10 questions about winter greenhouse growing

Winter presents unique challenges for growers. As the weather gets colder and snow becomes a potential issue, different preparations are required compared to other seasons.

Here, CropKing horticulturist Jake Emling shares his thoughts on preparing for cold weather and how to run a solid operation during the winter.

Produce Grower: What are some of the most common mistakes growers make when preparing their greenhouses for winter?

Jake Emling: One of the more common mistakes that growers make is not having the equipment functioning in time for the changing of the seasons. Generally, I like to see all of my heating units working properly at least three weeks before the first frost date. During this time of year, we typically start to have some cooler nights, and by having the equipment ready, the greenhouse will function properly.

PG: What is your top tip for growers as they prepare their greenhouse for the winter season?

JE: Inspect the glazing material. Look for anything that needs to be repaired or replaced before the foul weather comes. The glazing on the greenhouse is the main line of defense when protecting the plants from the outside climate.

PG: Weather can be fickle. How else can growers properly prepare for winter without knowing exactly what's to come?

JE: Start with maintenance on all equipment inside the greenhouse. Be sure to check that the vents are working properly, exhaust fans have the belts properly tensioned and are showing no signs of wear. By being proactive in your maintenance, you’ll be better prepared for what’s to come.

PG: Should growers clean their glazing materials before winter to compensate for lower light levels and gain optimal light transmission?

JE: I would make sure to check to see if the glazing is in good shape. Cleaning will help, but for example, with an older poly film, the age of the poly will impact the quality of light coming in more so than the amount of dirt on it.

PG: How can growers keep their operations running in the winter?

JE: Keeping the operation going in the winter takes lots of planning in order to be effective. I look at the crops that I want to produce during that time of year, the light and heating demands of that crop, and the market value. All of these factors are considered before doing much in the winter. These factors can vary from location to location. If the greenhouse is not equipped to handle the weather during the winter months, I would find limited use once the temperatures start to decrease.

PG: If growers are going to grow in the winter, what are some heating adjustments they should make?

JE: For heating adjustments in the winter, I look more toward crop selection as my main way of changing the heat required in the greenhouse. By the time fall starts, I would be working on transitioning the greenhouse from warm-season crops to crops that perform better at lower temperatures. This allows the greenhouse to be operated at lower temperatures in most cases, and still be able to produce a crop. A great example of this is switching from oak leaf lettuce to spinach.

PG: Are there cold-weather disease issues or any pest risks growers need to be aware of?

JE: In wintertime in cooler climates, having the greenhouse closed up can cause some issues with the plants. Often the plants will use up the oxygen available in the greenhouse within several hours if no air changes happen on a cold sunny day. Having a carbon dioxide burner can help by increasing the levels of CO₂ that is available to the plant during the times when no fresh air can be pulled into the greenhouse. As for pests, look out for insects that like to move indoors such as ants, stink bugs and box elders that will likely come into the greenhouse to look for a warm place to overwinter.

PG: What type of greenhouse structures is best suited for winter growing?

JE: The best option is an engineered structure designed to handle the snow load and winter conditions in your area. The most common values that CropKing designs for include a 20-pounds-per-square-foot snow load in the northern portions of the country. Using a higher value will be useful in areas with high amounts of wet snow or frequent snow storms.

PG: What can growers do to lighten or eliminate snow loads on their greenhouse?

PE: To help with snow load on the structure, make sure that the gutters are clean of debris and are flowing properly. With a heated structure, the snow should melt once the flakes touch the glazing and the melted snow should flow down the gutter and away from the structure. By having a blocked gutter full of snow, it will cause excessive strain and can lead to bending parts or more severe and costly damage. Also, if the structure has energy curtains, open them to allow more heat to come into contact with the roof glazing.

PG: When does it become economically feasible to use supplemental lighting in the winter to produce more crops?

JE: This can be a difficult question to answer. When adding lights to the crop, you have to make sure that the crop you are producing can justify the increase in input costs. For example, if the crop values do not increase to cover both the cost of the lights or the operation of the lights, I would not decide to add lights to the crop. Usually the better decision is to change to a crop that prefers lower-light conditions for that time of year.

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