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Arnold Arboretum

Warm weather encourages winter annuals

January 10, 2012

by Peter Del Tredici

Alliaria petiolata

Alliaria petiolata. Photo by Peter Del Tredici

The amazingly warm fall and winter we’ve been experiencing in southern New England this year has provided welcome relief to people who prefer more mild conditions at this time of year. By analogy, the same can be said for a group of weedy plants known as winter annuals. The seeds of these herbs typically germinate in early fall and the seedlings grow slowly throughout the autumn and winter, whenever temperatures are above freezing. Winter annuals flower in early spring, as the days are getting longer, and they die by the end of the spring, leaving behind a crop of seeds that is proportional to the size they have attained.

Most of the winter annuals common in the northeast come from Europe, and their numbers across the urban landscape seem to be increasing as our winters get warmer and our springs arrive earlier. To wit, mild fall weather lasts longer than it did in the past, which gives these plants more time to accumulate carbohydrate reserves; and at the other end of their life cycle, the earlier arrival of warm weather in spring allows the plants to start growing earlier and to grow for a longer time before flowering. Winter annuals occur most abundantly on disturbed, rich soil like that of the typical vegetable garden, and common examples include: yellow rocket (Barbarea vulgaris), shepherd’s purse (Capsella bursa-pastoris), hairy bittercress (Cardamine hirsuta), Virginia pepperweed (Lepedium virginicum), chickweed (Stellaria media), knawel (Scleranthus annuus), groundsel (Senecio vulgaris), and red sandspurry (Spergularia rubra).

Verbascum thapsus rosette

Verbascum thapsus rosette. Photo by Peter Del Tredici

This same pattern of opportunistic growth also holds true for many biennial species that complete their life cycle over two growing seasons, beginning with seed germination in spring or early summer. For these plants, the protracted fall and mild winter provides the evergreen rosettes a longer period to develop, resulting in a bigger, more prolifically seeding plant in late spring or summer. Typical examples of biennials include burdock (Arctium minus), celandine (Chelidonium majus), mullein (Verbascum thapsus), bull thistle (Cirsium vulgare), and garlic mustard (Alliaria petiolata).

Taken together, these plants illustrate how changes in the weather can favor those species with the capacity and flexibility to respond to the opportunities presented by our rapidly changing climate. Indeed, the weeds are well positioned to inherit the earth.


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