Colutea arborescens

Colutea arborescens, cousin of Tree Medick, flowers in spring, in the form of clusters of papilionaceous flowers of a somewhat dull yellow, speckled with brown. The peculiar appearance of the fruit gives it the name "bladder senna". While not particularly spectacular, this deciduous bush is robust and easy to grow in any well-drained, even poor, limestone and stony soil. It suits a dry garden and enjoys the company of other scrubland plants.

Colutea arborescens belongs to the Fabaceae (Leguminosae) family, and is native to southern Europe and North Africa. It grows quickly, and its adult size will reach on average 2 m (6.6 ft) in height, with a spread of 1 m (3.3 ft). Its habit is bushy, erect, well-branched during its early years, then it becomes bare at the base and forms a small trunk supporting a bushy crown.

Its branches bear deciduous leaves divided into 7 to 13 oval or elliptical leaflets. They are strongly veined, glabrous, and of a bright green colour. Its flowering takes place in summer. The inflorescences are like small bouquets of papilionaceous flowers (up to 5 flowers) born in the axils of the leaves. They are yellow in colour, marked with 2 brown-ringed spots at the base, and attract pollinating insects. This flowering is followed by the formation of translucent and swollen pods, 5 cm to 7 cm (2 in to 2.8 in) long, changing from greenish-yellow to brown-red when ripe. Its root system is equipped with nitrogen-fixing nodules present in the air. These are small balls the size of a pinhead, which are often found in plants of the Fabaceae family. This is an adaptation to very poor soils. Another specificity of this bush lies in the fact that it naturally layers quite frequently. It is the only plant whose fruits play host to the caterpillars of a very pretty blue butterfly called Blue Argus or Giant Blue (Iolana iolas).

Colutea arborescens has the advantage of thriving in difficult conditions, including severe frosts. It remains attractive throughout the summer. It is valuable for gardens where the soil lacks depth. It can be planted in a small hedge (less than 2 m (3.3 ft)) alongside Coronilla glauca, Cotinus, Juniperus oxycedrus, Lonicera tartarica, Phillyrea angustifolia, or even Pistacia lentiscus (in mild climates). All these low-maintenance bushes are perfect for bringing a little life to tricky areas of the garden. You can plant small shrubs around its base, for example, Alypum Globularia, which flowers in winter, low cistus (Cistus pulverulentus), helianthemums or shrubby perennials with summer flowering like Salvia microphylla, S. greigii or S. jamensis with red flowers (Royal Bumble, Red Dream, Raspberry Royal) or blue like Salvia 'Blue Armour' or S. chamaedryoides, which will take over in summer.