Between shimmering foliage and decorative fruiting, autumn has treated us to exceptional colours this year. And the colours still linger, thanks to the very last flowering of the season, that of the Isodon longituba, also called Rabdosia longituba. If you do not yet know it, here is an introduction and some tips to grow it successfully...
This large Japanese perennial, native to the island of Honshu, is the last perennial in the garden to flower. This attractive exotic, uncommon in our gardens, belongs to the sage family. Exotic in appearance for the bush it forms, literally covered with a mass of intensely electric-blue tubular flowers rarely seen on other perennials, Isodon remains easy to grow.
Although its exotic habit contrasts with other plants in the garden, it is a perennial that can be used with sages or grasses in a naturalistic border or in a large pot on a terrace. Easy to grow, it prefers a sunny position and will grow in any good garden soil; it is as straightforward as a sage or a lobelia.
However, it requires a warm, sheltered position, not because it is fragile, but so its flowers can reach maturity. Its late flowering occurs between late October and early November, while in many regions frost is already starting to appear. And as with many herbaceous perennials, its flower buds are damaged from −2 to −3 °C.
The planting site must therefore be chosen with great care: plant it ideally on a south- or west-facing spot, sheltered from the wind, among other plants, to provide the best thermal protection. If, however, you live in a mild climate the issue will not arise and you can plant it wherever you like.
Late to flower in autumn, it is also late to restart in spring. It is often one of the last to reshoot... no need to worry! Nevertheless, like many late perennials, its young leaves emerge at the time when slugs are rampant. So watch it carefully around April and do not hesitate to spread some slug pellets around the stump as a preventive measure.
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