Winter Weeds

By Laura Gardner, Master Gardener

At this time of year there are two kinds of easily identifiable weeds: “winter annuals” and “biennials.” Biennials begin their lifecycle by germinating from seed in the first growing season. Winter annuals germinate from seed in the fall. They both form a vegetative basal rosette that lies in a suspended state over winter. The rosette protects what is known as the shoot apical meristem—stem cells of the plant that are responsible for the generation of shoots and leaves later in the spring. They will then grow more upright, flower, produce seeds, and then die, finishing their lifecycle. Some species may function as either a winter annual or a summer annual. Summer annuals germinate from seed in the spring and complete their life cycle that same year. One of the challenges of managing summer annuals is that they can reproduce more than once a year, potentially contributing to a large seed bank. Biennials generally take two years to complete their lifecycles. However, some so called biennial species may extend into subsequent growing seasons and be more like perennials if they have sufficient root energy stores and have not had the opportunity to flower and go to seed. This can happen if you do not remove at least the root crown of the plant when weeding.

In the Peterborough Public Library’s native plant garden, I took some photographs of some basal rosettes with the idea of identifying them later at home. Coincidentally, a copy of a book that I had on order for more than a year finally arrived: Weeds of the Northeast, 2nd Edition, by Joseph C. Neal et al. (Cornell University Press). In addition to the US Northeastern, Mid-Atlantic, and Upper Midwestern states, the book includes Southern Canada. In the 26 years since the first edition was published in 1997, more than 200 new species have been added. Many of these new species are invasive plants from the horticultural trade [e.g. Common Periwinkle (Vinca minor)]. It is strictly an identification guide and so if you are looking for weed management guidance, this type of information will need to be found elsewhere.

The book has a dichotomous key that is a bit different from others. It relies on identifying the plants through their vegetative parts such as leaf lobes, leaf arrangement, leaf margins, leaf hairs, etc., but not on their floral traits. It also does not lead you to an immediate single species identification, but rather enables you to compare your plant with several possible matches through their photos and descriptions. The photos are particularly valuable for identification purposes—showing plants in various stages of life—from seedling to maturity. The glossary is helpful for those that are unfamiliar with some of the botanical terms. This is a great resource for anyone to use to confirm the findings of a plant identification app.

Here are some of the weeds I found and identified using this guidebook:

Shepherd’s Purse (Capsella bursa-pastoris): A summer or winter annual.

Shepherd’s Purse (Capsella bursa-pastoris)

Field Pennycress (Thlaspi arvense): A Summer or Winter Annual

Field Pennycress (Thlaspi arvense)

Canada Horseweed (Erigeron canadensis): Summer or Winter Annual

Canada Horseweed (Erigeron canadensis)

Bull Thistle (Cirsium vulgare) and Common Mullein (Verbascum thapsus): Biennials

Bull Thistle (Cirsium vulgare) and Common Mullein (Verbascum thapsus)

3 thoughts on “Winter Weeds”

  1. You can lump me in with people who don’t like weeds. Your post was really interesting and got me thinking. By further researching the Latin names, I was wondering how many of these are present within our region in Finland. At first glance, I would have said no. But through the internet review and seeing flower heads, I would be more inclined to say, yes, I think so!


    1. I think all the weeds I mentioned are native to Finland except for Erigeron canadensis. I like to use Kew’s Plants of the World database to see where plants are native or non-native globally:

      Generally speaking, native plants are desirable because they have co-evolved with other species in that particular environment. There are natural controls in place such as being host plants for native insects. Erigeron canadensis is one of those native plants whose status here as a desirable plant is mixed. Being native to here it has ecological benefits—supporting a number of different insect species. However, it isn’t favoured in agricultural settings. And although the library garden is a native plant garden, it has the potential to be weedy and crowd out other plants. We can encourage other less aggressive native Asters that will still have benefit.

      I understand that Solidago canadensis (Canada Goldenrod) and Solidago gigantea (Giant Goldenrod) are considered invasive in Europe. They were introduced in the horticultural trade. They are native here and they support a lot of different insects but can be weedy in small gardens or agricultural settings. They spread by seed and rhizomes but there are a number of Solidago species that have fibrous roots and don’t spread in the same way.

      I think it’s great that you follow our blog from Finland. Thank you for reading!


      Liked by 1 person

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