Honey Locust

Issue: 
July-August 2004

HOPEFUL MONSTERS' REFER TO VARIETIES of species that arise via mutation events and differ so conspicuously from the norm that they may be thought to represent different species or at least macromutations, or mutants (which used to be called ‘monsters’). Richard Goldschmidtmith (1940) used the term in his influential and controversial book The Material Basis of for Evolution. For the honey locust, Gleditsia triacanthos, what is the hopeful monster? Is it the parental species type Gleditsia triacanthos triacanthos with its forbidding armament of spines, or is it the horticultural favourite cultivar, spineless and podless, Gleditsia triacanthos var. inermis?

Range and Popularity
Wild growing honey locust in Canada is quite rare, restricted to the extreme southwestern part of the Carolinian ecoregion zone and within it, to the extreme southwest and Niagara area in Ontario. The natural range of the tree extends south to Louisiana, west to Oklahoma and Kansas, and east to the Appalachians. Trees grow to a height of about 25 metres with a trunk diameter of over 1 m. Larger trees sometimes have characteristically flattened crowns, reminiscent of savannah trees in Africa. One of the most spectacular examples of this is a female tree with a trunk diameter of over 3 m at Point Pelee at the most southerly tip of mainland Canada. As the trees age, individuals may be split to make multiple stems.

The beautiful, feather-like, bright green, doubly compound leaves provide a dappled shade. That is what makes the species such an attractive ornamental tree on lawns and in avenues. Moreover, the leaves turn golden yellow in autumn, and when they fall they naturally break apart so little raking is needed to clear up after them.

Pleistocene Remnants?
What about the branched spines? Strong and stout enough to puncture tractor tires, it is supposed that the fearsome horrendous array once provided protection from large herbivores, perhaps ground sloths, now long extinct. But considering the more than 6,000 years absence of the Pleistocene megafauna, could the production of such an extravagant defense mechanism (when it must come at the expense of allocating resources to other functions such as growth and reproduction) still be worth it? Perhaps such protection applies today to discourage deer and bears from browsing and squirrels, raccoons and other mammals from eating the maturing fruit. Or perhaps the thorns are simply an evolutionary anachronism still waiting to be purged from the species’ genetic makeup.

Fruit, Seeds and Nectar
The fruit of the honey locust is long (sometimes up to 40 cm) with a somewhat curved, flattish and twisted pod. Within that pod are up to 20 large (1 cm long), bean-like, chocolate brown seeds surrounded by a sweet gel. Presumably the dropped pods, after falling to the ground in autumn to early winter, are attractive food for mammals and birds which disperse seeds away from the parent trees. The seeds are durable and adapted to passing through guts. Scarification by teeth, beaks, and animal digestive juices may be a requirement for quick germination (within a year or two), but seeds can persist in the ground for years and still germinate.

As the name may suggest (although it is from the pod pulp that the name originates), the honey locust is a copious producer of nectar from its pale greenish and rather inconspicuous flowers. Individual trees are unisexual. The species is bisexual (dioecious) with racemes (inflorescences) of male and female flowers on different plants (dioecy). They start to flower when the trees are about 10 years old. Inflorescences are about 5 cm long, but are composed of many flowers on males and fewer flowers on females. Very occasionally, trees are hermaphrodite with both kinds of inflorescences. This breeding system is termed leaky dioecy.

The flowers are heavily and sweetly scented and attract many bees and other pollinators. After pollination, the male flowers are cast and a few of the female flowers give rise to the pods. In its wild state, the species is considered a ‘mass’ fruiter, that is, it typically only produces large quantities of fruit sporadically, although in some parts of its range it may produce fruit every year. Mass fruiting is often erroneously called ‘mast’ fruiting – mast is merely the word for forest tree nut-like fruits such as those of oaks, beech, hickory and such. Honey locust is a mass master.

Home and Garden
The horticultural value of honey locust derives not simply from its spreading form, shade and foliage, but also from the ease with which it can be propagated and transplanted, rapid growth, and tolerance to various conditions. Although the trees grow best in moist and acidic soils, they are tolerant of salt and pollution, and do well in full sun.

The horticultural forms, sunburst (with new foliage that is yellow), shademaster and majestic are spineless, podless and seedless (not much chance of a future outside the safety of human culture for these). Although spineless forms can produce flowers, horticultural stocks are mostly clones of one sex, so cross-pollination can not take place. They must be propagated vegetatively through cuttings, or from suckers which arise often in both spined and spineless types.

Odds and Ends
Historically, the spines from naturally occurring trees have been used as nails, the pod pulp has been used for brewing, and the wood is heavy and hard. It has been used for fence posts and railroad ties where the trees are common. The brittle wood takes a high polish and is attractive when used by skilled cabinetmakers for furniture. It was used in the past for making bows. Native peoples likely spread the tree as they used the pulp for food and the seeds for a coffee-like drink.

We’ll let you form your own opinion. Which is the hopeful monster now? The parental species with its forbidding weaponry, rare in Canada, and indeed not common throughout its range; or the far ranging and common asexual, spineless, useful beauty of our city parks, streets and gardens?

— Peter Kevan, University of Guelph and Helen Murphy, University of Windsor. Photos from Trees of Ontario, Lone Pine Publishing, 2001.

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