Helianthemum bicknellii (Hoary Frostweed)

Plant Info
Also known as:
Genus:Helianthemum
Family:Cistaceae (Rockrose)
Life cycle:perennial
Origin:native
Habitat:part shade, sun; sandy or rocky soil; dry prairie, open woods and clearings
Bloom season:June - July
Plant height:8 to 24 inches
Wetland Indicator Status:none
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: 5-petals Cluster type: raceme

[photo of flower] 2 to 10 stalked flowers at the top of the plant, only 1 or 2 open at a time. Flowers are ½ to 1 inch across, with 5 yellow petals and 10 or more orange-tipped stamens fanning out around a small center column, the stamens often clustered together on one side. Behind the flower are 5 red-tinged sepals, the inner 3 broadly triangular, the outer 2 very narrow and usually about as long as the inner. Sepals are covered in short star-shaped hairs.

[photo of cleistogamous flowers] Later in the season, tight clusters of petal-less, self-pollinating (cleistogamous) flowers develop in the axils and tips of the numerous branches.

Leaves and stems: Leaf attachment: alternate Leaf type: simple

[photo of leaves] Leaves are alternate, up to 1 inch long and ¼ inch wide, oblong-elliptic, toothless, blunt or pointed at the tip, wedge-shaped to tapering at the base, and stalkless. Surfaces are covered in short star-shaped hairs. Stems are round and hairy, often brown, multiple from the base, and initially unbranched, developing numerous erect to ascending branches after the yellow-petaled flowers have finished blooming. The branches do not rise much above the top of the stem.

Fruit: Fruit type: capsule/pod

[photo of developing fruit] Fruit is a round to egg-shaped capsule enclosed by the persistent sepals, very much resembling the cleistogamous flowers. Fruit developed from yellow-petaled flowers are larger and contain more seeds than fruit developed from cleistogamous flowers. The reddish capsules open from the top, dispense the seed, and persist through much of the winter.

Notes:

Hoary Frostweed is often found in small colonies, with clustered stems developing from a branched, persistent stem base (caudex). After a hard frost the stems can burst, sap oozing out and freezing into ribbons of white ice crystals (hoarfrost), and is where the common name “frostweed” comes from. H. bicknellii is similar to the more rare Canada Frostweed (H. canadense) with 2 distinct differences: H. canadense has a mix of long, unbranched hairs with the short, star-shaped hairs on leaves and sepals, and its later branches rise far above the tip of the stem. H. canadense starts and ends blooming 2 or 3 weeks earlier than H. bicknellii, but there is overlap. Many references also note on H. canadense the base of leaves is tapering (not wedge shaped) and the narrow, outer sepals are about half as long as the inner, but these traits are not always distinct or consistent. Our observations were the length of outer sepals in particular can be variable on both species. We also observed that H. bicknellii tends to be more leafy than H. canadense, particularly on the later branches. In some references, H. bicknellii goes by the synonym Crocanthemum bicknellii.

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More photos

Photos by K. Chayka taken at Helen Allison SNA, Anoka County, and Rice Creek Trail Regional Park, Ramsey County. Photos courtesy Peter M. Dziuk taken at Whitewater Wildlife Management Area, Winona County.

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