Caulophyllum thalictroides

Caulophyllum thalictroides, better known in its native North America as the Blue Cohosh, is a rather unusual rhizomatous perennial, long used by native people and then in modern pharmacopeia, but extremely rare in cultivation. For us gardeners, it is remarkable for its ability to grow in the darkest areas of the garden, in soil that does not dry out too much. Its young purple-green shoots are almost black in colour, and accompany an equally dark flowering in spring. This plant then reveals beautifully cut foliage, which has earned it its other name of False Blue Cohosh, displaying a beautiful glaucous hue once unfurled. After the flowers, which are quite insignificant, deep blue berries appear, in a shade that seems somewhat unreal in the dense shade of large trees. The berries remain decorative for a long time.

Caulophyllum thalictroides, also called squaw root, yellow ginseng, or blue ginseng, belongs to the Berberidaceae family. It is native to eastern North America, from Manitoba and Oklahoma to Quebec and a zone extending to the Atlantic Ocean. It is found in the deep shade of deciduous forests, sometimes on hills, in moist, rich and light soil. It is a rhizomatous perennial, whose annual vegetation disappears in winter and reappears in spring. It is extrremely hardy. The plant spreads slowly through the production of non-wandering rhizomes that remain in a 'clump'.

False Blue Cohosh grows in a clump of leafy stems reaching an average height of 65cm (26in) with a spread of 50cm (20in). Each rhizome produces only one to three purplish stems, each with a large leaf at the base and a smaller one higher up, below the inflorescence. This vegetation appears almost black at first. The leaves unfurl, revealing a strongly cut structure with leaflets, similar to that of the leaves of cohosh. Each leaflet, 3 to 8cm (1 to 3in) long and 3 to 10cm (1 to 4in) wide, is obovate to ovate in shape, with 3 to 5 lobes. At this stage of development, the leaf is a slightly bluish green with a glaucous hue. In April-May, the inflorescence evolves parallel to the leaves. It takes the form of a large terminal cyme, composed of 5 to 70 small blackish flowers, 1 to 1.5cm (1in) in diameter. Each flower is composed of 6 thin, fleshy, slightly curved sepals, which are purple, green, or yellow. The centre of the flowers is occupied by short yellow stamens. After pollination, the flowers give way to clusters of small round fruits, 8mm (1in) in diameter, in a very dark blue colour. The seeds will only germinate very late in the summer of the following year.

The Blue Cohosh will delight gardeners looking for plants capable of thriving without care in the deep shade of deciduous trees. Its deep blue berries remain decorative for a long time, and take on an almost unreal reflection in the absence of light. Amusing, unusual, and easy to grow, it is content with any good light garden soil that is not too dry. It will grow in acidic, neutral, or even alkaline soil. It can be combined with other shade plants such as variegated ivy, lily-of-the-valley, and Liriope muscari.

Medicinal properties: This plant is widely used in Native American pharmacopeia, and is now one of the flagship products of homeopathy. It is known for its emmenagogue, antineuralgic, and antirheumatic properties. Native Americans used it to facilitate childbirth, and some used its seeds as a coffee substitute. Its rhizomes and roots contain a variety of molecules, responsible for the plant's medicinal properties.