Package of Johnny's Selected Seeds
Package of Johnny's Selected Seeds
Get Set for Winter Harvest & Overwintering | Pt 1

Part 1.  Get Set for Winter Growing

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You've no doubt heard the terms winter harvest crops and overwintered crops. In this 3-part article, we'll explain what we mean when we use those terms — there is a distinction! We'll review the basic methods for growing in winter and make some specific crop recommendations for each type of growing.

Let's start with some definitions.

The first group you pick in winter, the second group you leave in place to produce an early spring crop.

Ideally, your winter harvest crops will be tapering off just as the overwintered crops become marketable. Following Johnny's guidelines will assist you in achieving a smooth and steady supply throughout the coldest months of the year. We also recommend that you test timing in your location before making large plantings.

Protection Methods for Winter Growing

Crops grown for winter harvest and overwintering both need some protection from the elements in colder climates. Here is an overview of the two best methods we have found for growing in winter.

Low Tunnels

QuickHoops Bender

The QuickHoops™ Low Tunnel Benders were developed specifically to create hoops that can support heavy winter snow loads. Other materials such as wire or PVC will fall short. QuickHoops made of bent steel conduit or EMT (electrical metal tubing) are placed over crops to be overwintered.

QuickHoops Bender

The hoops are covered initially with row cover (AG-19 or heavier) to extend the crop into the fall. These coverings protect the crop from frost but allow them to "breathe" and self-ventilate as temperatures fluctuate throughout the day.

Snow Cover Seals the Film

Later on, when "real winter" threatens, a 4-mil (100-micron) greenhouse film is added on top of the row cover.

Initially on warmer days, this plastic may have to be manually ventilated (lifted on the sides) to prevent internal temperatures from getting too high.

In the dead of winter, the tunnel will effectively be sealed shut by the snow load on top of it.

Post Solstice Warming

After the winter solstice, as days become longer and warmer, the plastic may again have to be vented intermittently to prevent overheating.

Early Onions

Eventually the plastic can be removed completely, with the row cover remaining to protect the crop until it is time for early spring harvest.

High Tunnels

Row Cover in High Tunnel

There are many variations in procedures when it comes to overwintering in high tunnels, but one facet (at least in colder climates) is very common, and that is the use of one or more layers of row cover inside the tunnel for additional protection in colder months.

Late Winter Solar Gain

This row cover (especially when lighter weights are used) is sometimes left in place all the time. Some growers leave the crop covered at night and remove it on warmer days when the tunnel's internal temperature has risen sufficiently. This results in increased solar gain, and raising the temperature of the ground mass, with the hope of faster growth.

Spinach Under High Tunnel

On some crops such as spinach, the row cover can simply “float” or be placed directly on the crop. Otherwise, some type of support is required, especially if multiple layers or heavier fabrics are chosen. From QuickHoops™ and wire wickets to cables and metal suspension frames, various methods can be deployed to support the row cover and to make the daily process of removal more efficient.

To make the most of stationary hoophouse and high tunnel space, you can transplant crops that are normally direct seeded. Start your seeds early in a greenhouse or inside a building and grow them to transplant size. You can then let your summer-fruiting crops go as long as possible before you turn the space over to winter harvest crops.

Scheduling Guidelines for Winter Harvesting & Overwintering

Watch Conditions

As is the case with all growing, careful scheduling allows for a smooth transition from one harvest phase to the other. For a steady market supply across winter and into early spring, plant during specific time slots to control growth incrementally. The exact seeding dates depend heavily on location. Eliot Coleman's Winter Harvest Handbook is an excellent resource to use as a guideline, but careful observation of individual conditions remains key.

For Winter Harvest

Dormant but Harvestable

Seed so the plants are at marketable stage before the "Persephone period," when day length is below 10 hours and plant growth essentially stops. Mature plants can continue to be harvested even though growth has ceased, and you won't have to worry about the crop becoming overmature or bolting until the days lengthen in late winter.

For Overwintering

Orion Fennel
  • Seed so that germination and first stages of growth occur before the plant goes dormant during the Persephone period. Growth will begin again when days lengthen. Be aware that wide fluctuations in spring weather patterns may cause premature bolting in overwintered crops.
  • Seed so germination does not occur until after day length increases to greater than 10 hours per day. Take note that in some instances there is a higher occurrence of seeds rotting in the soil due to prolonged wet conditions.

As you go about planning and implementing your winter crop program, keep in mind that your goal is to have winter harvest crops tapering off just as overwintered crops become marketable, providing a smooth and steady supply.

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Next Steps

These are the basic recommendations from Johnny's Research Team for methods, equipment, and scheduling your winter growing program. To inform your next steps, you can refer to the following resources.

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