By Kylie Gettleman
How do you keep your backyard greenhouse supplying food year-round? There are two fundamental factors to consider when planning your greenhouse planting schedule: temperature and day-length.
Day length is arguably the most important factor when determining when to plant in your greenhouse. If you are not using supplemental lighting, then it will be critical for you to know what average day lengths are for your area throughout the year. Where we’re located in Colorado, early February marks the time when our day length begins to reach 10 hours per day, which is generally enough daylight for seedlings to grow. And mid-November marks the time when our days drop below 10 hours and plant growth significantly slows. Plants will continue to survive throughout the winter, but will typically enter a semi-hibernation. If you plant your winter garden early enough, plants will be nearing maturity by the end of November and you’ll be able to slowly harvest all winter long from the semi-dormant plants, even without supplemental lights.
Temperature inside your greenhouse and the multiple micro-climates within will also affect growth. The coldest parts of your greenhouse will typically be close to your greenhouse glazing and right next to your vents. That’s where you’ll want to place your cold-hardy vegetables throughout the winter, like spinach and kale. The warmest part of your greenhouse will typically be along the north wall where the sun reflects off and hits the plants in that vicinity. By planning your planting schedule according to day length, managing your plant’s location based on temperature, and choosing the right crops and varieties, you’ll be able to harvest year-round vegetables.
Here is a rough planting calendar that we use for our location near Denver, Colorado at about 40 degrees latitude.
The days are beginning to lengthen as we move towards the equinox. Where we’re located in Colorado, February marks the time when there is now enough daylight (about 10 hours a day) to start to seed new crops without needing to use supplemental lighting.
Begin seeding your first spring round of cold tolerant crops (lettuce, kale, radishes, beets, carrots, peas, etc).
Begin seeding warm-loving, long-season vegetable crops in the greenhouse (tomatoes, peppers, eggplant, etc). These crops typically take 100-150 days to mature and they do not tolerate cold weather, so plan to give them the longest possible time in a warm growing environment while they grow and ripen their fruit. You can start these crops in your greenhouse and transplant them outside once the nighttime temperatures are consistently above 55 degrees. Or you can keep them growing in your greenhouse all summer long and into the fall.
Near the spring equinox, daylight lengthens and plants begin to grow more quickly in the greenhouse. Begin to seed warm-season crops with shorter days to maturity (beans, basil, cucumbers, squash).
Begin to harvest from your first round of cold-tolerant crops and continue planting cold-tolerant, quickly growing crops to replace them.
The lengthening days and warmer nights allow for much faster growth in your greenhouse.
You’ll likely be harvesting a lot from crops like lettuce, kale, spinach, and peas.
If you have started transplants in the greenhouse, you can now start to plant cold-tolerant transplants outside (broccoli, cauliflower, cabbage) once the night time temperatures are consistently above 45 degrees and warm-season transplants outside (tomatoes, peppers, eggplant) once the night time temperatures are consistently above 55 degrees.
This is generally a hot time for greenhouses, depending on what type of cooling systems you have. Your warm crops, like peppers, eggplant, beans, and tomatoes will be very happy in the greenhouse, but you’ll still have to watch for overheating and make sure you have proper ventilation and that you have enough humidity to keep the plants from transpiring too much and wilting. (To learn more about the relationship between temperature and humidity in your greenhouse, see our blog post about VPD).
August and September is usually the time to start to plant your winter garden. November through January, day length shortens such that plants will grow very slowly without supplemental lighting. Your goal with a winter garden is to plant early enough that most plants are close to reaching maturity by November or December. As plant growth slows, your crops will go into a semi “hibernation” and you will be able to harvest slowly throughout the winter even without seeing very much new growth.
Before the first frost in your area is also the time to move some potted plants inside the greenhouse for the winter. Citrus, figs, peppers and tomatoes that are in pots outside can survive the winter in pots in the greenhouse. We’ve seen potted pepper plants that are over 3 years old grown this way, and are still producing fruit.
Days are beginning to shorten. You’ve already gotten your winter garden planted, but October can still provide enough light to start very short cycle crops (like radishes that only take 20-30 days to mature) or to start crops that you plan to harvest in the late winter/early spring, knowing they will grow very slowly throughout the winter.
Hardy vegetables like spinach, lettuce, and kale planted now will typically have time to germinate and become small plants, overwinter, and grow rapidly as days lengthen in February. Winter and early spring vegetables taste sweeter than at other times of the year as vegetables begin to store sugars in their cell walls to protect against frost damage.
A time of rest and dormancy before the next season really begins again. You can continue to harvest slowly from those plants that are mature, pulling off kale leaves, digging carrots and beets, or cutting spinach. Now is also the time for winter pruning your fruit trees, reading seed catalogues, and planning next year’s garden.
If you do choose to use supplemental lighting, then greenhouse growth does not need to slow down during this time. Leafy greens and root vegetables will thrive in your winter greenhouse with a little added light. We’ve also seen winter greenhouses grow warm-weather crops, like tomatoes and peppers, with supplemental lighting if the greenhouse nighttime temperatures can stay at least above 62 degrees. Because a Ceres greenhouse harvests light through reflecting off of its north, east and west walls, we are able to use less supplemental lighting for strong winter growth.
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