Sunday, April 12, 2015

Purple (Not really) Bitter Cress (Cardamine douglassii)

Cardamine douglassii (04/08/15)
Durham Co., NC
Cardamine douglassii is sometimes called Limestone Bitter Cress, or Purple Bitter Cress. Neither name seems to be that appropriate (but common names rarely are),  First, it only rarely (maybe never?) grows directly on exposed limestone, and certainly never does so in NC. Second, the flowers are rarely purple, but nearly always appear pure white (but see below). For these provincial reasons, I prefer to call it "Douglas's Bitter Cress".

Douglas's Bitter Cress,
atypical "purple flower" variant
Granville Co., NC
Douglas's Bittercress is another plant species with restricted distribution in North Carolina, and which seems to be most abundant or widespread in the mid-western United States.  In North Carolina it occurs only in a few Piedmont counties, including Durham & Granville; these locations and the those in the Virginia Piedmont are significantly disjunct from the nearest populations to the west.  The habitat where I have observed the species is infrequently flooded floodplain forests. One of the sites supporting the species is shown below:

Cardamine douglassii habitat in Durham Co., North Carolina
Bottomland Hardwood Forest, lush cover of Spring Beauty (Claytonia virginica)
04/11/15, Cardamine just finished flowering

Like other Bitter Cress species, Cardamine douglassii is part of the Mustard or Brassicaceae family and produces slender tubular seed capsules, or "siliques", that are much longer than wide.

Cardamine douglassii in full fruit

Stems usually lack leaves for most of their length. Several accounts from other states indicate stems are hairy, but I have not observed that in North Carolina. Cauline leaves are usually found on the lower third of the stem, unlobed and usually toothed. Basal leaves are more heart shaped. Younger, non-flowering plants of these perennials produce only rounded leaves
Fortunately, the species seems to require little in the way of active management.  Localized threats to populations in North Carolina include construction & maintenance of water and sewer easements. installation of waterfowl impoundments, beaver flooding, and competition with invasive exotics, especially Japanese Honeysuckle. 

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