Robinia have been beset by problems over the past decade. The most widely grown variety, the golden-leaved ‘Frisia’, which until now has been a popular tree for streets and parks, has been suffering from an as yet unidentified disease.
Regardless of the identity of the exact set of factors leading to the decline of these trees, the symptoms are clearly seen — new leaves not unfurling in the spring or leaves falling prematurely, then there is branch dieback and, in the worst cases, tree death.
Growers are reducing their stocks of this variety in particular so the decision for gardeners and landscapers now is whether to completely shy away from the genus or to continue growing those species and varieties that seem to be escaping the fate of ‘Frisia’.
Robinia is part of the highly attractive legume family Papilionaceae and therefore produces pinnate leaves and racemes of those lovely butterfly-like pea flowers. It is a small genus, with just 10 species of these fast-growing trees and shrubs. They all hail from the USA and northern Mexico.
They have a suckering habit and often spiny stems that can be seen as a problem when cultivating them. But when used as a landscape tool to stabilise banks or loose ground, these features prove useful.
The parent of ‘Frisia’ — and several other excellent cultivars — Robinia pseudoacacia has a great pedigree. It was named by Carl Linnaeus in honour of the French botanist Jean Robin and his son, royal gardeners to Henry IV of France. A grand specimen of this variety can be found in the Royal Botanic Gardens, Kew.
It was planted as far back as 1762, making it one of the oldest trees to be found in the gardens.
R. pseudoacacia is an imposing tree when mature, with a wide trunk covered in deeply furrowed bark. It has mid-green pinnate foliage and clusters of scented, white flowers in June and July.
It is as yet unaffected by the same troubles affecting ‘Frisia’ so can still be considered for planting in parks and streets. The varieties ‘Bessoniana’ and ‘Umbraculifera’ can also still be considered good street trees with their small-to-medium size, neat habit and spineless branches.
Other species and varieties that are worth considering include R. hispida and several hybrids — R. × ambigua, R. × margaretta and R. × slavinii.
R. hispida is a shrub or small tree that produces fragrant, rose-pink pea flowers from May to June. It can be quite brittle so is best given a sheltered position — growing it against a sunny wall is ideal. The hybrids are all small to medium-sized trees with attractive clusters of flowers in shades of pink, suitable for a small garden, particularly the elegant R. slavinii ‘Hillieri’ Award of Garden Merit (AGM).
Robinia are not fussy about soil conditions so are they adaptable to ordinary soils, being especially useful in dry, sunny situations. They are also tolerant of atmospheric pollution, which is why they make such good street trees. Perhaps because of their quick growth they produce brittle branches. This means that hard pruning is recommended for small species or varieties.
For those of you who already grow ‘Frisia’ and are experiencing dieback, the RHS has a fact sheet on its website. But the long and short of it is that there is no control yet identified for the decline.
It is not yet known whether it is the fault of a single disease. A combination of factors including the wetness of the last few winter seasons leading to a rise in leaf spot fungus and root rots, including Phytophthora and honey fungus, as well as a wilt fungus could be the cause. Until more is known, landscape managers will have to deal with the symptoms as they see fit.
What the specialists say
Mike Glover, managing director, Barcham Trees, Cambridgeshire
"Robinia has suffered greatly as the until now popular variety ‘Frisia’ has shown itself to be particularly susceptible to terminal disease. The genus makes a poor urban tree as it’s habit is variable and its branches are weak — prone to breaking out.
"We have stopped growing ‘Frisia’ and now only stock ‘Bessoniana’ and ‘Umbraculifera’, but in small numbers for customers who need to continue with existing avenues. We stock more than 120,000 containerised trees ranging in size from 10-12cm to 40-45cm girth and of these fewer than 200 are robinia.
"As alternatives, R. pseudoacacia ‘Bessoniana’ is probably the pick of the robinia clones for durability, but R. × margaretta ‘Casque Rouge’ is the most attractive, though haphazard and weak in structure. They are best planted in any well-drained soil."
Matthew Pottage, garden manager for hardy ornamentals, RHS Garden Wisley
"The naked silhouette in the winter months of an old R. pseudoacacia is an unforgettable sight, with fissured bark and a rugged branch outline. The compound leaves of this genus are complemented by racemes of flowers, borne typically in June, and are either pink or white.
"Warm summer temperatures can help increase flower profusion and heat generally helps to ripen the wood and bring the tree into leaf, which can be late in a cold spring.
"Robinia have vibrant, large leaves when young, therefore pollarding plants can show this quality off to full effect. But be cautious of disturbing the roots, which can trigger the undesirable suckering habit.
"My fond favourite is R. × slavinii ‘Hillieri’, which has a delicate leaf and is peppered with fragrant pink flowers in June, eventually making a small tree, perfect for a lawn centrepiece in a sheltered garden.
R. hispida var. rosea is a smaller subject, ideal for a sunny border, bearing striking pink flowers on stems that are typically free of spines.
"For the smallest of spaces, look out for R. pesudoacacia ‘Lace Lady’, which can be grown in a pot and forms a mound of twisted stems and lightly rippled leaves — interesting in both summer and winter.
"At Wisley, we sadly lost the four striking R. pesudoacacia ‘Frisia’ that stood outside the main restaurant to the undefined pathogen that is causing the decline of this once splendid cultivar.
However, golden leaves can still be enjoyed from Gleditsea triacanthos ‘Sunburst’, Catalpa bignonioides ‘Aurea’ and Quercus rubra ‘Aurea’."
Species and varieties
R. pseudoacacia — the black locust — is a large, suckering, deciduous tree native to North America. It has a picturesque oriental appearance with rugged and deeply furrowed bark. Its twigs are covered in spiny stipules and it produces mid-green foliage and clusters of scented white flowers in June and July, followed by pods. Popular for parks and streets but can spread aggressively by suckering from the roots and can become invasive due to its prolific seed production. Height: 25m.
R. pseudoacacia ‘Bessoniana’ is a thornless false acacia that is smaller than its parent, also producing foliage that is paler yet a more vibrant green. With its compact, rounded crown of virtually thorn-free branches, medium height and tolerance of urban pollution, it is considered perhaps the best robinia cultivar for street planting. Not suited to windy sites, however, because its branches are brittle. Height: 10-15m.
R. pseudoacacia ‘Frisia’ has been a highly popular tree for parks, gardens and streets because of its attractive golden/yellow pinnate leaves. But sadly over the past decade it has been hit by diseases leading to foliage loss, dead branches and ultimately decline of the whole tree. As a result, growers have severely reduced their stocks. Height: 15m. Spread: 8m.
R. pseudoacacia ‘Lace Lady’ Plant Breeders’ Rights (syn. ‘Twisty Baby’) is a dwarf variety with twisted branches and stems and pinnate leaves. It can be grown in a garden border or a large container, either as a shrub or a small standard tree. Best planted in moist, well-drained soil. Height and spread: 2.5-4m.
R. pseudoacacia ‘Rozynskiana’ is a rarely seen variety that becomes an elegant large shrub or small spreading tree with a pendulous habit. It produces profuse fragrant white flowers in June. Use in a natural setting with plenty of space. Ideal for tough sites such as dry, gravelly soils.
R. pseudoacacaia ‘Umbraculifera’ is a small mop-headed tree as a result of being a top grafted clone. Ideally top grafted 1.8-2m from ground level so they can easily be walked under, they form a rounded and compact crown ideal for urban piazzas and town gardens. More regularly seen in French and German cities than here in the UK. It thrives in most soils and is tolerant of urban conditions. Mature height: 5-10m.
R. pseudoacacia ‘Tortuosa’ is a slow-growing, small to medium-sized tree with contorted branches, making it an interesting feature. Usually grafted on a high stem, but sometimes on a low stem to produce somewhat shrub-like growth. A multi-trunked tree then develops later.
The thorny branches grow in remarkable curves and young shoots often grow like corkscrews. The more-or-less hanging leaves are also somewhat twisted. Flowers appear only occasionally, and only on old trees. Sensitive to strong winds. Selected for parks to provide striking winter silhouettes. Height: 10-15m.
R. hispida is a shrub or small tree with arching stems clothed in bristly purplish/brown hairs that produces short, wisteria-like trusses of fragrant, rose-pink pea flowers from May to June. Dark-red/brown seed pods follow in the autumn.
Ideal for a warm, sheltered wall offering protection against strong winds. Its long slender shoots can be quite brittle. Best in moist, well-drained fertile soil but tolerates poor soils and drought. Height: 2.5m. Spread: 3m.
R. × ambigua is a small tree with somewhat sticky shoots that produces short racemes of pale-pink flowers in June. Sometimes flowers again in the autumn after a warm summer. Height: up to 4m.
R. × margaretta ‘Casque Rouge’ (syn. ‘Pink Cascade’) is a very profuse and exotic rich-pink flowering form of the false acacia tree. Becomes a large shrub or medium-sized tree. Produces dark-green pinnate leaves and loose clusters of fragrant, purplish/pink flowers in June. Height: 6-10m.
R. × slavinii ‘Hillieri’ AGM is an elegant small tree with delicate foliage, developing a rounded head of branches. Produces plenty of lightly fragranced lilac/pink flowers in June. Pruning hard after flowering is recommended. Needs protection from strong winds. Ideal for a small garden or courtyard. Fairly unusual — only small numbers of this lovely tree are produced each year. Height: 8m. Spread: 6m.
Thank you to Floramedia, which supplied the images for this article from its photo library