Hoary Alyssum
Stemonitis sp slime mold

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Hoary alyssum (Berteroa incana) is in the mustard family. It is native to Europe and thought to have been introduced to North America as a contaminant of forage and clover seeds in the late 19th or early 20th century (Stopps 2012). In some states it is considered a noxious weed. In Wisconsin, it carries no such label although it should. It isn’t good for native ecosystems and is known to be toxic to horses if more than 30% is in their forage (Noack et al. 2020). Hoary alyssum is a strong competitor establishing quickly to outcompete native forbs and grasses. If not controlled, hoary alyssum can reduce the biodiversity and diversity of native pollinators as it is not particularly attractive to insect pollinators (Stopps 2012).

Hairy alyssum is considered an annual, a short-lived perennial, or a biennial, depending on whether it’s mowed early or when it is in flower. In Wisconsin it acts more like a biennial, which makes it sound like one can pop the flower off and that plant will die (Hastings and Kust 1970). Yet, this is not the case. If hoary alyssum is mowed while in the early stages of flowering it will easily regenerate, with 56-80% of plants producing new shoots, flowers, and seed (Stopps 2012). These new shoots, like Queen Anne’s lace, will bloom and set seed at the shorter height, making it harder to see them and the shorter height could easily remain after mowing.

It reproduces only from seeds and these do not normally travel far from the plant (Warwick and Francis 2006, Stopps 2012). This makes it sound innocuous, but this plant is aggressive, and it produces a BUNCH of seeds. While the viable time length of the seedbank isn’t known for certain, anecdotally it’s been said to last from 1-9 years (Stopps 2012). Even more important is to never let this go to seed. Each plant can produce up to 2500 seeds (Stopps 2012). Yikes!

Hoary alyssum, Berteroa incana

Source: Warwick and Francis 2006

Hoary alyssum has been reported as a species with potential allelopathic properties because many members of the Brassicaceae family contain allelochemicals known as glucosinolates (Stopps 2012). This allelopathic ability allows hoary alyssum to produce a dense monoculture (Stopps 2012).

 Unfortunately for weed managers, very little is known about its biology and less about how light, temperature, atmospheric carbon dioxide concentration, wind, relative humidity, soil water, and nutrients allow hoary alyssum to persist (Stopps 2012). It is adapted to a variety of climates and will happily grow in various habitats from stream banks to dry, sandy soil.

Hoary alyssum, Berteroa incana

This shows how the stem grow out and away from the root crown.

Hoary alyssum, Berteroa incana

This is a close up of how the stems grow out and away from the root crown. Some describe the tap root as deep, but most of the ones I’ve pulled are about 4″ and I wouldn’t call this deep. 

How to Manage

Pulling is the best option.

Mowing and Fire are not effective.

Effective Herbicide use is about Timing

Pulling is the best option. It can be easily pulled but one needs to be able to gather all the widely spread stems. Pulling on one stem may result in snapping it from the root ball. If the entire root crown isn’t pulled, it could regenerate. Clearly this option is only available for small infestations that are intermixed with native plants. If this plant has been allowed to reproduce unchecked and you were faced with a “sea of alyssum” (aka a monoculture), management becomes a bit easier as you could herbicide the area with a foliar broadcast spray, but this would not be a one-time application as there will be a seedbank.

Mowing is not effective. Mowing when it flowers merely allows it to regenerate and flower shorter. If mowed before flowering, it can persist as a short lived perennial, regenerate from its carbohydrate storage reserves, and set seed (Stopps 2012).

Fire is not a good option for controlling this plant. Hoary alyssum loves disturbance and can be one of the first rosettes appearing in early spring.

Its growth makes it difficult to manage with herbicides. As the stems grow out (rather than straight up), foliar spray would cause too much collateral damage if hoary alyssum is intermingled with desired native plants. One could carry a spray bottle with Garlon 4/Element 4 in bark oil and spritz the root crown. If recognized in the rosette stage, a spritz of this herbicide mix would be effective.

Carry Out Any Plant with Seeds on it

Plants with any seed formation must be carried out and bagged. Immature seeds, if they have begun the ripening process, may continue to mature into viable seeds after their stalks have been mowed (Monaco et al. 2002). It is unknown whether seeds of hoary alyssum can continue to mature in such a way after their stalks have been mowed, but other mustards such as shepherd’s purse (Capsella bursa-pastoris) do show this ability (Stopps 2012).

Notice the oval, green, immature seeds below the blooms along the stem of this plant.

Overseed

Overseeding in areas where hoary alyssum has been removed is a must. Mowing short before frost seeding is better than burning when you overseed the first year.Native plants can outcompete with a little help from us humans!

Hoary alyssum, Berteroa incana

References

Hastings, R. E. and Kust, C. A. 1970. Reserve carbohydrate storage and utilization by yellow rocket, white cockle and hoary alyssum. Weed Sci. 18: 140–148.

Monaco, T. J., Weller, S. C., and Ashton, F. M. 2002. Weed science principles and practices: New York, NY: Wiley & Sons Inc.

Noack, R., M. Pokorny, J. Jacobs, and J. Mangold. 2020. Plant guide for hoary alyssum. USDA-Natural Resources Conservation Service, Bridger Plant Materials Center, Bridger, MT.

Stopps, Gregory James. 2012. Biology of the rangeland weed hoary alyssum. Doctoral dissertation, University of British Columbia.

Warwick, S. I., & Francis, A. 2006. The biology of invasive alien plants in Canada. 6. Berteroa incana (L.) DC. Canadian journal of plant science, 86(4): 1297-1309.

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