May 1, 2019
Winter hoop house production set by timing

Hoop houses extend the growing season, but planting date is key if one plans to grow vegetables through the winter.

Base the planting date on the last day that has 10 hours of daylight as the days shorten before winter. Determine that date for the farm’s latitude. Determine how many days the crop needs to grow to the desired size. Count the days back into summer to determine your planting date. The goal is to get the crops at least 75% mature by the last day that has 10 hours of daylight.

“You can use a guide or you can do your own calculations. You want your crops up to size before going into winter,” said Kate Heflick, community supported agriculture (CSA) manager at Michigan State University’s (MSU) Student Organic Farm. Heflick was a speaker at the most recent Great Lakes Fruit, Vegetable and Farm Market EXPO in Grand Rapids, Michigan.

“A common question I hear is, ‘Do all plants stop growing in less than 10 hours of daylight?’” Heflick said.

“Growth really slows for all crops, but you’ll get some growth when you get sunny days. Crops like kale and spinach are going to continue growing in winter when you get some sunlight.”

With limited growth, the plants hold maturity for harvest as needed throughout the winter.

The website is one source of the information on the last 10-hour day for your area. That last 10-hour day is frequently called the Persephone Period.

Persephone is a figure in Greek mythology. The early Greek farmers knew day length and sunlight strength affect agriculture. They created the Persephone myth to explain why winter comes with its shorter days and reduced sunlight.


MSU’s Student Organic Farm offers a 48-week CSA program. The CSA program is part of an eight-month training program in organic production for local markets. The farm also offers Farmer Field Schools, which are intensive, one-day and two-day workshops.

The number of CSA program participants drops in the winter, but the program has wholesale accounts, such as MSU Campus Dining, to utilize any extra production. About one-third of the weekly share provided to CSA participants in the winter is grown fresh in hoop houses. MSU’s Student Organic Farm has nine hoop houses with a total area of 19,000 square feet.

Spinach and kale are the mainstays of the CSA’s winter production. These crops store more sugar in their leaves in the winter than in the summer. “It’s like a natural antifreeze,” Heflick said. “The plants perk right back up on warm days.”

This degree of winter hardiness, or freezing-thawing capacity, is a key factor when deciding what to grow.

Vegetables are usually grouped into three tiers based on their suitability for winter production. Tier 1 vegetables are the ones most likely to be successfully grown in the winter.

These include crops like spinach and kale. Tier 2 vegetables are less likely to be grown successfully and include crops like arugula and cilantro. Tier 3 vegetables are more challenging to grow in the winter and include lettuce, chard, radishes, turnips and carrots. MSU’s Student Organic Farm grows some challenging crops, like lettuce. “That’s definitely the one we lose the most of over winter,” Heflick said. “Kale or spinach work out the best for us.”

Timing is everything

Make timely planting in the late summer or early fall a priority. The plants must get up to size by the Persephone Period.

“We have ripped out tomato plants that were still producing to get winter spinach in because that’s our money crop,” Heflick said. “You want to be really creative about extending the summer season while also getting your winter plants up to size.”

Try to plant on a schedule of seven to 10-day intervals. This helps allow for a more drawn out harvest. It also reduces the risk of planting date variabilities in weather and the risk of crop failures over winter.

Using transplants can get your crops a head start. “I really recommend you use transplants for the crops you’d grow from seed, like spinach,” Heflick said. “We sacrificed some space to use transplants so we could stay on our planting schedule.”

The days of growth time as a transplant must be added in when counting back the days from the last 10 hour day. “Adjust for your transplant time in your planting dates,” Heflick said.

Crops that overwinter for harvest next spring, like radishes, can also be planted in the fall based on the last 10 hour day. The plants will establish in the fall, be dormant over winter and be ready for a fast start the next spring.

“They’ll start growing again when you get to your 10 hours of daylight in the spring,” Heflick said. Days lengthen quickly in the spring. Plants should be ready so daylight is not wasted.

Planting crops for overwintering until next spring is especially important when targeting early-season markets, but growers should be cautious with the strategy. The wide fluctuations in temperature in the spring – from late winter lows to sunny day highs – are a challenge.

Warming up

Adequate ventilation, minimizing temperature fluctuations and managing soil moisture are all management considerations in winter production.

Keeping the hoop house warm isn’t as important as minimizing temperature fluctuation. “Ventilation is very important,” Heflick said. “It can be cold, but on a sunny day, temperatures could spike up to 85 degrees and that’s pretty stressful on plants. The plants are used to 20 to 30 degrees, so you want about 40 degrees, maximum. You want to minimize temperature fluctuation as much as possible.”

Maintaining adequate air flow is an issue since diseases and insects can build up in the hoop house when the temperature gets too high. Scouting and promptly removing any diseased plants are essential. Wider row spacings can help, too.

Soil moisture must be brought up to the right level before winter. You want the plants to have enough moisture between planting and reaching the goal of at least 75% mature by the Persephone Period. Most crops, however, don’t use much water over winter. This makes it easy to water too much or too little.

When you do water, you don’t want to leave any water on the leaves when temperatures are falling toward freezing. “We only water on sunny days,” Heflick said, and only between 11 a.m. and 2 p.m. With crops like head lettuce, only the area between the rows is watered to avoid getting water in the head and causing disease or quality problems.

MSU’s Student Organic Farm is only using supplemental heat in one hoop house starting in late January when transplants are being established.

Under cover

Various types of internal covers are used to add a layer of warmth and protection to the crops.

Row cover fabric and old hoop house plastic have both been used as internal covers. Some are draped over the rows on low hoop frames or on wire supports. This allows smaller, more manageable sizes of cover to be used.

Frames and supports are hard to work around. Removing them also adds one more step to get ready for spring planting.

Another system is for one sheet of plastic to cover the width of the hoop house. “It’s hard for one person to remove,” Heflick said. “It can be a good option – especially if you have two workers all of the time.”

Overwinter production is a complicated system. One strategy will not work for every situation. The Persephone Period for your area gives you a set date, but you still must have some flexibility.

“Go on either side of that date. Leave yourself some room to make adjustments,” Heflick said. “Sometimes growing in the winter works out well, sometimes it doesn’t. It all depends on the winter.”

– Dean Peterson, correspondent

Above: Kate Heflick, community supported agriculture manager at Michigan State University’s Student Organic Farm. Photo: Dean Peterson

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