As we head toward the end of June, we venture out of the blooming period of one of the eastern United States' loveliest flowering trees, the Catalpa. There's still time to see it in all of its glory, though.
What it is. The two native species in the genus Catalpa most commonly found in the eastern U.S. are the southern catalpa (Catalpa bignoniodes) and the northern Catalpa (Catalpa speciosa).
The northern Catalpa is sometimes called hardy catalpa because it can withstand cold winters better than its southern cousin. The two species are almost identical, but there are a few tricks to telling them apart, which I’ll detail below.
Both northern and southern can grow in New Jersey, and both are beautiful trees. They have enormous heart-shaped leaves, some as long as a foot, and in June bear vast numbers of sweet-smelling, orchid-like flowers that grow in upward-pointing clusters, or panicles.
The flowers, white with yellow spots and purple speckling near the throat, can bloom so thickly that, as beloved botanical biographer Charlotte Hilton Green says in her wonderful 1939 book Trees of the South, “the tree as a whole looks almost like a great tropical nosegay that some giant in seven-league boots, stopping for a moment to rest, might pluck for his buttonhole.”
The Catalpa’s other distinguishing characteristic is its fruit. The long, skinny pods – up to 20 inches in length – give the tree a few of its other nicknames: cigar tree or Indian bean. Despite the bean moniker, the tree isn’t actually in the bean, or legume, family (some that are include locusts and red bud).
The pods, which turn from green to brown and drop off in the fall, contain thin papery seeds that drift easily on the breeze.
Where to find it. Both southern and northern catalpas are native to areas south of the Mason-Dixon line, but have spread and been planted as far north as New England.
Catalpas aren’t common in our native forests, but despite the fact that they cover the ground beneath them with their large numbers of flowers in the spring and pods in the fall, they are quite popular in New Jersey and elsewhere as a suburban street tree.
They’re also present in just about every arboretum I’ve ever visited. The Sister Mary Grace Burns arboretum at Georgian Court University in Lakewood – an underrated plant destination in our area, if you ask me – has both northern and southern Catalpa trees.
The Ocean County College arboretum in Toms River and the David C. Shaw arboretum at Holmdel Park are also good bets.
Why bother. Because our two common Catalpa species are so much alike, they offer you the chance to style yourself as resident plant expert among your peers, a role which some of us take – ahem – rather seriously.
C. bignoniodes has smaller flowers than C. speciosa, as you can see in this photo from the Georgian Court aboretum website. Bignoniodes flowers’ throats are also more deeply speckled with purple, and the edges of the petals are lacier.
This isn’t a very useful way to tell the two apart, though, since it’s entirely relative, and the tree only flowers for a couple of weeks in late June.
The real trick is the shape of the septum, the membrane on the inside of the Catalpa pod that divides the pod in half along its vertical length, and to which the seeds are attached. Northern Catalpa has a thicker, rounded septum that takes up more of the volume of the pod; southern Catalpa has one that’s basically flat. Here's a more detailed explanation.
So the next time you stroll by a catalpa with some friends, pluck a pod, split it open, bend over it and say, while stroking your chin, “Ah yes, just as I thought – bignoniodes.” They’ll be amazed, I swear.
Once upon a time, it was actually quite important for foresters to know this trick. Catalpa wood is highly rot-resistant, making it a useful choice for railroad ties. But the wood of the northern catalpa is stronger and harder and thus more useful, so it paid to know which tree was which.