Curious, pretty shrub with distinctive pods
Bladder senna is a very pretty shrub or small tree in flower at the moment. Indeed, it has been in flower for a few weeks and will continue to produce its pretty yellow or orange flowers into late summer. The flowers are carried in short bunches of three to 10 flowers, each about two centimetres long.
It is part of the pea family and the flowers are the typical pea-flower shape, familiar from laburnum, broom and peas themselves, including sweet pea flowers, which, though much larger, are the same shape. The name bladder senna is unusual but when the 'bladders' are seen, it is very obvious.
The bladders are effectively the pods that normally form after flowers of the pea family fall, but in the case of this plant, they take on a very distinctive swollen form. They are pea-pod shape, about 8cm long, but so bloated that the sides of the pod become translucent as they mature.
When mature, in late summer and early autumn, the shrub is covered with pale brown or pale white-green bladders and the effect is quite decorative, especially as they are nicely set off by the slightly ferny-looking leaves. Each leaf is divided into five or six pairs of leaflets, which gives the ferny appearance.
There are two forms seen in gardens, neither very frequently, and occasionally it is seen as a very pretty small street tree. One form has yellow flowers and makes a large shrub or small tree to about 3m tall and wide. About the same size, the other form is a hybrid and it has orange flowers, often with a brown mark at the centre. The other parent, which is not much grown, has red-copper flowers.
These plants' botanical name is colutea. The yellow species originated in southern Europe and the red-flowered one is native to Iran and the surrounding region. It gets the senna part of the name because it is related to the herb senna which is used as a natural relief for constipation. However, colutea is not the true senna and consuming its leaves or bladders can cause stomach pains.
Like most members of the pea family, bladder senna likes well-drained soil, ideally gravelly or sandy, and it is very happy to grow on a dry bank where it will flower even more freely. It thrives by the seaside and it likes full sunshine. The leaves fall in autumn, turning yellow, and the papery bladders often hang on until the winter winds shatter them.
The plant is easily grown from seeds or cuttings. It is hardy and has been grown in northern Europe for hundreds of years. It is such a pretty shrub, and a curious one, that it should not be ignored.