The other important factor to consider is the lack of natural pollinators. Insects that naturally carry pollen from flower to flower will disappear in the winter, and they may have a difficult time getting into your greenhouse throughout the year if you never open it up. Get to know the pollination requirements of the fruits and vegetables you want to grow. Facilitating pollination may be as simple as setting up an oscillating fan to mimic the wind, or you may need to transfer pollen by hand with cotton buds.
Finally, think about how to efficiently use your greenhouse space as you make your year-round vegetable garden plan. Plant vegetables in containers that can rest on sturdy shelving units within the greenhouse, and set up a trellis system for vines. You might also be interested in upside-down planters that you can hang in the center of your greenhouse. Keep in mind that the use of containers will require more frequent watering and fertilizing, since the soil dries out faster and nutrients leach out of the soil with every watering.
Planning Your Year-Round Garden
Unless you plan to set up a heating system to maintain summer-like conditions in your greenhouse in the winter, it’s wise to still follow seasonal planting schedules. According to Burpee and Practical Self Reliance, vegetables that thrive in cooler weather include:
All members of the Brassica family, including broccoli (Brassica oleracea var. italica), cabbage (Brassica oleracea var. capitata), cauliflower (Brassica oleracea var. botrytis), kale (Brassica oleracea var. sabellica) and brussel sprouts (Brassica oleracea var. gemmifera).
All members of the Allium family, including onions (Allium cepa), garlic (Allium sativum), green onions (Allium fistulosum), leeks (Allium porrum) and chives (Allium schoenoprasum).
Salad greens, including lettuce (Lactuca sativa), spinach (Spinacia oleracea) and arugula (Eruca vesicaria ssp. sativa).
Root vegetables like carrots (Daucus carota subsp. sativus), parsnips (Pastinaca sativa), beets (Beta vulgaris), radishes (Raphanus sativus) and potatoes (Solanum tuberosum).
By protecting these vegetables from frosts and providing artificial lighting, you can enjoy salads and fresh vegetables starting in late fall and continuing through winter well into spring. By the time spring arrives, more sunlight and warming temperatures will naturally make your greenhouse inhospitable to these cold-weather veggies. It’s time to replace them with plants that thrive in summer, such as tomatoes (Solanum lycopersicum), zucchini (Cucurbita pepo), bell peppers (Capsicum annum), beans (Phaseolus vulgaris) and cucumbers (Cucumis sativus). Vegetables that may not do well in greenhouses include those that require a lot of space, such as corn (Zea mays), winter squash (Cucurbita maxima) and melons (Cucumis melo).
Use a Greenhouse for Seed Starting
You can also take advantage of your greenhouse to start seeds before they can be direct-sown outside. While you’re still waiting for that last frost to pass, your seedlings can be cozy and warm inside the greenhouse, ready to transplant as soon as conditions become suitable outdoors. Once you’ve transplanted the seedlings, direct sow another round of seeds in your garden to give you a staggered harvest.
Not all fruits and vegetables appreciate being transplanted, however. Some have sensitive roots or struggle to adapt to the changes in temperature, humidity and wind that exist between the greenhouse and the garden. By the time these sensitive seedlings recuperate and begin to grow strong, their direct-sown counterparts may have caught up to them, making the transplant process a futile exercise. For example, the University of California recommends direct-sowing all root vegetables (carrots, parsnips, beets, etc.), as well as cucumbers, melons, corn, peas (Pisum sativum) and beans.
However, you can create strong seedlings by following a “hardening off” process, using biodegradable seed starting pots that allow the roots to remain unmolested, and by giving the transplants a little extra pampering if needed. Tomatoes, peppers, leafy greens and cabbages all tend to transplant easily and are a good choice for beginners.
Creating a Tropical Greenhouse
If your vision for a greenhouse is to have a true “hot house” or tropical greenhouse that stays warm regardless of the cold temps outside, you’ll need to do a little more setup and be willing to pay for the monthly utility bills that will inevitably rack up. But the payoff will be worth it when you harvest delicious tomatoes, cucumbers and fruits in the off-season.
The specifications of your greenhouse will vary based on your needs and budget, but glass is a popular construction material for tropical greenhouses, thanks to its ability to let in plenty of light while also looking attractive. According to the University of Georgia, greenhouses should also have fans or windows that can easily be opened for ventilation when needed. It will need dedicated heaters for winter and a fogger to maintain adequate humidity levels, but in the summer it may need fans or a cooling system. Your tropical greenhouse will also need to be well-insulated to ensure your heating and cooling efforts don’t go to waste.
You’ll also need to outfit your tropical greenhouse with plenty of grow lights. The further north from the equator, the less sunlight you’ll receive in winter. Tropical and summer plants need as much sun as they can get, so grow lights will help to make up the difference and keep them thriving during the shortest days of the year.
Planting Schedule for Year-Round Harvest
Because most vegetables are annuals, pay special attention to your planting schedule to enjoy a year-round harvest in a tropical greenhouse. The amount of seeds you sow during each interval will depend on your available space. Johnny’s Selected Seeds recommends the following sowing schedule:
- Radishes: Every week
- Spinach: Every week
- Lettuce: Every week for baby greens, every 10 days for full size
- Peas: Every 10 days
- Bush beans: Every 10 days
- Beets: Every two weeks
- Carrots: Every three weeks
- Cucumbers: Every three weeks
- Melons: Every three weeks
- Summer Squash: Every 30 days
Tomatoes will keep on giving as long as they’re an indeterminate variety, and peppers will also keep growing because they are perennials, not annuals. You can also propagate tomatoes and peppers from cuttings, and carrots, onions, celery and lettuce from the crowns. Potato plants will keep sprouting tubers if they are continuously buried in deeper and deeper fertile soil.