Green Patch: Demystifying Dormancy

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Question: The plants in my herb garden have all turned brown. What should I do to help the hardy perennial ones come back in the spring?

Answer: All herbs know that winter is not a good time to grow, but they also know how to survive until spring. In winter, hardy perennials enter a state of dormancy. They do not merely stop growing. Rather, they are in the middle of a slow growth cycle in which most of their energy is used to survive the winter season. If you want to help them out, the first step is to understand how dormancy works, along with its strengths and weaknesses.

Dormancy is achieved very gradually. Your plants began preparing themselves for winter several months ago. As days became shorter and cooler, the lowest stems developed special modified leaves, called bud scales, which grew to cover the buds from which new stems will emerge in the spring. Many of the tiny latent buds on the lowest stems of lavender plants, for example, are now well armored with bud scales. Similar structures have been installed to protect buds present on the roots of mint, artemisia and other herbs that sprout from the roots. Whether on stems, roots or both, very hardy plants tend to have more or better bud scales than those less tolerant of cold.

We should mention annual herbs, too, in which the dormant life phase is represented by seeds. Most annual herbs are dead by late fall, but their seeds survive hidden in the soil. We tend to think of perennials as “higher” plants because they can live longer lives, but the ability of annuals to produce huge crops of seeds often gives them a distinct advantage, as any puller of crabgrass knows too well.

Back to your dormant perennial herbs: Your primary concern should be preserving the bud scales and root buds your plants have clad themselves with in preparation for spring. One of the easiest ways to assist in the survival of low buds is to leave some of the old stems on the plants through winter. Trim back very tall branches that are prone to waving about in the wind, but don’t prune back closer than 6 to 8 inches from the ground. Even if the stems are dead and brittle, they will shelter the crown of the plant from ice and drying winds.

Perhaps they will even help catch a little snow. A nice blanket of snow has excellent insulating properties, but in some years (and some climates), there is not nearly enough snow to buffer plants from the ravages of winter. If you don’t want to gamble with snow, mulch around your herbs instead. But be careful, because too much mulch, piled on top of dormant plants, can cause them to rot.

My favorite way to mulch herbs is to erect a small teepee made of sticks over them, tie the top together with string, and mulch around the outside of the teepee. A bale of oat or wheat straw, purchased from a farm-supply store, is the best material to use as winter mulch, though I’ve never had to buy it. Many people who use bales of straw to decorate for Halloween eventually set the bales out with their garbage, and I’m not too proud to pick them up.

In years when I buy a live Christmas tree, I wait until the holidays are over and use the limbs to protect perennial herbs through the second half of winter. To do this, drag the tree outside and use pruning loppers to sever the branches. Then lay the boughs around your herbs, like messy wreaths. I keep a few boughs in reserve and lay them over the tops of the herbs when serious ice accumulation is in the forecast. You can use boughs from other evergreen plants the same way.

Winter’s cold does pose dangers to plants, but frequently herbs are more seriously injured in spring, when they begin growing too soon. Indeed, a second miracle performed by bud scales involves the accumulation of hormones that tell the latent buds to wake up and start growing. If winter tends to linger long in your garden, leave winter mulches in place until freezing weather is almost past. Then, on some fine day in spring, you can gather up the mulch and prune plants to relieve them of their dead stems. A few weeks later, when new growth appears, help your plants celebrate the end of their dormancy by giving them a gentle spring feeding.

Barbara Pleasant lives in the mountains of western North Carolina, where she enjoys gardening, garden writing and cooking. To contact Barbara, e-mail   Barbara Pleasant