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Transitioning the High Tunnel to Fall & Winter Crops

September is in full swing in the Northeast – the mornings feel fresh and crisp, the days are still sunny and warm, and those glorious heirloom tomatoes are still sweet and juicy, but the nights are undeniably cooler and the daylight hours are lessening by a few minutes each day. Sadly, the dog days of summer are over and winter is just around the corner. But the change of seasons does not mean we need to abandon our high tunnels for the outdoor fall harvesting and stockpiling of storage crops like winter squash, onions and potatoes in preparation for a long, cold winter. On the contrary, with dependable cold-hardy crop selection and smart planning, a high tunnel will produce an abundance of fresh vegetables throughout the winter months, and now is the time to begin transitioning the beds to these fall and winter crops.

Most outdoor plantings of fall crops such as broccoli and other brassicas, beets, and carrots have already been planted during the late summer, and many will begin to require the protection of a row cover over low tunnel hoops as the nights dip into the low 40’s. Radishes, salad turnips, and salad greens such as arugula, lettuce mix, Asian greens, and spinach can be successfully grown outdoors with sowings beginning in early September and continuing as late as September 19th for arugula and Asian greens.

High tunnels have the advantage of extending the season and protecting summer crops like tomatoes and peppers into the late fall, and they also provide an ideal growing environment and extra protection from cold temperatures for late fall/winter crops, but it is important to anticipate the transition of the beds from summer to fall planted crops to ensure adequate growth before too many daylight hours are lost. Here in the Northeast, we can lose over three hours of daylight from the time period of the summer solstice in June until the fall equinox in September. When anticipating crop planning in the high tunnel, this fall transition can prove to be challenging for many farmers since most are reluctant to pull out a productive marketable crop and replace it with a temporarily un-harvestable crop. Furthermore, the growing conditions in the high tunnel can be too hot and dry to successfully germinate and grow many of the longer maturing crops which prefer to grow during the cooler temperatures of autumn. A movable high tunnel, which can be moved over the outdoor planted crop in late autumn, can easily remedy these issues so it is an important option to consider when investing in a new high tunnel (See Blog post ‘To Move or Not to Move’ for more info).

Without having access to a movable tunnel, the best method to transition from summer to fall/winter crops is to perform it in stages. For instance, the center beds of the high tunnel may still have productive trellised tomatoes in September, whereas the edge beds may have grown an earlier crop of cucumbers or summer squash now at the end of the productive phase. For an unheated or ‘Cold’ tunnel, these former cucumber/squash beds would be selected to be re-planted with the longer maturing fall/winter crops such as fresh winter carrots, kale, chard, scallions, or even direct seeded with a fall crop of spinach. Then, once the tomatoes have finished producing at the beginning of October, the empty beds can be direct seeded with faster maturing salad greens like baby leaf lettuce or transplanted with two-week old spinach seedlings. With the additional protection of an inner row cover over low tunnel hoops, these crops would then be harvestable around the end of November and then weekly throughout the winter depending on the severity of the weather conditions and its effect on regrowth for ‘cut-and-come-again’ crops.

For a ‘Cool’ high tunnel, which is minimally heated to 35 degrees F at night, the benefits of adding a little supplemental heat include expanded diversity to more cold-sensitive crops, increased productivity with more consistent regrowth and succession plantings, and slightly more flexible planting dates for certain crops. Arugula and baby leaf lettuce, for example, can be sown as late as October 20th and succession sowings can be continued all winter long. Radishes and salad turnips can be successively sown starting in late September and through late October, and will provide fresh harvests through the end of December. Fresh herbs like parsley, if sown by mid-July and transplanted in the fall, can also be productive in the ‘Cool’ tunnel all winter long. Carrots can even be sown on November 30th and then will be ready to surprise customers with freshly harvested baby carrots on April 1st!

With additional forethought and proper planning, a high tunnel can be organized to be productive with a diverse array of crops during the entire year. By following these general guidelines, continuing to experiment with varying planting dates and cold-hardy varieties, a four-season high tunnel is guaranteed to delight and surpass your expectations all year long.


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