September. A month of transition. Flower-filled summer hands the baton over to the fiery foliage of autumn, and there is no plant that bridges this gap better than the Chinese plumbago, Ceratostigma willmottianum.
Where most plants with autumn colour wait until they finish flowering, this plant often goes unnoticed earlier in the year. Saving all its energy for one big display from July to September, it throws out sprays of cobalt-blue flowers which sit upon leaves rich in red, plum purple and orange shades.
Chinese plumbago is a small deciduous sub-shrub growing to 1m by 1m at a medium rate. It has bristly, mid-green stems with green to purple margined leaves, turning red in autumn. It has 5-lobed salverform rich blue flowers, borne in dense terminal clusters. These flowers look great viewed at dusk, being very vivid in the failing light.
C. willmottianum originates from dry open situations in West Sichuan, China. It is very easy to grow, providing it is planted in the correct position. It is happiest in a warm, sheltered location, ideally a south-facing slope or against a building (in cold spots it may not flower at all). If you can’t provide is with a south facing position, protect it by a hedge or wall. It requires a sunny position and cannot grow in the shade. C. willmottianum prefers well-drained, nutrient-poor soil, so improve drainage by digging grit or gravel into heavy clay soil. The poorer the soil the more compact its habit. It will happily grow on acid, neutral and alkaline (chalky) soils.
Flowering is best on the current year’s growth, so if the winter is mild enough and you wish to train it as a shrub, prune in spring. Thin out a third of the plant's growth, leaving a framework of the strongest stems, removing frost-damaged shoots at the same time. If you have a small garden and wish to train it as an herbaceous perennial simply cut it to the ground late autumn or winter.
To propagate you can either sow from seed, divide rooted suckers in the spring, and remove self-layered stems. Cuttings of half-ripe wood can be made in autumn. All young plants should be overwintered in a frost-free frame or greenhouse for their first winter.
There are only a handful of specimens of this AGM award winning plant growing here at RHS Garden Harlow Carr, and you can see good specimens planted in the rock wall behind the Alpine House. This plant is readily available to buy in the RHS Plant Centre.