Boechera collinsii (Collins' Rock Cress)

Plant Info
Also known as:
Genus:Boechera
Family:Brassicaceae (Mustard)
Life cycle:biennial, short-lived perennial
Origin:native
Habitat:part shade, sun; dry sandy or rocky soil; prairies, savannas, dunes, cliffs, open woods, along railroads
Bloom season:May - June
Plant height:10 to 30 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: 4-petals Cluster type: raceme

[photo of flowers] Loose, elongating raceme at the top of the plant, usually a single cluster, occasionally branched. Flowers are about ¼ inch long with 4 narrow, spreading petals and 6 yellow-tipped stamens. Petal color is usually white, occasionally lavender.

[photo of sepals] Surrounding the flower are 4 sepals half or more as long as the petals, oblong-elliptic with rounded tips, light green often tinged reddish and with white edging, and covered in a mix of branched and unbranched hairs. The flowers are nodding to drooping on slender stalks up to about ½ inch long that are hairless to sparsely hairy.

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

[photo of basal and lower leaves] Basal leaves are less than 2 inches long, less than 1/3 inch wide, widest above the middle, tapering to a short stalk. Edges are toothless or with a few small teeth and may be fringed with hairs, sometimes just along the stalk. Basal and lower stem leaf surfaces are moderately to densely hairy with a mix of unbranched and branched hairs with up to 8 rays. Leaves on the lower stem are crowded, hiding much of the stem, and become more widely spaced and less hairy as they ascend the stem.

[photo of upper stem and leaves] Upper stem leaves are erect, narrowly lance-oblong, hairless to sparsely hairy, mostly toothless or rarely with a few teeth, blunt to pointed at the tip, stalkless and with 2 lobes at the base (auricles) 1 to 3 mm long. Stems are usually single from the base and hairy. Hairs at the base of the stem are fairly dense and all short-stalked and branched with 2 rays (i.e. T or Y-shaped), becoming a mix of 2-rayed and unbranched (simple) hairs, then all simple hairs, and hairless on the upper stem.

Fruit: Fruit type: capsule/pod

[photo of fruit] Fruit is a slender, 2-sectioned pod up to 3½ inches long, drooping and mostly hugging the stem. Fruits are sometimes all on one side of the stem (secund) but usually not.

[photo of seeds in one row] Seeds are up to 1.4 mm long, oval with a narrow papery wing all around the edge, and arranged in 1 row in each section of the pod.

Notes:

Collins' Rock Cress was formerly known under a number of names: Arabis collinsii, Boechera holboellii var. collinsii, Boechera retrofracta var. collinsii, and others. It is understandably very similar to what are now Holboell's Rock Cress (B. holboellii) and Second Rock Cress (B. retrofracta), but is distinguished from all other Boechera species by hairs on the lowest part of the stem all 2-rayed, becoming a mix of 2-rayed and simple hairs then mostly hairless on the upper the stem, uppermost stem leaves hairless to sparsely hairy, the drooping and appressed fruit, and a national distribution of mostly east of the Rocky Mountains. By comparison, B. holboellii is restricted to Greenland, and B. retrofracta is mostly west of the Great Plains, its hairs on the lower stem are 2 to 8-rayed and the uppermost stem leaves are more consistently hairy. Other Boechera species may have a different arrangement of fruit/flowers and/or are hairless.

Collins' Rock Cress has an interesting history in Minnesota. It was long thought to be present only in northern Cook County, a single population discovered on a cliff face in the Boundary Waters Canoe Area in 1938. Meanwhile, its cousin B. retrofracta was recorded in sandy prairie areas around Halma in Kittson County, in the northwest corner of the state, and subsequently listed as a state Threatened species due to its rarity and small population sizes. We've visited some of those sites and studied those plants, and came to a surprising conclusion: what we saw was much more consistent with descriptions of B. collinsii than B. retrofracta.

We sought guidance with Dr. Anita Cholewa, curator at the Bell Herbarium, who came to the same conclusion. She also noted that Dr. Ihsan Al-Shehbaz, author of the Boechera treatments for Flora of North America and expert on the mustard family, had already annotated one of the Kittson County specimens as B. collinsii. After a review of all the specimens from the northwest part of the state, she determined they are all B. collinsii. Those from Lake and Cook counties have yet to be reviewed but it will be interesting to see what becomes of them...

At the time the specimens were collected and the Threatened designation made, B. collinsii may have still been treated as a variety of B. retrofracta, so the use of that name is understandable. Will B. collinsii replace B. retrofracta in the rare species list? Only time will tell.

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More photos

Photos courtesy Peter M. Dzuik taken in Kittson County and in South Dakota.

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