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Styphnolobium - Growing Guide
The taxonomists have recently decided that the commonly grown Sophora japonica should now be renamed and given the unpronounceable name Styphnolobium japonica. It is probably easier still to refer to it as the Japanese Pagoda Tree or S. japonica whose care and treatment is much in line with the other species of sophora.
This is a spreading deciduous Chinese native tree which is widely grown in Japan. It has large pinnate leaves which are composed of up to 17 ovate to lance shaped dark green leaflets. The leaves produce very attractive yellow autumn colours but the late summer or early autumn flowering of this tree is dependent on hot, baking and dry summers. In the West Country we do not often see a good show of flowers but it performs better in the south east and in city gardens. It can be a bit frost tender until it develops a woody stem and frost pockets should be avoided.
Young trees do not produce flowers. However, when you do get a mature enough tree to produce some, they can be spectacular. Large terminal panicles of creamy-white pea flowers may be more of a rarity in UK gardens than in hotter Mediterranean countries but they are well worth waiting for. Borde Hill Garden have a very fine specimen growing in a hot dry location on top of a hill.
In colder areas this may be a plant for the back wall of a conservatory or, outside, for a hot sheltered sunspot beside a wall or fence.
Like all trees and shrubs with ‘pea’ flowers and ‘peapods’ of seeds the seed is easily germinated when sown fresh in a cold frame in late autumn.
I remain puzzled as to why this tree is not still classed as a sophora?
The hardiest species and the easiest to get away with outside in warmer counties or in warmer city environments are S. japonica and S. microphylla ‘Sun King’. They need full protection from cold north or east winds and are perhaps best grown alongside a warm sunny south facing wall. In the garden here we hide them away in a sunny glade to surprise spring garden visitors with Sun King’s large clusters of dark yellow pea flowers which appear before the leaves. This is a semi evergreen small tree in our climate.
S. tetraptera has long drooping clusters of yellow flowers in late spring and should be trained onto a wall for best effect. S. davidii is smaller growing and deciduous with terminal racemes of purple-blue and white flowers in late May or early June.
S. davidii and S. prostrata grow to a more suitable size for the greenhouse or shrub border. They like high levels of humidity under glass to prevent leaf drop and are best potted on once a year with a gentle haircut to maintain their shape. If you let them get a little potbound they seem to flower better in the next season. Use a peaty compost with added grit for best results. Sophora will flower away in pots at a very young age.
S. microphylla is an evergreen New Zealand native as is S. tetraptera. S. davidii comes from China. S. japonica is perhaps the largest growing of these trees and shrubs. It is also deciduous and can readily withstand a bit of dieback after a cold winter. In maturity it has grey ash-like bark but it is green-brown and spotted in immaturity.