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Robinia - Growing Guide
Robinia are a small genus of North American trees and shrubs. Their name commemorates Jean Robin, herbalist to Henry IV of France (d. 1629). They are amongst the most ornamental of all hardy trees both in leaf and in flower. The leaves are pinnate and the pea shaped flowers are borne in pendulous racemes followed by flat brown seed pods with many seeds. The stems and branches are often spiny.
Robinia are suitable for any ordinary soil and grow best in dry sunny situations where soil drainage is good. In improved soils they tend to grow exponentially which usually causes their brittle branches to snap off in the wind or when branches get too heavy with new growth. For this reason, in a smaller garden context, they are best pruned hard after flowering to maintain the shape of the tree at a reasonable size and to encourage more flowering on the newer growth in the next season.
Robinias are suckering trees which can provide opportunities for lifting suckers with roots to transplant elsewhere. In reality the suckers need to be removed at the end of each season to preserve vigour in the main tree. Seed is the easiest method of propagation and these should be sown in a cold frame in the autumn.
All Robinia are extremely tolerant of polluted atmospheres and grow especially well in cities on poor soils.
Robinia hispida, the Rose acacia, is more of a shrub than a small tree and grows to only 8-10ft with a similar spread. The flowers are rose pink in 5in long racemes and the stems are extremely bristly or spiny. The pinnate leaves are around 12in long and composed of nine to thirteen ovate leaflets. This plant thrives when grown on a hot wall and will need firm staking initially.
Robinia pseudoacacia ‘Frisia’ is a smallish ornamental tree with gorgeous golden yellow foliage in spring turning yellow-green in the summer and orange-yellow in the autumn. The pinnate leaves are 12in long with up to 23 ovate leaflets. Fragrant white flowers appear in midsummer in long hanging racemes followed by long brown seed pods. Robinia pseudoacacia is known as the False acacia or Locust tree.
Robinia x slavinii ‘Hillieri’ has made an attractive small spreading tree of around 15ft in height and width at Caerhays after 30 years or more. The spines or prickles are quite impressive but so are the fragrant lilac-pink flowers which appear in June but often not all at the same time. Strong gales have split branches off over the years and we have not pruned it as much as we probably should. The pinnate leaves are up to 8in long and are dark green with up to 19 ovate leaflets.