A weed that prospers on degraded pasture

Noxious Weeds: Hoary alyssum

The seed pods just below the blooms make hoary alyssum 
easy to identify.
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Thought to be brought to North America as a seed contaminant from Europe, hoary alyssum is toxic to horses and other livestock. It has a variety of life cycles depending on site and weather conditions, which makes this weed tricky to control.

A prohibited noxious weed in Alberta, this pest can be easily identified. Hoary alyssum grows from the base and sends out several stems. Along each stem sits alternate leaves that decrease in size moving up the stem. Sitting atop each stem is a crown of clustered white, four-petal flowers with deep notches for each petal.

Slightly inflated seed pods give this plant away. They are located just below the crown of blooms and have a sharp spike at the tip. Hoary alyssum only reproduces from seed and generally enjoys degraded range and pasture. Seed banks can be viable for several years and when ready, can germinate throughout the growing season.

As anyone who’s battled this weed may know, hoary alyssum is not a great competitor. Controlling this pest can be as simple as maintaining healthy pasture and providing desirable vegetation, which usually is capable of outcompeting this weed.

Livestock will not graze this plant — affected horses exhibit swelling of the legs, depression and diarrhea, which normally subsides a few days after the animal being removed from the infested pasture. There are no selective herbicides registered for use, so other means of control are needed. Hand pulling seems to be most effective on small infestations, but may need to be done repeatedly. Once pulled these weeds should be bagged and disposed of. Burning mature plants will also be effective at destroying the seeds.

For more information on this or any invasive plant, contact your local Agricultural Fieldman or the Alberta Invasive Species Council.

Aimee Delaney is a Conservation assistant for Red Deer County.

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