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Catalpas: What’s Old is New Again

One of the showiest of flowering trees in June are the Catalpas.

Editor’s Note: We’re celebrating our Season of Trees with Tree Tuesday on the Nature Notes blog. Come back each Tuesday for stories exploring the Arboretum’s trees and tree collections. This is an extended version of a story appearing in our June/July issue of Arboretum Magazine.

Catalpa tree in bloom at the Arboretum. Photo by Alan Branhagen.

By Alan Branhagen, Director of Operations

One of the showiest of flowering trees in June are the Catalpas. In Victorian times they were all the rage with the large, pillowy white, orchid-like flowers set on candelabras above the bold, tropical-esque foliage. In the post war years they fell out of favor, considered too messy for the immaculate lawns that were common (the flowers, large leaves, and bean-like seedpods drop through the year). Today catalpas are enjoying a renaissance for various reasons. First, we are determined to diversify our landscapes to guard against diseases like Dutch elm and Emerald ash borer and secondly, the mess is all organic and compostable. We are more apt to have beds of groundcovers or shrubs underneath our trees and let the “mess” decompose as mulch.

There are two American species of Catalpa that are commonly planted in our region: Northern Catalpa (Catalpa speciosa) and Southern Catalpa (Catalpa bignonioides). Northern Catalpa is more common and its pre settlement range was very small: centered where the Mississippi and Ohio Rivers meet, northwestward into Indiana and southwestward into Arkansas. Southern Catalpa was originally found along rivers from Virginia to Texas, and it’s surprisingly fully hardy in Minnesota. Both have since naturalized well to the north. With climate change, we are giving more thought to how trees migrate and some, for whatever reason, did not migrate back north as the Ice Age subsided – some of this is because the megafauna (mammoths, mastodons, giant ground sloths, etc.) of North America also died off during this time period.

Plant both species of catalpas because Northern Catalpa bloom first with larger flowers with fewer in a cluster – the Southern Catalpa begins to bloom as the Northern fades and has smaller flowers with more of them in a cluster. Both are great for bumblebees!

Northern Catalpa becomes a very large, upright shade tree, readily growing over 50 feet tall while the Southern Catalpa is usually around 35 feet tall, rarely over 50 feet in height at maturity. The bean-like seedpods are also longer on the Northern Catalpa, but hybrids can occur and some trees, including at the Arboretum, are hybrids.

The Arboretum also contains the Chinese Catalpa (Catalpa ovata), which has far less showy, yellowish flowers that bloom at the same time as the Southern Catalpa. Chinese Catalpa has also naturalized on rare occasion in the Upper Midwest.

Young Catalpa tree in the Arboretum’s Pillsbury Shade Tree Walk. Photo by Alan Branhagen.

Look for the Arboretum’s catalpa collection between the Dahlia parking lot (opposite the Chinese Garden) and the Grass Collection.

Catalpas host a specialist moth (its caterpillars will feed on no other trees), the Catalpa Sphinx and its caterpillars can be so prolific they can defoliate smaller trees. The moth has spread along with the range of the trees as far north as Winona in southeast Minnesota (let us know if you have seen these in the Twin Cities!). The caterpillars are prized by fishermen as bait!  Sphinx moths are large moths that look like hummingbirds when in flight – often seen in gardens feeding on flowers at dusk but about half of the nearly 40 regional sphinx moth species, including the catalpa sphinx, do not feed as adults. Attend Moth Night at the Arboretum on July 25, during National Moth week, to learn more about moths.

Catalpa trees are also very susceptible to verticillium wilt which is a pathogen that is very prevalent in Illinois but is spreading into our region and has been confirmed at the Arboretum. For that reason, please never mulch a catalpa with ground or chipped trees from an unknown origin (potentially infected tree), which is one way the pathogen spreads.

Share your Tree Stories | We want to hear about your favorite trees at the Arboretum! Do you have a regular picnic tree? Or a special tree you like to visit? Tell us about your favorite tree at the Arboretum and include a photo if you have one. Send stories to arbpr@umn.edu, and read Tree Stories from Arboretum members, visitors and staff.

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