Isodon longituba

Isodon longituba, also known as Rabdosia longituba, is a delightful autumnal bush. This tall and rare perennial produces a cloud of small tubular corolla flowers in a clear porcelain-blue, generously distributed among its somewhat loose vegetation, starting from October. This marvel brightens up the damp undergrowth and cool areas of the garden, while the colours of chrysanthemums explode in full light. Sometimes difficult to acclimatise, this plant thrives in moist soil. It is quite hardy, but it takes a little time to establish itself. 

Isodon longituba is a large deciduous perennial belonging to the Lamiaceae family. It is native to Northeast Asia, particularly the mountains of Japan. This magnificent and little-known plant was awarded at the Courson plant fair. Resembling a plectanthrus (it was previously called Plectanthrus effusus v. longitubus), it forms a bushy mass, with a slightly loose habit, as tall as it is wide. When fully grown, it will reach 80cm (32in) to 1m (3ft) in all directions. Emerging from the ground, its upright and relatively stiff stems bear opposite, triangular and elongated leaves with very serrated edges. They are initially light green, then turn a more greyish colour, and finally take on reddish hues towards the end of summer. The light blue, clear, and abundant tubular flowers appear in October and continue to bloom on the plant until the first frost. The stems die back each year in winter, and new growth starts relatively late, from April onwards.

Perfectly hardy, withstanding temperatures of around -19°C (-2.2°F), and undemanding in terms of soil composition, Isodon longituba thrives in a partially shaded position that protects it from the sun during the hottest hours of the day. It also tolerates complete shade. These conditions can be found in cool undergrowth, under the cover of deciduous trees or deep-rooted shrubs that do not deplete all the available nutrients at the surface of the soil. The plant will also thrive near a north or east-facing wall, alongside Japanese anemones, Fuchsia magellanica 'Alba', Strobilanthes, aconites, and foxgloves. Plant bulbs of Naples cyclamen or colchicum at its base. A combination with purple foliage in autumn, such as Physocarpus, Hydrangea Endless Summer® Twist and Shout, or Viburnum dentatum, is simply stunning. Ferns make perfect companions.