Late late show - autumn bloomer
The striking blue flowers of ceratostigma make a wonderful contrast with the predominantly yellow and orange hues of autumn foliage. While the name is unlikely to be familiar, the plant is reasonably frequently seen in gardens. There are not many shrubs that flower after mid-summer and it is even more valuable for that reason. It has a common name of leadwort but this is not much used, if at all.
Ceratostigma starts to flower in August and continues into September and October. Although the brilliant blue flowers are small, only 2cm across, they are carried in clusters at the tips of the shoots. Individual flowers don't last very long, shrivelling up after a few days but they are produced in large numbers, and seem to be unusually plentiful this year. The shade of blue varies but is clear and bright.
There are two main kinds grown: Ceratostigma plumbaginoides and Ceratostigma willmottianum, both native to China. The first is a perennial with a woody base. It is about 40cm tall and has rhizomes that spread under the soil surface. It can be a bit of a spreader in ideal conditions, but it is easily controlled if necessary. Later, its leaves take on a rich red colour enhanced by the bright blue flowers.
The other commonly grown species Ceratostigma willmottianum is much bigger, a shrub that can reach 1m tall and more in width. It has red-brown stems and carries bright light-blue flowers. Its leaves change colour to fiery shades of orange and red before falling.
This plant is easy to grow in any reasonably fertile but not too rich soil, which tends to make the plant leafy at the expense of flowers. The soil should be fairly light and well-drained in winter. Because the plant can be damaged by severe frost, it has a better chance when grown in light, well-drained soil which makes its stems harder and more resistant to frost.
Both kinds can be trimmed back to ground level in early spring to keep them low or the shrubby kind can be allowed to grow a bit taller. Hard frost may cut them down but they readily re-sprout and flower the same year. Although they tolerate some shade, they flower best in sunshine and the blue colour is more vivid.
How to increase my dahlia flowers?
Q: My dahlias, which I have had for several years, produced a lot of leaves but not a lot of flowers this year. Why was that and how can I increase flowers next year? I don't lift them in winter. M Munroe, Co Mayo
A: When dahlias do not flower well, it could be due to the soil being too rich and moist, or too shaded, or too much competition from other plants, and possibly a late start due to slug damage to the new shoots of plants overwintered in the ground. Address one or more of these factors, and use a high-potash feed in early summer.