Can ornamental greenhouse space be utilized for off-season greens?

Lots of high-quality greenhouse space designed forgrowing ornamentals is unused between Novemberand February. Would it pay to use that space to growgreens during the coldest, darkest time of the year?

Becky Sideman, Ph.D., associate professor and extension specialist in sustainable horticulture at the University of New Hampshire, thought it a subject worth investigating. Her initial research, conducted with Brian Krug, extension specialist in greenhouse production, focused on the rate of growth and the cost of producing winter greens.

On bench tops in two double-poly greenhouses with Lexan ends at the Woodman Horticultural Research Center in Durham, N.H., Sideman and her students seeded 12 varieties of greens on several dates from September through October 2010 and again from February to April 2011. The final harvest from the September-October planting was December 24. The February seeding was last harvested May 20. Production was approximately 40 pounds of greens a week, all utilized in the university’s Dairy Bar.

Choosing varieties

“We wanted to grow greens that other northern New England growers were producing successfully,” says Sideman. “We also wanted our mesclun to have a balance of pungent and nonpungent varieties.” Cold tolerance was also important, especially for greens seeded in the fall. Fall-seeded greens included mache, spinach, kale and lettuce (a less cold-tolerant green). All were grown in UNH greenhouses using organic materials and methods.

As part of an experiment to test the feasibility of utilizing ornamental greenhouses in winter, University of New Hampshire students Nate Suhadoinik and Elisabeth Hodgon tend winter greens that will be served at UNH’s Dairy Bar.

For the February seeding, mache, endive and escarole were dropped from the mix because they grow too slowly. “Spinach didn’t do particularly well in this production system, and it was much more laborious to harvest than other greens,” says Sideman. Based on availability, all seeds for this experiment were purchased from Johnny’s Selected Seeds. Of the greens seeded in the fall (listed below), all except spinach did well:

  • Tokyo Bekana (Brassica rapa, Chinensis group) – Looks like lettuce and grows relatively quickly in cool temperatures and low light.
  • Five Star Greenhouse Lettuce Mix – A blend of red and green downy mildew-resistant varieties including oakleaf, romaine, red and green leaf lettuces.
  • Mustard – Ruby Streaks and Scarlet Frills (Brassica juncea) – Regrow well and make mesclun mix fluffy. Dark green and maroon leaves of Ruby Streaks are sweet and slightly pungent. Scarlet Frills’ red ruffled leaves are spicy.
  • Red Russian Kale (Brassica napus pabularia) – Tender-leaved (compared to other kales). Used in mesclun and also good for light cooking.
  • Mizuna and Early Mizuna (Brassica rapa, Japonica group) – Mildly flavored Asian green good for multiple cuttings.
  • Vitamin Green (Brassica rapa, Narinosa group) – Both cold and heat-tolerant; will grow for a second harvest.
  • Arugula (Eruca sativa) – A pungent green that becomes very pungent when temperatures warm.
  • Tatsoi (Brassica rapa, Narinosa group) – Leaves form a rosette. Staple of early UNH mixes, but seeds difficult to obtain.
  • Spinach (Spinacia oleracea space) – Spinach did not grow well in this setting, and it compacted when packed.

Greens seeded in a University of New Hampshire greenhouse in February were continuing to produce in mid-May. Becky Sideman, Ph.D., associate professor and extension specialist in sustainable horticulture, is exploring the feasibility of growing greens in greenhouses that might otherwise go unused in winter.

Low light, high light

Plants react quickly to decreasing daylight. The very short time between the September 15 and October 15 seeding dates made “a world of difference in harvest dates,” says Sideman. While greens seeded September 15 were ready for harvest in three to five weeks, many of those seeded October 15 were not ready for harvest in eight weeks. With decreasing daylight, the later-seeded plants quickly slowed down to no growth at all. Increasing the greenhouse temperature could not compensate for the reduction in daylight.

“High levels of light, on the other hand, have an interesting effect on greens,” says Sideman. High light levels affect appearance, flavor and texture. Pungent greens, milder flavored in lower light, become more pungent as light increases.

Low heat, slow grow

Because the greens in the UNH greenhouses were grown on benches (as they would be in greenhouses used for growing ornamentals), Sideman’s study did not replicate conditions in the greenhouses of most northern New England greens growers who, says Sideman, generally plant in the ground and heat minimally, often to 37 degrees Fahrenheit. Both of the UNH greenhouses used in this study were heated, one to a minimum of 40 degrees Fahrenheit, the other to 50 degrees Fahrenheit. In the future, rather than heating the entire greenhouse, Sideman would like to experiment with using under-bench heating or energy curtains (akin to row covers).

In this project, greens grew faster at higher temperatures but did not yield more. At what point production would offset the cost of supplemental heat is dependent on the price of fuel oil and on the specific greens grown.

Mesclun grown in lower light is lighter colored than that grown in more light.

The two biggest costs

As anyone who purchases fuel oil in northern New England would guess, the biggest cost was heat. Second was growing media. Sideman and her students used Vermont Compost Fort Vee potting mix and a compost-based mix without supplemental fertilization from Ideal Compost Co. in Peterborough, N.H. “We could have made our own growing media,” says Sideman, “although we are not set up to mix and store large amounts.” Labor was a significant investment, too. UNH Dining paid the cost of student labor in exchange for mesclun from the project.

To reduce the cost of producing greens in a commercial setting in northern New England, Sideman would consider under-bench heating. If growing in the ground were feasible, she would consider installing underground water pipes.

The biggest challenge

Spinach did not do well in the conditions of UNH’s winter greenhouse experiment.

“Watering was our biggest challenge,” says Sideman. “On sunny days, we sometimes had to water three times.” All water came from an overhead system, and all watering was done by hand. “If I were growing commercially, I would consider an automated watering setup,” she says.

Insects and diseases were not a problem, according to Sideman. The UNH greenhouses are generally fallow in summer, a good way to minimize insect and disease carryover. Sideman notes there was a little leaf spot on one Tokyo Bekana.

Winter markets

Where there are winter farmers’ markets, there is a big demand for greens, according to Sideman. She cautions, however, that market feasibility varies by region. In winter 2010 – 2011 in the New Hampshire seacoast area, mesclun was often packed in 4 to 6-ounce bags priced at $4 to $5 – $16 to $20 a pound. High-end restaurants are another good market for locally produced greens. “Because of the high cost of production in winter – whether in heated or unheated greenhouses – growers need to be adequately compensated for the long growing time and high production costs,” says Sideman. “Market opportunities are great, so tweaking your systems to grow efficiently is worthwhile.”

Ruby Streaks and Scarlet Frills, both mustards, add volume to UNH’s mesclun and regrow well.

All of the mesclun grown in Sideman’s research project went to UNH Dining to augment or replace less expensive mesclun from California. The locally grown mesclun was not only a more interesting mix, but also had a longer shelf life. Shelf life, easily two weeks, was not a factor in the choice of varieties grown in Sideman’s experiment. “The greens are delicious, and people really like them,” says Rick MacDonald, assistant director of UNH Dining.

Lettuce consider

While preliminary research has produced much anecdotal information, additional data from continuing research over the next three years is needed to decide whether or not it is economically feasible to grow greens in winter in unused commercial northern New England ornamental greenhouses. Preliminary data shows the following:

The double-poly greenhouse in which Professor Becky Sideman and her students grow winter greens on benches is heated by forced hot air.
  • Some species and some varieties do much better under low heat and low light than others.
  • Some varieties regrow, producing multiple cuttings.
  • Results differ in different temperatures. Heating does not increase yields, but does increase the rate of growth.
  • Fuel oil use was much greater in the spring plantings than in the fall plantings.

Next year a chambered greenhouse will be used to evaluate the effect of various light levels on the rate of growth. Sideman, Krug and a graduate student will collect data to determine whether it pays to add light and heat, and how much.

For now, Sideman suggests producers interested in exploring the costs of winter greenhouse growing consider using Virtual Grower, decision support software for greenhouse growers, available from the USDA at The program helps users evaluate how greenhouse temperature settings, heating systems, fuel types and construction decisions affect overall costs of operation by integrating user data with a database of hourly temperatures, light and wind for selected locations.

Kathleen Hatt is a freelance writer and editor and has been a frequent contributor to Growing since its inception in 2002. She lives in Henniker, N.H.