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The Monday Garden
Catalpa: Great American and Invader?

Issue No. 162 - May 1, 2005
by Sue Sweeney

The northern catalpa (Catalpa speciosa) is a beautiful, tropical-looking North American native shade tree with a mysterious past. At one time, its range was confined to part of the Mississippi Valley but sources are admittedly fuzzy about which part. Likewise, the name is definitely Native American in origin but there's a big disagreement over which tribal language is the source.

RANGE AND FAMILY: Today, the northern catalpa (Bignonia family) is found through out the U.S.A. and southern Canada, as well as in other countries where it has been imported as a combat-ready urban tree. Also found in temperate urban areas are the northern catalpa's southern Appalachian kin, the southern catalpa (Catalpa bignioides), and its Asian kin – particularly the clearly invasive empress tree (Paulownia tomentosa). Those of you from the southwest might know the southwestern native cousin, desert willow (Chilopsis linearis). Many gardeners are familiar with the lovely orange-flowered native trumpet vine, which is the family's signature member. The Chinese catalpa (Catalpa ovata) is also sold for landscaping.

picture: Northern catalpa in bloom at Stamford High. Stamford CT June 2004
INVADER OR RETURNING ALUMNI? Some say that outside of its native range, the northern catalpa is an invader. My home state of Connecticut has labeled the northern catalpa as having "demonstrated invasive tendencies", which is a no-no of the third degree. (The first degree has been reserved for the worst of the worst, like ailanthus). Being "invasive" means that the plant doesn't come from the area, can escape cultivation and survive on its own, can spread widely, and can out-compete local native plants. There is no question that the northern catalpa does not stay where it is planted. My own Hoyt Street Alley hosts numerous northern catalpa specimens both large and small. The northern catalpa's spread, however, is limited by its natural fussiness as to location – it likes rich, moist, slightly acid soil and just a touch of shade. In other words, it is an edge-of-forest tree that won't survive in deep forest, and which does well in the open meadow and lawn only with plenty of water, as in the Alley.

But is an invader? How come the northern catalpa is built to withstand the -30F temperatures in Montreal, if its original range was only in the southern Mississippi Valley? The black locust may be similarly situated. Many say the black locust an invader in the Northeast, but geologists have found that black locust was here before the Ice Age. So, the black locust is only reclaiming its original territory. Is the same true of the catalpa? Oddly, it is hard to come by popular-level research publications on the catalpa. The northern catalpa has even been neglected by the U.S. Forest Service, whose web site is normally the source of much good information.

picture: This catalpa, along the Mill River, at Scalzi Park, has been repeatedly cut down by the Parks Department. Stamford CT, April 2005.
Does it play nicely with others? To me, this is the more important question. Regardless of whether the northern catalpa has a pre-Ice Age "green card" permitting it to be outside of the Mississippi Valley, does it add or detract from bio-diversity? Again, popular-level research is hard to come by. Anecdotally, in my area, I have not observed it taking over. It is weedy in that it makes lots of seedling-babies.. However, I don't see pure stands of catalpa where it has crowded out the competition. (I wish that could say the same for certain others such as the ailanthus and Norway maples). In Hoyt Street Alley, the northern catalpa shares space with a variety of trees including the box elder and Norway maples, American elm, eastern cottonwood, red oak, shaggy bark hickory, American sycamore, chokecherry, white mulberry, ailanthus, and native crab apple. The catalpa saplings are hardier than those of most of the other trees (as hardy as the ailanthus in fact), but even the big catalpa trees are being smothered in places by that dreaded invader porcelain vine.

Allelopathic: There is authority that both the southern and northern catalpas are "allelopaths", in that they emit chemicals from their roots that prevent other plants' seeds from germinating. The work done on the southern catalpa indicates that the allelopathic properties only become a problem when the tree is allowed to grow in large, pure stands.


pictures: northern catalpa seedlings in Hoyt Street Alley, Stamford CT 2004- 2005

HUMAN-ANIMAL USE: Naοve America humans used catalpa for medicinal purposes. Their kids, like ours today, probably played with the large heart-shaped leaves, orchid-like flowers and cigar-shaped seed pods.

As early as the 1750's the Euro-settlers recognized the value of the catalpa. The tree grows fast and fairly straight, producing a brittle but rot-resistant, light-weight wood, good for farmers' fence posts and for railroad ties. So where the farmers and railroads went, so did catalpa planters. The catalpa was thus spread over most of the Midwest and Southwest. However, the catalpa, as a native tree, has native predators and, over time, the catalpa did not well commercially because in large, pure stands, the predators get out of hand. The chief predators are "catawba" worms (see below), several non-serious leaf fungi, including powdery mildew, and verticillium wilt, an internal fungus that is a killer.

Today, the northern catalpa is recognized as a useful, fast growing, medium-to-large ornamental shade tree that stands up well to urban pollution. This is despite it being a bit weedy with its seedlings and a bit messy with its flowers, leaves, and seed pods. (Note: There is no such thing as a litter-free tree except over-hybridized, high-pollen male street trees. In my view, if you will tolerate only litter-free trees, get a plastic one, and go live where only plastic trees grow.)

Strangely enough, northern catalpas are also grown for fish bait. No, fish do not eat the tree. But some good eatin' fish do adore the larvae of the catalpa sphinx moth, known as "catawba" worms. The catalpa sphinx moth is a very ordinary looking gray-brown moth whose younger self is a caterpillar with yellow and black longitudinal stripes. The moth is more of a problem (or blessing, depending on whether or not you're a fisher-person) in the south. In any case, when the little guys get going they often completely strip trees of foliage and if they do this several times, they can kill the tree. Meanwhile, both the fisher-person and the birds, perhaps the fish, enjoy the caterpillars.

Pollinators, including hummingbirds, love the flowers. Few animals munch on the leaves and bark; even the white-tailed deer don't like it much. The thick, shady canopy is good for birds' nests but bad for lawn grasses. Plant it where you'd like a shade garden or a mossy lawn.

As a lawn tree, give it plenty of space and water, and a touch of shade. The leaves will get iron chlorosis (yellow between the veins) in high PH soil.

picture: Northern catalpa in bloom at Stamford High. Stamford CT June 2004
DESCRIPTION: The northern catalpa has big (7-10" or so), heart-shaped, velvety, medium green leaves that are in opposite pairs on the branch or in whorls of three. The leaves are pinnately veined, smooth edged and have soft hair below. Clusters of the showy flowers cover the tree in early summer. The bell-shaped flowers are white with yellow or purple spots, and come in branched, upright clusters. The flowers are followed by long, bean-like reddish-brown seed pods which persist over the winter. The pods split in the fall to release flattish, papery, fringed seeds that can be wind-borne quite some distance.

The tree, itself, is medium-sized with a short, stout trunk, spreading branches, and a rounded crown. The bark is relatively smooth, brown and reddish-brown; and, with age, develops scales and/or shallow vertical fissures.


pictures: northern catalpa against the winter sky in Hoyt Street Alley, pods hanging over the Mill River,
and a detail of the pods in Hoyt Street Alley. Stamford CT January- April 2005

IDENTIFICATION CHALLANGES: In the winter and early spring, believe it or not, catalpa saplings can be mistaken for ailanthus and stag-horn sumac saplings that grow in the same conditions. In the summer, catalpas of any size can be mistaken for the Asian invader, the empress tree (Paulownia tomentosa).

The empress tree v. the catalpa is easy to resolve, while both trees have the family's big heart-shaped leaves, both kept their seed pods most of the year. The catalpa, has the long, bean-like pods. The seed pods of the empress tree are ovoid and only an inch or so in size. The empress tree also has purple, rather than white flowers.

Distinguishing ailanthus, sumac and catalpa seedlings is more of a challenge and requires careful observation of the twig's buds and leaf scars.

• The catalpa leaf scars are round to oval and there's no terminal bud, so the bare twig looks kind of like a traffic stop light; next year's bud sits on the "shoulder" above the leaf scar and has a rosette of tiny dark, pointed bud scales.

• The ailanthus and sumac leaf scars are more heart or shield shaped. The ailanthus' buds are fat, rounded, fuzzy lumps sitting in the notch at the top of the leaf scar.

• The sumac's bud is more pointed and is at the center of the leaf scar.


pictures: a northern catalpa sapling in Hoyt Street Alley, Stamford CT January- April 2005

All three have dramatic new foliage in lime green to dark red. As the leaves come out though, you will be able to tell the catalpa's simple heart-shape from the ferny compound leaves of the ailanthus and sumac.

pictures: ailanthus twig and new leaves; sumac twig, northern catalpa sapling.
All in Hoyt Street Alley, except sumac is from Bedford Street, Stamford CT March- April 2005

Cross breeding: the Southern catalpa and the Asian kin are often sold in the northern temperate area by landscapers, and, guess what? The catalpa trees cross breed. So if the tree you're trying to ID does not quite look like it should, take a look at the foreign cousins – you may be looking at a mutt.
picture: Northern catalpa leaf in the rain at Stamford High. Stamford CT, summer 2004
From The Monday Garden. Copyright © by Sue Sweeney. Reproduced with permission. 

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