By Ellie MacDougall
If you have a cold frame or other sun-filled, wind-protected, unheated structure, consider setting aside space for herbs that won’t survive in the open. You may find yourself enjoying some pleasant surprises this winter.
We grow herbs for a living, blending them by hand into seasonings and vinegars. One of the constraints to increasing production without a significant increase in labor has been finding flavorful species of somewhat tender, slow growing, difficult to germinate herbs (rosemary is a perfect example) that we can overwinter. Another challenge is the short period of harvest dictated by our 140-day growing season, which makes it difficult to achieve our standard of product freshness year-round.
Working with a High Tunnel
In 1997, we decided to build an unheated greenhouse, also known as a high tunnel. We built the structure with clear end walls (one end even uses salvaged glass sliding doors), not the recommended solid wood, because we saw an opportunity to build shelving into the end wall framing to accommodate flats for hardening off and germinating seedlings. We also equipped the greenhouse with a 50-gallon plastic barrel, cut a spigot into the bottom and filled it with water to provide a ready supply for times when we didn’t want to haul 350 feet of hose down from the house. The structure measures 17' wide x 32' long x 10' high. It is covered with a single sheet of 8 mil plastic. The house is ventilated through flaps in the roof peaks and doors at each end, plus roll-up sides that are easily worked using a t-handle. Narrow growing beds line each side, and a wider bed runs down the middle. The structure sits in an area with a relatively high water table. We hoped this location would allow us to water from overhead less frequently and thus avoid fungal problems on leaves that could otherwise occur in a closed greenhouse. The location, however, also is subjected to punishing westerlies, and wind chills that hit –75 degrees are not unusual. During our first winter, we planted the middle bed to salad greens, carrots, parsley, celery and leeks that were made even more delicious by being harvested during snow storms and carried back to the house in plastic bags tucked inside our jackets.
Planting the Herbs
Last September, a little later than we planned but, fortunately, not too late, we interplanted 3" cuttings of Arp rosemary (hardy in zone 7) and tiny clumps of English thyme (hardy in zone 6) into one of the side beds. We thoroughly soaked the soil each week until November when, as night temperatures flirted with freezing, we stopped watering and closed the greenhouse. A couple of rips in the plastic did not keep it completely weather-tight, though – probably a good thing for air circulation. Down to about 20 degrees outside, we let the plants inside fend for themselves. When the weather got colder, we draped a frost blanket over them.
How Did It Work?
There is nothing like crunching through the snow on a sunny winter day, opening the greenhouse door and stepping into the fragrance of a Mediterranean garden. When protected from desiccating winds, the plants tolerated far lower temperatures than their cousins who were exposed in the field. Last winter wasn’t that cold, but night time air temperatures did dip just below zero inside the greenhouse and should have killed the plants. The ground, however, never froze. The plants did just fine. We even had an unexpected bonus. Despite colder temperatures, our latitude is close to that of Provence. The plants apparently considered location more important than degrees. The rosemary bloomed in mid-March and the thyme a month later. By then, we could open the greenhouse vents, doors and, occasionally, sides. The bees came in droves. As I write this in early July, the greenhouse is fully open, temperatures are mercilessly hot and the herbs are thriving. The thyme is becoming a living mulch under the rosemary, with some plants a foot in diameter. The rosemary slips have sent out multiple branches and average more than a foot in height. We cultivate and hand water deeply twice a week.
We’ve concluded that an unheated greenhouse shows enormous potential for extending the harvest period for our herbs and, as a consequence, expanding our time frame for product freshness. It also allows us to overwinter more tender perennials that would perish in the open. We’re giving serious thought to significantly increasing the length of our greenhouse. This also may allow us to sell fresh herbs in the off-season, when the only herbs in the market are from Florida and California.
With careful management, home gardeners can overwinter somewhat tender herbs in a cold frame or other protected environment. Clearly, this won’t work for very tender annuals such as basil, but there are other herbs that we have yet to try in our high tunnel. We’ve just set in sage, tarragon, chives and oregano seedlings. They’re hardier than Arp rosemary and English thyme, so we’ll be interested in seeing how much longer we can keep them from dormancy in the fall and how much earlier they wake up in the spring. We’ll keep you posted.