Overwintering Culinary Herbs in Unheated Greenhouse
Answered by: Conrad Richter
Question from: Cathy McGregor-Smith
Posted on: September 26, 1999

We purchased several plug trays from you in the spring. We have a winter greenhouse and a couple of unheated polyhouses. Do you have or can you suggest culinary herbs which we could raise all winter or have overwinter for early march restaurant customers? We have an organic vegetable farm and on-farm market near London, Ontario.

No culinary would grow over winter in London without heat. Although the solar heating effect will raise inside temperatures above 10 degrees Celsius needed for growth on some days, the temperature lows would be too extreme too often to get any production.

If heat is not an option, you can overwinter hardy perennial herbs such as tarragon, mint, and chives for late winter or early spring forcing. There are several ways this can be done. If you have soil beds in your greenhouse, you can plant direct in the ground. If you need greater flexibility to be able to move other crops in later in the year, you can plant in boxes. These boxes should be large -- at least 60 cm length and width and 30 cm deep. The size is important in order to give the plants enough room to grow and to ensure survival during the coldest periods of winter. These containers should be on the ground, not raised off the ground, to minimize temperature swings in the soil mass.

The plants could be from stock dug up from the field. Field grown plants have root systems that are large and strong enough to generate plenty of shoots in the spring. If you are planting with purchased potted or plug stock, you need to allow enough time for the plants to generate good root systems. That 60cm by 60 cm box should be half full (or more) with roots.

We have potted and plug stock available for many culinary herbs that can be overwintered and forced in this way. At this writing (late September), it may be a bit late for potted or plug stock to generate the growth needed for next spring. Your best bet may be to use field grown material if you already have some in your fields.

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