Catalpa bignonioides Seeds | Southern Catalpa
LATIN NAME: Catalpa bignonioides
COMMON NAME: Southern Catalpa, Indian Bean Tree
Plant Profile
In the southern United States the catalpa was not planted for shade. The caterpillars of the catalpa sphinx moth feed on its leaves and nothing else, and a tree carrying a good brood of them was worth guarding. Anglers shook the branches over a sheet to collect the larvae and kept them in tins for catfish bait, and some farmers planted whole groves for the purpose. A tree that feeds a moth so completely that people grew it as farm stock is worth the space on its own.
The leaves are the size of a dinner plate, up to twenty-five centimetres long, heart-shaped and softly downy when young, arranged in whorls of three around the stem rather than alternately. At the base of each leaf blade, and on the stalk, sit small glands that secrete nectar outside the flower; ants and wasps visit them and in return patrol the shoot against caterpillars. The leaves give off an odd, faintly burnt smell when bruised, and they hold a great deal of water, so a tree in a dry site will wilt visibly at midday.
The flowers open in early summer in upright panicles up to thirty centimetres high, each bloom four to five centimetres across, white and bell-shaped with frilled edges. Inside the throat run two rows of yellow ridges, and around them lie blotches of purple-brown; these are nectar guides, directing bees to the base of the tube. The fruit is a slender capsule that ripens through autumn into a long brown pod, packed with small seeds that carry a fringed, papery wing at each end and travel a long way on the wind. The pods hang on the tree through winter, which is where the names cigartree and Indian bean tree come from.
Catalpa seed oil carries catalpic acid, a conjugated octadecatrienoic acid in which the double bonds sit in alternating order. Conjugated fatty acids cure quickly on exposure to air, so they have been studied as renewable feedstocks for coatings, inks and resins; the best-known plant example is the calendic acid of pot marigold seed oil. The oil also contains the ordinary linoleic and oleic acids in quantity.
Science & Biotechnology
The genus gave its name to catalpol, the iridoid glucoside first studied in these trees and later found in plantain and in several medicinal plants. Catalpol belongs to a large family of bitter, water-soluble monoterpene derivatives, and its route from geraniol through iridotrial and the enzyme iridoid synthase is one of the better-mapped secondary pathways in plants. The Asian species also carry naphthoquinones, among them catalponol and catalpalactone, and catalpol has been examined for its effects on nerve tissue and on glucose handling.
Because the catalpa sphinx is faithful to this genus, the pair has become a study system for induced defence. Leaves that have been chewed raise their content of phenolic compounds and iridoid glycosides, and the timing of an attack matters more than its size: a tree stripped before midsummer can refoliate and still set some fruit, while one stripped late loses the season. Workers also use the tree to test how much defoliation a fast-growing deciduous species can absorb.
Uses & Benefits
In the garden the tree has two quite different uses. Left to grow, it makes a generous shade tree whose flowers are a serious nectar source: catalpa honey is light and mild, and beekeepers in the southern states value the tree for the summer flow when little else is in bloom. Cut back every winter to a permanent framework of knuckles, however, and the tree answers with shoots carrying leaves larger than the normal ones, an effect that has made pollarded catalpas a feature of European parks. The timber is light, durable in contact with soil, and was long used for fence posts, rails and crates.
Growing from Seed
Catalpa bignonioides is a medium-sized deciduous tree, usually ten to fifteen metres, with a broad, rounded crown and a trunk that is smooth and pale grey when young. It is short-lived by tree standards and grows fast while it lasts, which is why it suits a large garden better than a small one.
Mix two parts loamy topsoil, two parts garden compost and one part grit, firm it into the pan, and scatter the winged seeds on the surface under a covering of no more than three millimetres, since light helps them start. Keep the pan warm and moist and out of direct midday sun. Space the young plants thirty centimetres apart in a nursery bed and water steadily rather than heavily, because the seedlings rot easily at the base.
Move the seedling out when it is about fifteen centimetres tall and has three or four true leaves; after that it grows quickly and needs a stake for its first two seasons. Train a single stem to two metres, then let the head form, or cut it back to a knuckle each winter if you want the enormous leaves. The gall midge that twists the tips of new shoots in late spring is the common disfigurement, and the sphinx caterpillars strip the tree in a heavy year; the more serious risk is verticillium wilt, which kills branches from the inside, so avoid planting where tomatoes or potatoes have wilted with the same disease.
Our Take
Give it room and water, and the first flowers will appear while the tree is still young. The Asian member of the genus is smaller and flowers at a younger age, as described in Catalpa bungei, and any gardener planting a big tree for summer flowers should look at the tulip tree in Liriodendron tulipifera. For another tree grown for what hangs from its branches, the dove tree is worth a place in the same border, as covered in Davidia involucrata. All three want deep soil and space to spare.
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