US (MT): Insulating greenhouses for the winter

With temperatures beginning to plummet, SIFT operations have been squeezed into their greenhouses. These greenhouses were built in the 1970s for winter production. The angle of the Solexx on the greenhouse is meant to maximize the amount of sunlight during winter months. In fact, during the summer, the angle of the light is too steep and causes the plants to reach out for light. Unfortunately, SIFT have not been successful in growing through the entirety of the winter. That is why this year, they are increasing the insulation to push the greenhouse to its limit.


The SIFT farm under snow in early November. Photo: NCAT

Winter got an early start in Butte this year. We have barely stepped into November and the ground is already blanketed in snow and there have been temperatures below 0°F. The greenhouse has been able to hold the inside air temperature around 30°F warmer than the outside on days with clear skies. Cloud cover depletes the solar energy needed to heat the greenhouse, making it difficult to hold in the heat. Last year, the freestanding greenhouse held to 1°F on a morning that fell below -30°F. Though this temperature is well below the capabilities of growing produce, there is significant potential to keep the inside of the structure above freezing with a small input of heat.

The growers have added extra insulation on the back walls of the freestanding greenhouse by using water-heater covers to keep the colder, north side from cooling the building. As well, they have hung blankets on the front that can be drawn back during the day to let light in and closed at night to keep heat in. As a final defense, they have used a poly film to drape over trays of microgreens and lettuce so that below-freezing air does not settle onto the plants.

SIFT are looking at ways to supplement light and heat that will not significantly increase inputs. One option is to find a way to use solar panels to charge a battery that can run small lights and a heater. An oil heater may be the next best option; however, they want to hold true to their objective of sustainability.

The other greenhouse is attached to the south side of the building, from which it receives residual heat. Though this area has not shown to operate effectively throughout the winter, it still holds quite a bit warmer than the freestanding greenhouse. Hardy mint and herbs are currently growing in here and have survived all of the weather we have gotten. One potential for this greenhouse is an area that recesses in the back corner. This area does not receive much direct light and will need to be supplemented with lights.

The plan is to build a microgreen stand that can operate off of fluorescent lights. Microgreens do not need much solar energy because most of their energy comes from the seed itself. They are also looking into using a sodium lamp or a metal halide light for larger plants. Low-pressure sodium lamps give off a red hue that is effective for mature, fruiting plants. The reddish light simulates fall sunlight when the sunlight comes in at an angle to the atmosphere. On the other hand, metal halide lights give off a bluer hue that simulates the direct light during summer. The combination of these allows for a wide spectrum of light to infiltrate the plants leaves and stimulate rapid growth.

Source: SIFT

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