Greenhouse Herbs for Year-Round Gardening

“Every home should be a greenhome–a space for living connected to or wrapped around a greenhouse, a space full of life. It harvests warmth, oxygen, humidity, ions, nice smells, fresh food, and a continuous panorama of vitality.”

–Amory Lovins, co-CEO of the Rocky Mountain Institute, a resource efficiency consulting center near Aspen, Colorado

The gardens at Heartsong Farm in northern New Hampshire host snow well into March, and soil temperatures are too cold for most garden seed germination until early June. Nancy Phillips used to have a three-month season in which to grow more than eighty varieties of culinary and medicinal herbs on 1 acre to supply the town of Lost Nation. But everything changed when she installed a greenhouse ten years ago. “Now I can’t imagine living without one,” she says. “We wouldn’t be able to afford growing at our scale in this climate.”

Siting the Greenhouse

A greenhouse should be placed on a south-facing, stable site that gets at least six hours of light a day in winter, is protected from high winds, is near water and electricity, and is accessible from the home and garden. Having a long face side facing south allows for maximum heat gain from the winter sun. A lean-to is most cost-efficient if the house makes up the northern wall. In the South, attaching a greenhouse to the east side of the house will shelter it from the hot afternoon sun.

Phillips’s 10-by-16-foot south-facing lean-to is made from reglazed cypress-framed glass windows salvaged from an 1860s schoolhouse covered with a layer of clear rigid plastic. It relies on its proximity to the house, a small woodstove surrounded by loose bricks and flanked by water containers that hold heat, and an electric space heater (when temperatures dip into or below the thirties) to create a “warming room” that extends the growing season by four months. Four-inch fiberglass batts fill the studded wall cavities, and 10-inch batts insulate the back ceiling. Heating the greenhouse costs Phillips about $15 during March and April, and she burns about a quarter cord of wood, most of it pruned from her apple trees.

The greenhouse nurtures more than just plants. “Going into the greenhouse in March is a balm for my soul,” Phillips says. “It helps me make it through until the crocuses and daffodils start poking up. One of the greatest pleasures is going in there when the rest of the world is covered with snow and my heart is yearning for some green.”

Greenhouses have been much appreciated as climate controllers and ­indispensable additions to the garden and home since Roman times. During the nineteenth century, large steel and glass home conservatories were a sign of wealth in England and America, but with the advent of plastic construction materials, today’s greenhouses come in styles, sizes, and prices to meet nearly every herb grower’s needs.

Green House

Greening the Greenhouse

Greenhouses are versatile. Some growers use them to give herbs a head start. Phillips sows seeds in flats of potting soil in her greenhouse in March, when snow covers the ground outside, but daytime temperatures in the greenhouse may range as high as 90°F on a warm, sunny day. By June, the greenhouse is full of healthy seedlings that are ready for hardening off and transplanting into the garden. Phillips’s favorite medicinals–echinacea, St.-John’s-wort, valerian, nettles, catnip, skullcap, and lemon balm–thrive under this regimen.

Merry Gardens in Camden, Maine, keep its greenhouses working year-round. The nursery offers a wide ­selection of culinary and medicinal herbs along with ornamental ivies, geraniums, and rare tropical plants. Stock plants of perennial herbs spend the winter in a pit greenhouse, a 20-foot-high glorified cold frame attached to the back of a building that is held at 35° to 40°F with a built-in oil furnace. Low light and water, together with the cold temperature, keep the plants dormant through the winter. In January, Beth Jensen, who oversees herb propagation, takes cuttings from the stock plants, brushes the ends of the stems with rooting hormone and antifungal solution, and sticks them in plastic plug trays filled with sand or soil. The trays are placed under shade cloth in a standard greenhouse kept at 50° to 60°F, where a misting system keeps the cuttings moist. Rooting may take as little as a couple of days for some mints to as long as a year for bay laurel. The cuttings are then fertilized with a seaweed-fish emulsion and transplanted into 21/2-inch pots or returned to the pit house to delay growth until they are needed.

The potted herbs are eventually planted in the nursery’s display gardens or sold at the farm stand. Before the first hard frost in October, the cycle begins again. “It’s a real juggling act,” says Jensen. “But it works amazingly well.”

Winter Herbs

“One of the greatest pleasures is going in there when the rest of the world is covered with snow and my heart is yearning for some green.”

Popular Styles

• The standard English greenhouse looks like a glass house with straight walls and a sloped roof. Some are glazed with rigid plastic instead of glass; some come with two layers of plastic for better insulation.

• The Dutch greenhouse has walls that slope outward from roof to ground, increasing sun exposure and adding wind resistance.

• The long hoop houses, made of plastic stretched over curved metal bars, tend to be the least expensive style but are not long on looks or durability.

• Dome-shaped greenhouses come in geodesic, hexagonal, and gazebo shapes.

• Wall garden greenhouses are small lean-tos for gardeners with limited space.

• Solar greenhouses are kept warm at night by heat given off by drums of water that has been heated by the sun during the day. These work well where winter days are sunny but are less successful where cloudy weather is the rule. These can be cheap to build and maintain and don’t necessarily depend on buying something from a greenhouse company.

Green House Pest Control

The secret to growing healthy greenhouse herbs is healthy soil, says organic greenhouse farmer Dave Chapman of Long Wind Farm in Vermont, who amends his soil with plenty of compost, which both improves aeration and contributes beneficial soil organisms. Chapman also releases beneficial insects into the greenhouse as needed to counteract pests. For every whitefly, aphid, thrips, spider mite, or fungus gnat, there is a parasitic or predatory insect to combat it. Chapman monitors his greenhouse crops daily, and at the first sign of pests, he has the right amount of the appropriate beneficial insects delivered to his door. He cautions that pesticides can’t be used because they will kill the good guys along with the bad ones.

Herbs themselves can help control pests, according to Jerome Osentowski of the Central Rocky Mountain Permaculture Institute northwest of Aspen, Colorado, whose greenhouse allows him to grow a sizable sweet basil crop that he sells to local restaurants. He tries to have a few plants of carrot family members such as parsley, dill, fennel, or lovage in bloom at all times to attract the beneficial wasps that destroy whiteflies and aphids.

Osentowski maintains that rosemary repels pest insects by its odor, as do garlic, dill, cilantro, coriander, lovage, chamomile, nicotiana, fenugreek, mints, hops, and oregano. He plants mint and nasturtiums at the greenhouse eaves, where they thrive on water runoff and, he believes, fumigate the greenhouse vents, thus keeping whiteflies from entering. He also says that planting onions around basil will get rid of the verticillium wilt fungus.

“My theory of integrated pest management is that nature works, so you try to mimic it in the greenhouse with a diversity of mutually beneficial plants,” Osentowski explains.

From Garden to Kitchen

Jerry Traunfeld, chef at the four-star Herbfarm Restaurant east of Seattle, Washington, has gained renown for his inventive herb-laden dishes (salmon smoked over basil wood with brown fennel Pinot Noir sauce, for example). His herbs come from the Herbfarm nursery, whose five hoop-style greenhouses produce some 600 varieties of culinary and medicinal herbs.

“We could not do this without greenhouses,” says grower Lisa Leckenby. “It’s cold here for nine months of the year. Anything that needs warmth to germinate would not work.”

The temperature in the Herbfarm’s 25-by-40 and 40-by-50-foot plastic-­covered hoop greenhouses is adjusted as necessary to protect the herbs from the cold and to give seedlings a head start. In winter, the hoop houses contain African blue and cinnamon basils, pineapple sage, and lemon scented geraniums (used in lemon sorbet), as well as culinary standbys such as sage, rosemary, and thyme. Marjoram, basil, dill, cilantro, and parsley are started from seed in the greenhouse, then moved out to cold frames or directly to the garden, as spring weather allows.

Melissa Coleman grew up playing in a greenhouse attached to her home in Maine. Now she is a freelance writer in Aspen, Colorado.

Ventilation, heating and lighing

A wide variety of herbs can be wintered over in a greenhouse that maintains a night temperature of 45° to 50°F, but your climate and your greenhouse’s insulation will determine whether you need a heating system. The cost of heating fuel in your region should help you decide what kind of system to buy. Check with local utility companies and other greenhouse owners for an estimate of heating costs.

“The key is to explore the limits of what you can do ­without heating,” says Jerome Osentowski of the Central Rocky Mountain Permaculture Institute, who gardens at 7,000 feet in ­Colorado. In his case, it’s worth having a back-up. His ­homemade heating and ventilation system draws warm daytime air into the soil through pipes that run beneath it. At night, the warm air is discharged back into the air. He put together this contraption from parts readily available at the hardware store; it uses only a couple dollars a month in electricity to run the fans.

Most greenhouse companies offer a variety of heaters that are controlled by a thermostat. (They also sell alarms that warn of power failures.) Ventilation systems are available to keep plants from cooking when the weather heats up. You set the temperature, and a fan removes hot air while drawing in fresh air through shutters.

Smaller greenhouses and cold frames can utilize solar-powered vents. These contain a mineral wax that expands with heat to push a piston that opens the vent. As the air cools, the wax shrinks, and the vent closes.

Shade cloth made of fiberglass, knitted plastic, or black or green polypropylene can be rolled out or drawn like drapes along the greenhouse exterior to keep plants from getting sunburned or overheated.

For growing herbs and starting seeds or cuttings in winter, when little natural light is to be had, fluorescent lights are a reliable and relatively inexpensive source of light. Many gardeners find that ordinary cool white tubes from the hardware store are perfectly adequate and much cheaper than tubes designed for indoor garden use.