Gleditsia

These deciduous, spiny trees can tolerate pollution, making them a good choice for parks and gardens, says Miranda Kimberley.

G. triacanthos ‘Rubylace’ - image: Floramedia
G. triacanthos ‘Rubylace’ - image: Floramedia

Plants with thorns get a bad rap, but they can be interesting features in their own right. Gleditsia trees are notable for the long thorns that often adorn their trunks and branches, and while you would not want to brush up against them you can admire them from afar.

Boasting fluttery, pinnate foliage in fresh shades of green, which in some forms turns yellow in the autumn, they are a good choice for parks and gardens and make excellent urban trees because of their tolerance of pollution.

There are around 12 species of Gleditsia, hailing from North America and Asia, with one species in Argentina. They are all deciduous, spiny trees, though there are several thornless cultivars that have proved popular. The foliage is pinnate or bipinnate, which provides a lovely open canopy. The flowers are pretty insignificant but afterwards long seed pods add a real feature.

Without doubt the most widely grown in the UK is G. triacanthos. It is also known as the "honey Locust" and comes from central and eastern USA. It is a large tree known for its tolerance of pollution and has a thornless form — G. triacanthos f. inermis — from which the highly popular ‘Sunburst’, with its golden yellow spring leaves, was bred.

There is also the dense, bushy G. triacanthos ‘Elegantissima’, which only reaches about 5m high but has attractive fern-like foliage.

G. triacanthos ‘Rubylace’ stands out for having red-tinged juvenile foliage that graduates to a bronze-red.

There are a few other species, but they are rarely available in the UK. Originally called G. horrida because of its spiny branches, G. japonica is a graceful, conical tree that has
fern-like foliage, made up of many small leaflets. Somewhat smaller is G. caspica, which has formidable spines of 15cm long and larger leaflets than most species.

These are not fussy trees — they can be planted in any types of well-drained soil, even doing well in dry soil. They are tough, hardy and need little maintenance. Given that Robinia pseudoacacia ‘Frisia’ has been beset by disease over the past decade, G. triacanthos f. inermis ‘Sunburst’ makes for an excellent alternative, offering the same lovely bright-yellow foliage that can be used as a foil against a darker-green shrub or tree layer.

Gleditsia have an interesting history. According to Deepdale Trees, beer has been made from the seed pods and the thorns have been used as nails. Honey locust wood is considered high-quality and polishes beautifully so there is a small market for using it to make furniture.

As horticulturists we know there is always so much more to a tree than meets the eye, but you certainly would not want your eye getting too close to a Gleditsia trunk.

What the specialists say

Mike Glover, managing director, Barcham Trees, Cambridgeshire

"Gleditsia is a very versatile tree, growing well on most soils, both wet and dry, when established. Its fine foliage and architectural habit give it a grace and beauty. Its spreading habit makes it more suited to verge and parkland planting.

"‘Sunburst’ has taken over from the disease-ridden Robinia pseudoacacia ‘Frisia’ as one of the best yellow foliage trees on the market. ‘Skyline’ and ‘Shademaster’ are both good green urban forms. ‘Rubylace’ lacks the vigour to make a tree so should be avoided for commercial plantings.

"Formative pruning when young is necessary to ensure that laterals don’t hold dominance over the tree and distort its shape."

Matthias Anton, managing director, Deepdale Trees, Bedfordshire

"Gleditsia is a very good tree for street and city planting as it needs very little water and will grow well in poor soil. I think with these characteristics it should be used more often than it is in inner-city planting projects.

"The species or varieties that stand out for me are the thornless forms G. triacanthos f. inermis, G. triacanthos ‘Skyline’ and G. triacanthos f. inermis ‘Sunburst’. Inermis and ‘Skyline’ both have green foliage. Inermis is a large tree with an open crown. ‘Skyline’ is an upright tree that is ideal for street planting. ‘Sunburst’ is much smaller and, as its name suggests, has bright-yellow foliage in the spring that turns to pale-green in the summer. It makes a good substitute for Robinia pseudoacacia ‘Frisia’, which can be brittle. These thornless types produce few, if any, seed pods."

Joanne Stott, partner, Larch Cottage Nurseries, Cumbria

"Gleditsia or honey locusts are striking trees, mainly from North America, many of which are thorny. G. triacanthos f. inermis ‘Sunburst’ is the most popular of the varieties we stock. It is a thornless, fast-growing tree of open airy habit. The fern-like golden foliage makes an excellent accent against darker foliage.

"This variety prefers sun because of its golden leaves, but others are shade-tolerant. They are useful in urban plantings as they are tolerant of pollution. They are also tolerant of drought and salt-tolerant, making them good for coastal areas. ‘Sunburst’ also makes an excellent alternative to Robinia ‘Frisia’.

"The fruit of the honey locust is a flat legume [pod] that matures in early autumn. The pulp on the insides of the pods is edible, unlike the black locust (Robinia pseudoacacia), which is toxic."

In practice

Robert Player, proprietor, Garden Associates, central London/Hertfordshire

"We grow G. ‘Sunburst’ a lot. If you want a splash of yellow I prefer it over Robinia ‘Frisia’ because although it is not vibrant it is much less brittle in more exposed areas. I find also it is quite hard to find juvenile robinias that are a good shape — they are always rather gangly and tend to die back, whereas ‘Sunburst’ more often than not are a good shape and establish quickly.

"I grow them well in the cold, wet clay of Maida Vale and also in the free-draining loam of Kensington, so they aren’t choosy. All I would say is that most tree surgeons avoid them like the plague because they are simply very dangerous to climb. It’s easy, if you slip, to impale yourself so work on them is generally expensive.

"The huge thorns that sprout from the trunk of G. triacanthos make them a difficult choice for public areas, but we just cut our lower ones off, leaving the higher ones to deter any unwanted climbers."

Species and varieties

G. caspica, or the Persian honey locust, is a rarely seen medium-sized tree that has pinnate leaves with racemes of greenish flowers, followed by long dark-brown pods reaching 20cm. Its trunk is covered in branched spines. Height: 12m.

G. japonica is a medium to large-sized deciduous tree that has a conical habit. It produces fern-like leaves made up of many small mid-green leaflets. Its trunk is covered in spines and its shoots are purplish when young. Height: 15-20m.

G. koraiensis, the Korean honey locust, is a rarely seen large tree that has pinnate leaves with greenish flowers in June. Its pods ripen from October to December. The flesh inside the pods is edible and used medicinally in its country of origin, Korea. Height: 20m.

G. triacanthos is a large, deciduous, fast-growing tree. It has pinnate leaves that are fresh green upon unfurling, mid-green during the summer and turn golden yellow in the autumn. Its stems and trunk can have large, branched thorns and it produces long, flat, slightly twisted pods. Height: 20m.

G. triacanthos ‘Emerald Cascade’ is an interesting cultivar with hanging branches. It is a small tree that only reaches 5m high, with an irregularly growing open crown. Its large, compound leaves are dark-green and pinnate or bipinnate. It sprouts late and turns golden yellow in autumn. Can be used as a specimen in parks and gardens, and can also be used in hard surfaces. Height: 5m.

G. triacanthos ‘Rubylace’ is a lovely form with ruby-red, fine, pinnate young leaves that gradually darken to an attractive bronze-green as they age. Becomes a bushy shrub or small mop-headed tree. Tolerant of dry soil conditions. Height: up to 10m.

G. triacanthos f. inermis ‘Sunburst’ has lovely pinnate, fern-like leaflets that are golden yellow in spring and paler green in summer. It forms a small mop-headed tree. Like its parent it tolerates pollution well so makes a good urban tree. Will only produce a few, if any, seed pods. Height: 8-10m.

G. triacanthos f. inermis Spectrum = ‘Speczam’ is another golden foliaged form that does not appear to be currently available in the UK. Becomes a broadly rounded tree with spreading, pendulous branches. Height: 8m.


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