Montana Weed Control Association

Hoary alyssum is native to Europe and Asia.  It is often confused with another noxious weed, perennial pepperweed.  They have similar shapes and seed attachment.  These plants are both included in the mustard family and are therefore related.  Hoary Alyssum is a newly state listed noxious weed in Montana.  It was listed in 2008 and seems to be popping up everywhere.  This plant can be found throughout Minnesota, the upper midwest, and the western states.  It prefers disturbed sites but can also be found in meadows and pastures.  It is adapted to dry conditions on sandy or gravely soils.  Hoary alyssum prefers direct sunlight but will tolerate shade. 

 

Concerns Hoary Alyssum can be an annual, winter annual, biennial, or a short-lived perennial and is spread only by seed.  It spreads rapidly because each plant produces many seeds.  It has been found to be toxic to about half of the horses that ingest it.  If a horse is going to react to it they will in 12-24 hours after ingested.  The signs include swelling of the lower part of the legs, fever of 103 or higher, warm hooves, pronounced digital pulse, stiffness of joints, reluctance to move, and very rarely death.  Horses have reacted to both fresh plants and plants that have been cut in hay. 

 

Identification  The rigid stems are hairy, 1-3 feet tall, and grayish-green in color.  These stems branch many times near the top.  The alternate leaves are oblong, also grayish-green, and covered in rough hairs.  The white flowers are clustered at the tips of the branches.  Each flower has four petals that are deeply divided.  There are seed pods located up the stems below the flowers and they are hairy, oblong, and appear to be swollen with a point on the end. 

 

What can you do?  A healthy community of native plants will definitely help prevent the establishment of hoary alyssum.  Hand pulling or digging and mowing can be effective in controlling small infestation but must be done before flowering.  There are several effective herbicides but often more than one application is necessary for control.  For best results, herbicide must be applied before flowering.  If you have infestations of hoary alyssum and they are already flowering hand pulling and mowing before seed set is the best option.  If you have questions about this plant or any other plants please contact your local county weed district.

Visit the MWCA Weed ID pages for additional information and pictures of hoary alyssum.

This series of articles was developed by Ravalli County.  If you would like to use these articles please contact Ravalli County Weed District Weed Coordinator at (406) 777-5842.