Bean Trees

by Nancy Rose, Editor of Arnoldia

September 14, 2016

flowers of Japanese pagoda tree

Bean Trees

northern catlpa seed capsules

Though its fruit capsules look like long beans, northern catalpa (Catalpa speciosa, 2776-B) is not a member of the pea family. Photo by William (Ned) Friedman

honeylocust pods

This common honeylocust (Gleditsia triacanthos, 14681-A) displays the pod fruits typical of the pea family (Fabaceae). Photo by Nancy Rose

As my vegetable garden winds down for the year, I am, as usual, astonished by the productivity-per-square-foot of the bush green beans. Green beans (Phaseolus vulgaris) are part of the pea family (Fabaceae), a large and diverse group that includes many economically important plants (e.g., soybeans, peanuts, alfalfa) as well as lots of choice ornamentals. Many of the nearly 20,000 species in Fabaceae (also known by the older family name Leguminosae) have a symbiotic relationship with soil bacteria that fix nitrogen in nodules on the plants’ roots, which can allow the plants to grow in nitrogen-poor soils.

flowers of Japanese pagoda tree

Japanese pagoda tree (Styphnolobium japonicum, 1239-83-C) bears loads of typical “pea flowers” in late summer. Photo by William (Ned) Friedman

The Arboretum collection holds over 300 plants within 19 genera of trees, shrubs, and vines in the pea family. Like my green beans, many have classic “pea flowers” including the lovely wisterias (Wisteria), fragrant black locust [pdf] (Robinia pseudoacacia), and late-blooming Japanese pagoda tree (Styphnolobium japonicum 216-35*A). Others have very different looking flowers, like the pink, powderpuff-like blooms of silk tree (Albizia julibrissin [pdf]). You’ll notice the family ties more, though, when you look at their fruit: typically an elongated, single-chambered pod containing a number of individual seeds.

All those dangling pods might make you think “bean trees” but, oddly, the species at the Arboretum that do sometimes go by the common name “bean tree” aren’t even in Fabaceae. Catalpa speciosa, best known as northern catalpa, and its North American relative, the southern catalpa (C. bignonioides), are also known as bean tree, Indian bean tree, or cigar tree for their prominent clusters of long, narrow fruits. Botanically, these podlike fruits are actually capsules that contain many fringed seeds. Catalpas [pdf] are members of Bignoniaceae, a family also represented at the Arboretum by several vines including trumpet creepers (Campsis spp.) and crossvine (Bignonia capreolata).

–Nancy Rose, editor of Arnoldia

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