding plants, making fuller use of greenhouse space.
Choose flavorful vegetable varieties that produce well under greenhouse conditions. Greens and tomatoes are my most popular crops and are always my largest. My favorite spinach variety is Tyee F1, the most bolt-resistant, savoy-leaf variety I have grown. Romaine, leaf, and butterhead lettuces are available in a variety of types and colors.
Some vegetable varieties, recommended for greenhouse production, have little flavor. I have yet to taste a tomato variety bred for greenhouse production that equals the flavor of a good garden variety. I use varieties of tomatoes, peppers, cucumbers, beans, peas, and summer squash that produce well in my garden, selecting those with shorter maturity dates for spring and early summer sales.
Determine when plants (plugs) need to be transplanted. Germinate seeds early so plants will be available at required planting times. Seeds can be planted in 1204, 1206, and 1801 inserts (or 4″ pots). Provide enough space for roots to grow rapidly. If root growth is checked before plants are transplanted, growth is slowed and production delayed.
Heat loving plants can be located in the center of the greenhouse, where it is warmer, or in a separate section, double or triple-insulated to lower heat costs. Maintain a minimum temperature of 40–45F at night and 55–60F during the day to grow tomatoes, peppers, and other warm-weather plants. Even on cloudy days, light filters into the greenhouse, raising the temperature.
Bean seeds can be planted and cucumbers transplanted in the greenhouse floor as soon as air and soil temperatures remain above 45F. Cucumbers require so much space that they should be trellised.
Lettuce, spinach, broccoli, peas, and other cold-tolerant plants grow quickly in cooler areas and can be safely located close to greenhouse walls as long as they do not touch them. Most need to be harvested before greenhouses get too hot in May. Select greens that will not tip burn or go to seed quickly during an occasional hot day. Because peas germinate easily in cooler soil, seed can be planted under a temporary greenhouse structure. To continue growing and selling all of these varieties, plant them along with successive crops of lettuce in a temporary greenhouse structure, which you can heat on cold nights. By the time they are ready to produce, the cover will be removed allowing a full harvest period.
The greenhouse is cooler in the winter than it is in the summer. Summer and fall plantings of cold-hardy varieties can safely mature in the greenhouse, as long as temperatures stay above freezing.
Moving cold-tolerant plants to temporary greenhouse shelters will leave more space inside the greenhouse for plants that prefer the heat. Vegetable production can be expanded or bedding plants added.
Many varieties are self-pollinating, but those that are not will require your assistance. Bumblebees can be used to pollinate early and late crops, but they are expensive for a small greenhouse. Zucchini require pollination, which can be done by touching a Q-tip to the pollen in the male blooms and spreading it to the female blooms. Tomato pollination can be improved by shaking the branches during the warm/dry part of the day.
For vegetables grown directly in the ground, soil preparation is similar to that in a fertile garden. You need plenty of organic matter broken down into useable nutrients. Animal manures are my favorite fertilizers. Properly composted manure will have fewer viable weed seeds. With the use of a ground cover, however, weed seed is not much of a problem.
I use Lumite 994GC ground cloth and secure it with large staples made from leftover pieces of high-tensile fence wire. The black ground cover absorbs heat from the sun and helps warm up the soil. The organisms in the soil continue to work even in winter, breaking down organic matter.
Where pieces of ground cover overlap, they can be separated to grow double rows of beans and peas or intensively planted vegetables. In the rest of the greenhouse floor, a small X or circle can be cut in the ground cloth to transplant directly into the soil. When preparing each hole the soil should be loosened to allow extra space for roots to spread more easily.
My ground cloth has lines one foot apart, which makes it easy to measure and cut an X for each hole before planting. For tomatoes, peppers, basil, and other large plants, holes are spaced 18″ apart in rows 18″ or 36″ apart.
For smaller plants like lettuce, spinach, beans, and some flowers, holes are spaced 12″ apart in rows 12″ apart. Smaller early plants also can be grown between rows of larger plants and harvested before taller plants reach maturity.
For greens I grow 2–3 seedlings per cell. As plants grow, I thin and sell them as baby plants, allowing more room for the rest to grow. When there is only one plant left per hole, it can mature to full size.
Selling containers with vegetable plants bearing fruit, especially tomatoes, can be profitable. Hanging baskets filled with special varieties of cherry tomatoes (Tumbler), bush cucumbers (Ultra Pak), or everbearing strawberries (Tristar), are irresistible to some customers.
Water is needed to replace rainwater repelled by the greenhouse cover. Tomato plants require special care when watering. To discourage early blight, a potentially serious problem, tomato plants should be watered at ground level in the morning so plant leaves can dry out quickly. Their rows should be planted far enough apart to allow for good air circulation. If automatic watering is used, a drip or soaker hose is preferred to overhead watering systems.
Growing early vegetables creates more opportunities for pests to enter and multiply in the artificial environment of the greenhouse. Watch for the early arrival of the tomato hornworm, ants and aphids, sow bugs, and slugs.
Gini Coover is the author of The Natural Greenhouse, Growing Plants and Food for Profit. She has grown greenhouse plants and vegetables for twenty-eight years.