A plant for all seasons

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Season extension in greenhouse growing involves cultivating greenhouse plants beyond their normal growing season, which allows growers to offer several diverse products throughout the year. In areas of North America that have extreme temperature swings and colder climates overall, greenhouses are prime candidates for becoming season-extending businesses.

Pros and cons

According to the University of Nebraska in Lincoln, climate change alone has already extended the growing season of most crops by a whopping two weeks. Greenhouse growers can benefit from this longer season and can augment the advantage by growing marketable products year-round, especially in winter, when some crops are scant. Cultivating greenhouse season-extending plants offers several benefits, according to the North Carolina Agricultural and Technical State University (North Carolina A&T), including:

• Insuring a year-round income
• Higher yields overall
• The ability to provide a continuous supply of freshly grown products to regular customers
Retention of the customer base and a chance to gain new customers in the off season
• Higher quality plants due to perpetual technique refinements
• Uninterrupted work schedules for employees

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However, North Carolina A&T warns that there are also drawbacks to running a season extending operation, including:

• Increased management needed
• A relentless work schedule
• Higher production costs
• Higher utility bills
• Increased plastic-disposal demands

Perpetual produce

Salt Lake City’s Millcreek Gardens recommends familiar staples such as carrots, beets, broccoli, potatoes, radishes and turnips as good winter greenhouse (especially unheated) options because they do well at cooler temperatures, as do onions and garlic. More unusual winter veggies that grow well in winter greenhouses include leeks, parsnips, Chinese cabbage, bok choy and rutabagas, which make for exotic product offerings for customers in the off season.

Several varieties of leafy green vegetables do well in winter greenhouses as well, according to Millcreek, and provide a nice twist to winter salads, including endive, radicchio, kale and swiss chard. Cabbage, spinach and arugula are also good options for winter growing. Customers will be happy to have cool weather hardy broccoli, celery and peas available through the winter months. An asparagus bed will take two years to yield a harvest, but thereafter provides a high-end product for customers.

Recommendations for winter greenhouse growing, Millcreek says, includes:

• Space plants properly to maximize growth
• If using containers for the plants, space them apart enough so the leaves don’t touch
• If using raised garden beds, thin out the weak plants as growth progresses so that hardier plants thrive
• Perform frequent scouting for disease and remove sick plants immediately
• Scout frequently for pests, and interplant naturally repellant marigolds, or use ladybug and praying-mantis cases for year-round pest control
• Even on cold days, outside sunshine can cause the greenhouse temperature to spike enough to stress some plants; opening doors for a short period can help, as can using fans

Energy for lighting and heating a greenhouse year-round can be pricey, expenses that can be mitigated by use of modern technologies. Ground air heat transfer (Ground to Air Heat Transfer (GAHT) systems — also known as earth pipes or climate batteries — capture excess summertime heat that accumulates in the greenhouse and stores it in underground pipes for later use. The ground under the greenhouse is surrounded with insulation to prevent the warmth from escaping. GAHT systems can be built by growers, but consultation with a professional is advised as even small errors can lead to malfunction. Strategic use of fans and heaters can also maintain the proper temperature in the winter.

In contrast to winter growing in frigid climates, season extending can be applied to greenhouses in hot climates. Growers can be lured into a false sense of security by relying on warm temperatures to encourage plant proliferation. But in areas where summer temperatures regularly soar into the 90s and higher, even the most seasonal greenhouse plants can be destroyed by overheating.

Investing in an energy-efficient greenhouse can provide healthier summer crops and a wide variety of products, according to greenhouse giant Ceres. The company says that energy efficiency allows for the growth of heat-loving crops such as tomatoes, peppers and cucumbers alongside winter crops such as kale and chard. Getting plants started early, Ceres says, in the late summer/early fall when days are longer, allows seedlings to get plenty of light. By the time colder months come around, the plants have taken root, are more mature and provide a year-round harvest.

It is important for greenhouse growers to assess their profit margins and solvency before investing in season-extending plants: Higher costs of labor and materials must be weighed. But for many, season extension makes their greenhouse viable 24/7, provides their communities with fresh produce year-round, and cements their reputation as a creative, diverse source for their customers.