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
|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
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
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
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
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
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-
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
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