By Lynn Byczynski
Some of the most valuable herbs are perennials that won't survive most winters in Zones 5 and colder, especially if they don't have sufficient snow cover to protect them.
But growing perennial herbs in an unheated hoophouse under row cover will keep plants alive through extreme cold. That's what Karma and Michael Glos of Kingbird Farm in Berkshire, New York, discovered when they received a Sustainable Agriculture Research and Education (SARE) grant to research overwintering of perennial herbs. They built an unheated greenhouse, 30' x 96' with an inflated double layer of poly. They planted in ground beds and installed a data logger to monitor temperatures outside, inside, and under row cover suspended on wire hoops.
Their lowest winter temperature was -22.3F/-5.39C; the inside temperature was 3.2F/-16C; under the row cover, the temperature was 14.9F/-9.47C; and the soil temperature was 33.1F/0.61C. After experimenting with straw mulch and row cover, they concluded that they got the best results from just row cover. Plants tended to suffer from mold and were slower to recover in spring when mulched.
The herbs they grew were: Rosemay 'Arp'; Lavender 'Lady' and 'Hidcote'; French tarragon; purple sage; pineapple mint; oregano thyme; and lemon thyme.
With one layer of Agribon+ AG-19 suspended above the plants on wire hoops, the farmers recorded more than 95% winter survival of all their herbs. As a result, they have been able to make cuttings for plant sales and to sell fresh-cut herbs from April through December.
For photos and more details on Kingbird Farm's SARE project, visit their website.
Lynn Byczynski is the editor of Growing for Market and the publisher of The Hoophouse Handbook.