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CROSSVINE:
HAVEN FOR HUNGRY HUMMINGBIRDS

As we travel the country giving presentations about our hummingbird research at Hilton Pond Center, audience members often ask what plants we recommend to attract Ruby-throated Hummingbirds. In response we first refer them to our Top Ten Native Hummingbird Plants on the Web site for Operation RubyThroat: The Hummingbird Project, after which we always state the quintessential hummer plant across much of the RTHU's breeding range is Trumpet Creeper, Campsis radicans. If a person has to choose just one plant to put in to entice hummingbirds, this should be it. The long, orange, tube-shaped blossom of Trumpet Creeper makes the species a perfect hummingbird flower--especially when we learn it produces a particularly heavy nectar load. Unfortunately, Trumpet Creeper doesn't usually bloom around Hilton Pond until late May, so what's a hungry hummer to eat prior to that? One candidate is Trumpet Creeper's little cousin called Crossvine.

All text & photos © Hilton Pond Center

Crossvine, even though it's a native that thrives across the Southeast, is often overlooked as a viable component of the wildlife garden. With its flower of five petals fused into a tube, Crossvine resembles and is indeed related to Trumpet Creeper--both species are in the same family (Bignoniaceae) along with the Catalpa tree--but the corolla of Crossvine is usually bent slightly (below) while Trumpet Creeper's is essentially straight.

In addition, four-inch flowers of Trumpet Creeper are solid orange or orange-red, but in wild strains of Crossvine the outside of the two-inch flower tube is purplish-red and the petal lips and tube lining are bright yellow (below right). The specimen growing at Hilton Pond Center and photographed this week is mostly orange with only a hint of yellow, which leads us to believe this local plant may be "Tangerine Beauty"--an introduced cultivar perhaps planted by previous owners of the property. Several such varieties of Crossvine are available from nurseries, but to our eyes none of them are as attractive as the red-and-yellow wild type. This is one time horticulturists could have left well enough alone.

Crossvine, Bignonia capreolata (formerly Anisostichus capreolata or A. crucigera), is a native vine that grew originally in the southeastern U.S., although it was found in the wild north to southern Illinois. In the Carolinas, Crossvine is chiefly a plant of the Coastal Plain and Piedmont. These days, it is sold commercially and occurs in appropriate habitats as far away as California. It grows best in moist woods, floodplains, bogs, thickets, streambanks, and fencerows, and--given optimum moisture--is adaptable to the typical suburban backyard. As in its cousins Trumpet Creeper and Catalpa, the flowers of Crossvine produce long semi-woody "cigar-like" pods filled with dozens of flat, winged seeds that are wind-dispersed.

One of the more unusual things about Crossvine is its dark green foliage, which typically is retained all winter in warmer parts of its range. (Note #1: Leaves may turn red or purple during the cold months and be lost the next spring as they are replaced by fresh growth. Note #2: Crossvine is host to at least one caterpillar, that of the Rustic Sphinx Moth.) Crossvine leaves are opposite (above) and appear to sprout in pairs from either sides of the climbing stem. In reality, however, Crossvine's leaf is compound, and what looks like two leaves with lobed bases are leaflets. The really surprising thing, however, is that each leaf has THREE leaflets--not two; the middle one is highly modified into a corkscrew-like tendril (left) that itself divides into branches--each tipped with an adhesive pad that attaches to trees, fenceposts, trellises, and even buildings, thus allowing a mature Crossvine to climb as high as 60 feet or so in pursuit of sunlight. A Crossvine this large is likely decades old, may have a main stem 4" in diameter, and will share considerable canopy space with its supporting tree.

For years we thought the name Crossvine was given to this plant because--as shown in one of the photos above--its leaflets branch from the stem at right angles and form a cross. Instead, the "cross" in Crossvine actually refers to four symmetrical sections of tissue exposed when you cut a transverse section through a mature stem (right). Incidentally, we do NOT recommend this method of identifying Crossvine; after all, cutting kills the stem and the plant can be ID'd just as easily by looking at foliage, flower, or growth pattern.

As in Trumpet Creeper, the flower tube and reproductive structures of Crossvine show unusual configuration. In end view (above) we can see the single sticky-tipped pistil (translucent female part) and five pollen-laden stamens (yellowish male parts) --all of which arise 'way at the far end of the flower and twist toward the open end. It's very significant the near ends of the pistil and all the stamens cluster together on the top edge of the flower tube, for it is this location a hummingbird contacts when in search of nectar at the far end of the tube. As the hummer plunges its head and bill into the flower, pollen is deposited on the bird's forehead prior to being transferred to the pistil of the next Crossvine flower the hummingbird visits. Carpenter Bees and Bumblebees might be large enough to perform similar cross-pollination by picking up pollen on their thoraxes or abdomens, but it appears the adaptive relationship between hummers and Crossvine is much tighter and more dependable.

This turns out to be a good thing for Ruby-throated Hummingbirds since Crossvine begins to flower in the Carolina Piedmont in early to mid-April--at least a month ahead of Trumpet Creeper and right about the time when the first ruby-throats are returning to Hilton Pond Center after spending the winter months in Mexico or Central America. Although Columbine, Red Buckeye, and a few other wildflowers are abloom that early, we suspect an April Crossvine laden with tubular flowers and lots of nectar is a welcome haven for any hungry hummingbird just flying in from the Neotropics.

All text & photos © Hilton Pond Center
Stem cross-section from W.C. Grimm's How to Recognize Shrubs (1966)
Crossvine painting by Nicholas Joseph Jacquin (1809)

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Be sure to scroll down for an account of all birds banded or recaptured during the week, plus other nature notes of interest.

"This Week at Hilton Pond" is written & photographed
by Bill Hilton Jr., executive director of
Hilton Pond Center for Piedmont Natural History.

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