A Guide to Winter-Harvest Crops & Overwintering for Spring Harvest
Introduction to Winter Growing
Here at Johnny's Research Farm in Albion, Maine, the Persephone period begins on November 6th.
It ends on February 6th, when daylight at our latitude again reaches 10 hours in length.
When temperatures and daylength drop, your harvest season need not come to a full stop. As more growers add high and low tunnels to their operations and participate in winter markets, we are frequently asked:
"What can I plant to harvest in winter, and when should I plant it?"
Producing marketable crops in winter requires learning the correct planting window dates for your location. We've written this guide to provide a starting point, primarily for growing within unheated tunnels. You can use the charts and guidelines presented here while adjusting the techniques and timing to fit your own region and practice. Remember to keep records, to determine what works best and improve upon your successes.
To begin, it is helpful to distinguish between the two main winter growing strategies. The first group you harvest in winter, the second group you leave in place over the winter to produce an early spring crop.
- Winter-Harvest Crops are planted in late summer or early fall, primarily in high tunnels, for harvest throughout the winter.
- Overwintered Crops are planted in the fall or winter, often outside in the field or under low tunnels, and left in place for the earliest possible spring harvest.
There is plenty of flexibility and overlap in the methods employed, and many growers combine methods and practice succession planting to achieve four-season production.
Scheduling Guidelines for Planting the Winter-Harvest High Tunnel
Ten hours of daylight
The key to scheduling your winter-harvest plantings is to identify the date when your daylength has decreased to 10 hours on its trajectory to the winter solstice. It is during this darkest time of the year — referred to by Eliot Coleman as the Persephone period — that growth slows to a glacial rate for most crops. (You can obtain exact dates for your location using an app or website such as Sundial • Solar & Lunar Times or SunriseSunset.)
The goal is to seed your plants so they are about 75% mature by the time you enter the Persephone period.
Though plants may not grow appreciably thereafter — or, until day length has again increased to 10 hours plus — they can be harvested as needed as long as their maturity holds.
Careful scheduling allows you to control growth incrementally by planting at least two or three sowings at 7–10-day intervals, decreasing the time between plantings down to 2–5 days as you approach the Persephone period. Staggering the plantings in this way allows for crops to mature at different times and provide a longer harvest period. You might want to seed on September 20, September 27, and then October 1, for example. With well-timed, staggered plantings you can create a smooth transition from one harvest to the next for a steady supply through the winter. Multiple seedings also help you identify the best seeding dates for specific crops (which you could then record) and spread out the risk of crop failure due to unfavorable weather conditions.
Two neat tricks to get plants ready for the darkest days in high or low tunnels
- Transplant crops like spinach that are normally direct-seeded. Start the plants elsewhere and grow them to transplant size before planting them in your high tunnel, after your summer-producing/summer-fruiting crops have been removed.
- Establish hardy crops outside in late summer, then place a moveable tunnel over them or construct a caterpillar tunnel over the crop as winter draws near.
Squeeze in even one more crop
Anticipate and plan for any open bed space that may become available in late winter, once you have harvested your winter crops. Some crops, like lettuce, will be finished in early to mid winter. Other crops, like mustards, will bolt coming out of the Persephone period. As the end of the Persephone period draws near, they can, for example, be replaced with direct-seeded spinach or brassicas. These late-winter sowings will be ready for harvest by early spring, often long before the same crop would, if grown outside.
- Intro to Winter Growing
- Winter High Tunnel Scheduling Guidelines
- Winter-Harvest Crops • Planting Chart
- Winter Production in the High Tunnel
- Overwintering Scheduling Guidelines
- Overwintering Crops • Planting Chart
- Protection Methods for Overwintering in Low Tunnels
- Focus on Crops & Varieties for Winter Growing
- Overwintering Onions from Seed