Winter Annual Weeds

If you are seeing plants in your field that are currently flowering – or even setting seed – they are probably winter annuals. Winter annual weeds germinate in the fall, flower in the early spring, set seed, and die by mid-summer (Fig. 1)¹. See the UMass factsheet on weed lifecycles for more information on the differences between summer annuals, winter annuals, biennials, and simple and creeping perennial plant lifecycles.

This year there seem to be more winter annuals than usual. I have seen fields covered in Lamium purpureum (purple deadnettle), Draba verna (spring whitlowgrass), and Stellaria media (common chickweed). According to the UMass landscape weeds specialist, Dr. Randy Prostak, who has been working here much longer than I have, this bumper crop of winter annuals is unusual. It is probably due to our recent weather. Last summer there was record-setting rain, followed by a warm wet winter². Wet weather can help break the dormancy of winter annual weeds³ and a mild, wet, winter can increase the survival of winter annuals^4,5. With increasing global temperatures leading to changed weather patterns, this may become more common. In Massachusetts, we are predicted to experience warmer weather, more frequent droughts, and less frequent but more intense rainstorms⁶. These conditions may result in more years where there is a flush of winter annual growth, like we are seeing now.

What should we do when there is a flush of winter annual weeds? Well, maybe we do not need to do anything. For many annual cropping systems, the weeds are mostly killed when soil is prepared in the spring with cultivation and bed preparation. Winter annuals can even act as an unplanted cover crop – keeping the soil covered when it would otherwise be bare⁷.

However, winter annuals can also:

• Act as an alternative host for pests such as soybean cyst nematode⁸ and many Brassica pests⁹.
• Reduce soil temperature and decrease soil moisture in the spring, which could affect planting dates¹⁰.

Depending on your crop and the growing conditions, these effects could reduce annual crop growth during the season.

In perennial systems, winter annuals can become a more consistent issue. For example, maintaining a vegetation-free strip under apples from May through July leads to the highest yields¹¹. Some winter annuals will have completed their lifecycle by May, but others will not. Winter annuals can also proliferate in matted row strawberries, where early season weed management is most important for establishment of a recently planted crop¹².

If you need to control winter annuals in a perennial system, mowing in the spring may help reduce seed set and vole habitat in orchards. You can also use herbicides, but if you wait until the spring, some weeds may be too large for an effective post-emergent application by the time the temperature is warm enough to spray. It is actually easiest to control winter annuals in the fall, before they emerge (if using a pre-emergent spray), or when they are still small and most susceptible to herbicides (if using a post-emergent spray).

Here is a list of common winter annual weeds you may notice in your field with links to great sites with more photos, identification tips, and management techniques.

• Annual bluegrass (Poa annua)
  • Annual bluegrass can also be a summer annual, but right now you can see winter annual variants flowering (Fig. 2a). The flower is underwhelming – I would not put it in a bouquet – but the seed head (inflorescence) can help with identification. Grasses are notoriously tricky to identify, so we will take what we can get. You can also identify annual bluegrass by its prow-shaped leaf tip (Fig. 2b).
• Purple deadnettle (Lamium purpureum L.) and Henbit (Lamium amplexicaule L.)
  • These weeds are easy to confuse. Both are in the mint family, with spreading growth habits, purple flowers, and root at the nodes. To tell them apart I look for the upper leaves, near the flowers (if there are any). Henbit leaves will circle the stem and remind me of a ruff, popular in Europe in the mid-16^th and 17^th centuries. Purple deadnettle leaves will be downward-pointing and purple-ish.
• Many winter annuals are brassicas. Brassica flowers have 4 petals and are usually white or yellow. The flowers of the species I’m seeing the most right now are white.
  • Hairy bittercress (Cardamine hirsuta L.) You can find this already setting seed. The seeds pods are called siliques and are purple-ish when the seeds are mature. Once mature, a gentle touch will burst the pods, flinging seeds up to three feet from the plant, which is fun to do.
• • Shepherd’s purse (Capsella bursa-pastoris) rosette leaves can have a variety of shapes from toothed to deeply lobed. Leaves can be smooth, but most have star-shaped hairs. Once it sets seed, seeds are contained in a distinctive flat triangular or heart-shaped pod that resemble a Greek shepherd’s purse.
  • Spring whitlowgrass (Draba verna) also has branching hairs on the leaves, similar to Shepherd’s purse, but the stem is leafless and the rosette is smaller. The flower has 4 deeply lobed petals, that can be mistaken for 8 petals.
• Common chickweed (Stellaria media [L.] Vill.) is not a brassica, but has similar-looking flowers to Draba verna. There are 5 petals, instead of 4, but they are deeply lobed and can appear to have 10. It has a spreading growth habit and smooth, opposite leaves.

1. Bellinder, R. R., Kline, R. A. & Warholic, D. T. Weed control for the home vegetable garden. (1963).
2. Blue Hill Observatory. Monthly temperature. (2023).
3. Walck, J. L., Hidayati, S. N., Dixon, K. W., Thompson, K. & Poschlod, P. Climate change and plant regeneration from seed. Glob. Change Biol. 17, 2145–2161 (2011).
4. Ramesh, K., Matloob, A., Aslam, F., Florentine, S. K. & Chauhan, B. S. Weeds in a Changing Climate: Vulnerabilities, Consequences, and Implications for Future Weed Management. Front. Plant Sci. 8, (2017).
5. Peters, K., Breitsameter, L. & Gerowitt, B. Impact of climate change on weeds in agriculture: a review. Agron. Sustain. Dev. 34, 707–721 (2014).
6. Card, B. A. Massachusetts Climate Change Assessment. 22 https://www.mass.gov/doc/2022-massachusetts-climate-change-assessment-december-2022-volume-i-executive-summary/download (2022).
7. Breth, D., Tee, E., Donahue, D. & Wallis, A. Managing apple orchard weeds in the fall. N. Y. Fruit Q. 24, 9–15 (2016).
8. Basnet, P., Clay, S. A. & Byamukama, E. Reproduction of Soybean Cyst Nematode Populations on Field Pennycress, Henbit, and Purple Deadnettle Weed Hosts. Agronomy 12, 2027 (2022).
9. Chen, M. et al. Occurrence of the New Invasive Insect Contarinia nasturtii (Diptera: Cecidomyiidae) on Cruciferous Weeds. J. Econ. Entomol. 102, 115–120 (2009).
10. Bradley, K. Integrated Pest Management University of Missouri. Considering fall herbicide applications: It’s not just about the weeds https://ipm.missouri.edu/cropPest/2013/9/Considering-Fall-Herbicide-Applications-Its-not-JUST-about-the-weeds/ (2013).
11. Merwin, I. A. & Stiles, W. C. Orchard groundcover management impacts on apple tree growth and yield, and nutrient aailability and uptake. J Am Soc Hort Sci 119, 209–215 (1994).
12. Pritts, M. P. & Kelly, M. J. Early season weed competition reduces yield of newly planted matted row strawberries. HortScience 36, 729–731 (2001).