Caulophyllum thalictroides

Photos by Mark Dwyer

I remember my first exposure to this plant in a state park in Wisconsin many years ago. As I was following a trail in early fall, I noticed groupings of plants featuring what appeared to be “floating” blueberries for lack of a better description. I was mesmerized by what I later identified as blue cohosh (Caulophyllum thalictroides). At that time, I also noticed the similarity in the foliage as compared to meadow rues (Thalictrum sp.); hence the logical species name. The groupings I noticed were all in rich, moist and well-drained soils which is exactly what this undemanding plant requires and deserves for maximum vigor and appeal. I’ve since used this plant extensively in appropriate understory applications as it tolerates both part shade and shade while offering amazing texture. The late season fruiting display is a colorful ornamentation and the primary feature of interest.

Caulophyllum thalictroides is in the barberry family (Berberidaceae) and has a wide native range in the Eastern and Central United States and in parts of Canada. Spreading slowly by rhizomes and becoming bushy once mature, this many-stemmed, colonizing plant reaches 12-36 inches and features thrice-compound leaves with lobed leaflets. Spring foliage emerges smokey, purple-blue and matures to a bluish green by summer. The foliage can become bright gold in autumn. Inconspicuous, yellow-green to greenish-brown, star-shaped flowers appear in early spring in loosely branched structures. The blue, berry-like, fleshy seeds are held stiffly above the foliage and resemble small grapes (1/3 inch in diameter). It is important to note that the fruits, roots and leaves may cause skin irritation if touched and the fruits are mildly toxic to humans.

Once established in a favorable site, blue cohosh is long-lived and best left undisturbed. I’ve watched a patch of this perennial that I planted over 20 years ago continue to thrive and slowly expand from five original plants. That phrase, “moist but well-drained” is essential for this plant which prefers a neutral to slightly acidic pH but is quite adaptable. With no known insect or disease issues, this tough native is also deer resistant and has a long history of Native American use. While the fruits are toxic to humans, apparently birds and mice aren’t shy about nibbling.