Don’t tell Jennifer Glenister she can’t grow crops in winter. For her farm, it’s a “no-brainer,” especially growing in high tunnels.

Her farm, New Morning Farm in Hustontown, Pa., has sold fruits and vegetables year-round to farmers markets in the Washington, D.C., area for years. When she came to the farm in 2009, she wanted to give customers something fresher and greener — something they likely couldn’t find anywhere else.

“We had the infrastructure, and at that time of year, we have the labor,” Glenister said. “So, the high tunnels are really an expansion of that. How do we really have this attractive green stuff fresh, green stuff in winter, when customers are already coming to us for their potatoes and their carrots and their apples?”

But growing crops in winter, even in a protected high tunnel or greenhouse, has its own unique challenges. Glenister provided some perspective during a panel discussion at the Mid-Atlantic Fruit and Vegetable Convention in Hershey, Pa.

“When I’m thinking about winter in the high tunnels, I’m thinking about light, water and heat,” she said. “Depending on the kind of weather that we’re having that day in the winter, it might be a different limiting factor.”

Water is plentiful, but it can also be a limiting factor. It must be managed carefully because if there is too much, the soils won’t dry out enough. 

“The wettest we get our high tunnels in winter is drier than the driest we want our crops in the summer,” Glenister said.

“We might be putting on 15 minutes of drip irrigation every two weeks in late December and January, depending on the weather,” she adds. “We might be doing a half-hour every week, depending on the weather and how much the plants can grow and use that water, or if they can’t use that water because it is too cold and dark, then we don’t put it on.”

Irrigation is done via drip tape buried under black plastic in each tunnel. The goal, Glenister said, is to ensure water gets down 3 to 6 inches deep for the plant roots to get it. 

Most of the plants growing in the tunnels are transplants and, depending on the variety, will sometimes get a double-watering when established, she said.

“We don't want to put on more water to try to get moisture into the top layers of soil in October and November, because it will never dry out,” Glenister explained.

Heat management is also a big issue. The farm’s larger high tunnels — two tunnels measuring 180 feet by 34 feet — have automated ventilation systems that roll up the tunnel’s side walls when the temperature reaches 55 degrees F inside.

If the temperatures climb faster than the side-wall ventilation, then its peak ventilation opens and fans will kick on to circulate air.

But winter growing success, or lack thereof, ultimately comes down to the plants being grown. The farm’s winter crops are cold-tolerant varieties of lettuce, spinach, kale and parsley.

Glenister said lettuce is especially good as it provides three to four weeks of harvest in December and another three weeks of harvest in January, and some growth in February. The farm sells much of it through early March.

Getting the plants acclimated to cold before the winter growing season kicks in is also important, she said.

“I will ensure the tunnels open with ventilators on to ensure they get a few practice frosts before the outside temperatures really go down,” Glenister said. “I need them to make all those sugars so that they can freeze and thaw and give them a couple of days, and they’ll be perfect for harvest. They really can recover from a freeze. Of course, you have to have the right varieties that are cold-tolerant.”

Year-round management

The farm has 16,000 square feet of high tunnels — the two larger high tunnels and two smaller structures measuring 100 feet by 20 feet.

The high tunnels extend the growing season for a few months, but Glenister said she manages them as part of a year-round system. In the summertime, for example, the farm moves all its production outside — the farm grows 26 acres of organic crops. Cover crops are planted inside the high tunnels to keep the ground covered and provide nutrients.

"I think we've done just soybeans as a cover crop in a high tunnel and like that pretty well,” Glenister said. “And cow peas, our new favorite, are easier to incorporate cover crops for the hot summertime.”

Soils in the tunnels are tested twice a year, and tillage is done in the big tunnels twice a year. In the smaller tunnels, Glenister said she is doing relay cropping to reduce tillage.

“We’ve seen big improvements in not only organic matter, but also the feel and character of the soils, at least in the small tunnels,” she said. "We’re watching organic matter and looking for trends, and we want to improve it.”

Keeping a close eye on soil quality is crucial to making a high-tunnel winter system work, said Susan Scheufele, Extension educator with the University of Massachusetts, who took part in the panel discussion.

“My lesson learned is with more winter growing and doing the same amount of summer growing, pushing these tunnels and the soil to an extreme, it is important to find ways to address soil health in the tunnel,” Scheufele said.

“In tunnels where people have been growing all year-round for maybe 10 years, we just see some really weird nutrient imbalances and really high pH, just kind of some weird stuff,” she added. “So, I would just recommend people do lots of soil testing. In New England, we recommend a routine soil test, but also a saturated media test, so a water-based extraction, and making sure if things start to get off track, you address it sooner than later.”

Annette Wszelaki, associate professor and vegetable specialist with the University of Tennessee, said winter high-tunnel growing can open a lot of opportunities for producers willing to put in the work.

“Finding those crops that are niche that no one else has, this is very important, and in the winter it is even more important to drive sales,” Wszelaki said. “Being innovative and keep looking for those crops that you might not have thought you could grow.”

And having good help is also important. Glenister employs 15 people during peak season and eight people over winter, so the high tunnels keep everyone busy.

“I need to keep folks to do the markets, so it helps to have work right through the week,” she said. “High tunnels fit very well for us. We really are a year-round farm. There is no seasonal break.”