Growing green

While it may not be for everyone, growers of ornamental greenhouse crops that close down their greenhouses for the winter may consider utilizing an empty greenhouse to grow an alternative crop such as greens during the winter months.

Production systems for growing greens range from high tech hydroponic systems for growing lettuce to growing mixed greens in ground beds using minimal or no heat. One thing common to most (not all) systems is the use of a greenhouse structure.

When growing a crop for the first time, especially if new to vegetable production, begin on a small scale. Research the markets, including demands for certain types of greens, harvesting techniques and post-harvest handling, storage and packaging.

It may take some experimentation to develop a production system that works for your operation.

Scheduling: In crop production using minimal heat, plants are seeded from early September through the first week of October and harvested in November and December.

Early seeding is necessary because crops will accomplish most of their growth before short days and cold temperatures. The growth rate slows during the winter months due to cold temperatures, and low light due to cloudy weather and short day length. Using minimum heat, winter production of greens relies on plants making their growth throughout the fall.

One of the keys to success is to plant enough of a crop for harvesting through the cold season. For example, spinach may take several months to grow in colder months and should be planted in September and October, so it is nearly full-sized in December and can be picked through February.

Types of greens: There are several types of greens for winter production including, but not limited to: Asian greens (mizuna and tasoi), kale, lettuce (red and green leaf lettuces, oakleaf, romaine), mustards, gourmet cabbages, Swiss chard, spinach, arugula and claytonia.

Growers have found it best to plant varieties in different blocks rather than mixed together, since growth rates and times of maturity are different. Also, try several cultivars because some may grow better than others under various light and temperature regimes, and some are quicker to bolt than others. The mixture of greens can be created after harvest.

Irrigation: Automatic sprinklers or hand-watering can be used. Irrigate in the morning, to give plant foliage time to dry before temperatures drop at night, especially as days get shorter. As days get shorter and growth slows, use less water. Avoid overwatering, which makes soft growth. Plants will be less able to withstand cold, and less flavorful.

Temperature: Temperature will affect the growth rate and flavor of the greens.

Some growers produce a succession of greens harvesting every 14 to 21 days (micro-greens) at 50°F nights, 55°F days. Other growers provide minimal heat to maintain night temperatures of 37°F. On warm or sunny days, greenhouses are ventilated or sides rolled up to let the air in, depending on the structure.

Some growers grow in ground beds without supplemental heat. Some crops, such as lettuce and arugula, do not grow well without supplemental heat.

Light: Light also affects growth and flavor of greens. Decreased daylight will result in slower growth, and increasing the temperatures cannot compensate for the reduction in daylight. For example, greens tend to have milder flavor in lower light.

Harvesting: Growers may use a sharp knife, scissors, or hand pick with no tools, one leaf at a time. Some prefer a short knife while others prefer a longer one. Greens can be harvested by either removing outer, larger leaves at regular intervals or by cutting the entire plant back to within an inch or so above the soil so the growing point still remains. Leafy crops then re-grow to be harvested again.

Smith is a frequent contributor to Greenhouse Management and an extension floriculture specialist for the UMass Extension Greenhouse Crops and Floriculture program.

Have a question? You can write Tina at tsmith@umext.umass.edu.