Hoary alyssum is an annual (occasionally biennial) plant with white flowers, notched petals, and round seedpods. It generally grows in disturbed, dry areas and is not thought to be a large threat to intact native plant communities. However, it can be a nuisance in plant restoration efforts and it is toxic to horses.
Caution: Hoary alyssum is toxic to horses when they eat the fresh plant in a pasture or the dried plant in hay.
Hoary alyssum is an annual plant, but can occasionally be a biennial. Plants grow to two feet tall and have white flowers with deeply notched petals and round seedpods.
Leaves and stem
When hoary alyssum first germinates, its leaves are in a circular grouping (rosette) near the ground. The plants then send up stems. Leaves on the stems are alternate (come off the stem one at time at each leaf node). Leaves have smooth edges and can grow up to 3 inches long and ½ inch wide. Leaves and stems have fine, grayish hairs.
Hoary alyssum has small white flowers arranged in clusters. Each flower has four deeply notched petals, giving the petals a heart shaped appearance that can make it look like there are eight petals upon first glance. Flowers bloom from June through August.
Hoary alyssum seedpods are round and flattened and come off the stem on short stalks. Each seedpod can contain 4-12 seeds.
Hoary alyssum has a taproot.
Hoary alyssum is generally an annual plant that germinates in the spring, produces a rosette of leaves close to the ground, then sends up a flowering stalk, produces seeds, and then dies. Occasionally, plants germinate in late summer or fall and those plants will produce the rosette of leaves, but will not flower and produce seeds until the next year. Hoary alyssum grows well in dry soils with sparse vegetation. It is commonly found in disturbed dry areas such as along roads and railroads. It can be found in lawns, fields, and pastures. It displaces native species particularly in dry prairies and sand blowouts where vegetation is sparse.
Origin and spread
Hoary alyssum is native to Europe and Asia. It was likely originally introduced to North America as a contaminant in clover and alfalfa seed. It spread in North America as a contaminant in seed mixes, hay, and gravel as well as along roadsides. Hoary alyssum is widespread in Minnesota and is especially common in areas with sandy soils.
Refer to EDDMapS Distribution Maps for current distribution.
Don't be fooled by these look-alikes
- Shepherd's purse, Capsella bursa-pastoris (non-native) – Shepherd's purse flower petals do not have the deep notch of hoary alyssum flowers. Shepherd's purse seedpods are triangular and slightly heart shaped while hoary alyssum seedpods are round.
- Field pennycress, Thlaspi arvense (non-native) – Field pennycress flower petals do not have the deep notch of hoary alyssum flowers. Field pennycress leaves have toothed edges while the edges of hoary alyssum leaves are smooth.
- Green flowered peppergrass, Lepidium densiflorum (native) – Green flowered peppergrass flower petals do not have the deep notch of hoary alyssum flowers. Larger green flowered peppergrass leaves have toothed edges while the edges of all hoary alyssum leaves are smooth.
- Regulatory classification
Hoary alyssum is not regulated on a statewide level, but Becker County lists it as a county noxious weed.
- Threat to Minnesota
- Hoary alyssum does not pose a threat to intact native grasslands at this time. It is most abundant in disturbed dry areas.
- It displaces native species, particularly in dry prairies and sand blowouts where vegetation is sparse.
- It can be a nuisance in prairie reconstruction but declines as prescribed burns are administered.
- Hoary alyssum is toxic to horses when they eat the fresh plant in a pasture or the dried plant in hay.
- What you should do
One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.
PlayCleanGo: Stop Invasive Species in Your Tracks
- REMOVE plants, animals and mud from boots, gear, pets and vehicles.
- CLEAN your gear before entering and leaving the recreation site.
- STAY on designated roads and trails.
- PLANT non-invasive species.
- Native substitutes
- Pearly everlasting (Anaphalis margaritacea)
- Long headed thimbleweed (Anemone cylindrica)
- Field pussytoes (Antennaria neglecta)
- Heath aster (Symphyotrichum ericoides)
- Control methods
Mechanical control can be done by digging or pulling the plant by hand or with equipment such as a shovel. Mowing will not get rid of hoary alyssum but may reduce the number of seeds produced, as plants will be shorter than if they weren't mowed. Do not mow when seeds are present as the mowing will spread the seeds. There are reports that mowing may increase abundance of hoary alyssum as it reduces the height of other plants that may shade and compete with the hoary alyssum.
Revegetating sites with native plants and prescribed burning at the site may increase the vigor of native plants that can compete with the hoary alyssum.
Hoary alyssum can become abundant in overgrazed pastures. Contact University of Minnesota Extension for advice related to pasture management.
Herbicide control can be done using systemic herbicides which are taken up by plants and move within the plant, which can kill leaves, stems, and roots. Herbicides that can be effective on hoary alyssum include 2,4-D and glyphosate.
This species is unregulated, but you can add to the public information about this species by reporting new occurrences through EDDMapS.
- Biology, ecology and management of hoary alyssum (Montana State University Extension)
- Hoary alyssum and toxicity to horses (University of Minnesota Extension)
- Hoary alyssum identification (Friends of the Eloise Butler Wildflower Garden)
- Hoary alyssum management (University of Minnesota Extension)