A year-round greenhouse planting calendar is built around two factors: day length and greenhouse temperature. Day length determines when plants actively grow. Temperature determines which crops can grow in each part of the greenhouse at a given time of year. This guide covers the month-by-month schedule we use at Ceres headquarters in Boulder, Colorado, at about 40° N latitude. It also includes a printable chart you can adapt to your own location.
Where can I download the printable greenhouse planting calendar?
The chart below shows planting, growing, and harvesting periods for groups of crops across the year. Save or print it as a reference while you build a calendar for your own greenhouse.
How does day length affect a greenhouse planting schedule?
In a greenhouse without supplemental lighting, day length sets the active growing season. Days of roughly 10 hours or longer provide enough light for seedlings to establish and for crops to grow at a steady rate. When days fall below 10 hours, growth slows substantially and most plants enter a semi-dormant state. They survive the winter but produce little new growth.
In Boulder, NOAA sunrise and sunset data shows day length reaching 10 hours in late January and passing 10 hours 10 minutes by February 1. Days drop back below 10 hours in mid-November. Sunrise and sunset tables for your own area will show the equivalent dates where you grow. noaa
These two dates shape the winter garden. If you plant cold-hardy crops early enough to reach near maturity by late November, you can harvest them slowly throughout the winter without supplemental lighting.
How does temperature vary inside a greenhouse?
Temperature varies across a greenhouse, which creates microclimates you can use when placing crops. This matters most in an unheated or minimally heated greenhouse.
The coolest areas are typically next to the glazing and near vents. Place cold-hardy crops such as spinach and kale there through the winter. In a passive solar greenhouse with an insulated north wall, the area along the north wall is typically the warmest. It receives direct sun through the south glazing and light reflected off the interior wall. Use that zone for crops that need more warmth.
Plan around day length, place crops according to temperature, and choose cold-tolerant varieties for winter. Together, these allow a greenhouse to produce vegetables year-round.
What should I plant in a greenhouse each month?
The schedule below is based on our greenhouse in Boulder, Colorado. Adjust the timing for your latitude, local day length, and how your greenhouse is heated.
| Period | Conditions | What to plant | Harvest and tasks |
|---|---|---|---|
| February | Day length passes 10 hours | Cold-tolerant crops; long-season warm crops | Harvest overwintered greens |
| March and April | Growth accelerates near the equinox | Short-season warm crops; succession plantings | First spring harvests |
| May | Longer days, warmer nights | Transplant outdoors when nights allow | Leafy greens and peas |
| June and July | Highest heat load | Maintain warm-season crops | Manage ventilation and humidity |
| August and September | Days begin to shorten | Winter garden | Move potted plants inside before frost |
| October | Day length declining | Short-cycle and overwintering crops | Continue harvests |
| November to January | Days below 10 hours | Lit crops only | Slow winter harvest; plan next season |
February
Day length increases toward the spring equinox. In Boulder, February brings enough daylight, about 10 hours a day, to seed new crops without supplemental lighting.
Seed the first spring round of cold-tolerant crops, including lettuce, kale, radishes, beets, carrots, and peas.
Seed long-season, warm-loving crops such as tomatoes, peppers, and eggplant. These crops typically take 100 to 150 days to mature and do not tolerate cold, so they benefit from the longest possible time in a warm environment. You can transplant them outdoors once nighttime temperatures stay consistently above 55°F, or keep them in the greenhouse through summer and into fall.
March and April
Near the spring equinox, longer days increase growth rates in the greenhouse. Seed warm-season crops with shorter days to maturity, such as beans, basil, cucumbers, and squash.
Begin harvesting the first round of cold-tolerant crops, and seed fast-growing cold-tolerant crops to replace them.
May
Longer days and warmer nights speed up growth. Expect regular harvests of lettuce, kale, spinach, and peas.
Transplants started in the greenhouse can move outdoors as nights warm. Cool-season transplants such as broccoli, cauliflower, and cabbage can go out once nighttime temperatures stay consistently above 45°F. Warm-season transplants such as tomatoes, peppers, and eggplant can go out once nights stay consistently above 55°F.
June and July
Greenhouse temperatures are highest in June and July. How warm the space gets depends on its cooling and ventilation systems. Warm-season crops such as peppers, eggplant, beans, and tomatoes grow well in these conditions. Adequate ventilation prevents overheating. Maintaining enough humidity keeps plants from losing water faster than they can take it up, which causes wilting. For more on how temperature and humidity interact, see our introduction to vapor pressure deficit.
August and September
Late summer is the time to plant the winter garden. From November through January, short days slow plant growth without supplemental lighting. Plant early enough that most crops are close to maturity by November or December. As growth slows, these crops enter a semi-dormant state and can be harvested gradually through the winter.
Before the first frost, move outdoor potted plants into the greenhouse to overwinter. Potted citrus, figs, peppers, and tomatoes can all survive the winter this way. We’ve seen potted pepper plants more than three years old, overwintered in a greenhouse, that continue to produce fruit.
October
Days are shortening, and the winter garden should already be in. October still provides enough light for short-cycle crops, such as radishes, which mature in 20 to 30 days. It also works for crops intended for late-winter or early-spring harvest, which will grow slowly through the winter.
Spinach, lettuce, and kale planted now typically germinate and establish as small plants. They then overwinter and grow quickly as days lengthen in February. Cold temperatures also affect flavor: kale, collards, and Swiss chard become sweeter as cold converts their stored starches to sugars. msu
November, December, and January
Without supplemental lighting, these months are a period of slow growth. Continue harvesting from mature plants by picking kale leaves, digging carrots and beets, and cutting spinach. This is also the time to prune fruit trees, review seed catalogs, and plan the next season.
With supplemental lighting, greenhouse production can continue through winter. Leafy greens and root vegetables grow well with added light. We’ve also seen winter greenhouses produce tomatoes and peppers under supplemental lighting when nighttime temperatures stay above 62°F. Ceres greenhouses reflect light off interior wall surfaces toward the growing area, which reduces the supplemental lighting needed for winter growth.
How do I adapt this calendar to my location?
Start with three local inputs:
- 10-hour day-length dates: find the dates in late winter and fall when day length crosses 10 hours where you are.
- Frost and nighttime temperatures: know your typical first and last frost dates and the nighttime temperatures inside your greenhouse.
- Heating and lighting: decide whether you will use heating or supplemental lighting in winter.
At higher latitudes, the period of short days runs longer, so the winter garden needs to be planted earlier. At lower latitudes the window is shorter and may not occur at all. A heated greenhouse can support warm-season crops for more of the year than an unheated one.
To plan production for your own greenhouse, speak with the Ceres Grow Support team or view Ceres residential greenhouses.
Sources
- NOAA Physical Sciences Laboratory, Boulder, Colorado Sunrise/Sunset Data: https://psl.noaa.gov/boulder/boulder.sunset.html
- Michigan State University Extension, “Can I harvest vegetables after frost?”: https://www.canr.msu.edu/news/can_i_harvest_garden_vegetables_after_a_frost
