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Invasive species a member of the bean family

Published Tuesday, June 16, 2009 9:58 AM

Berkeley Independent

Hunter Desportes/USC Herbarium

The flowers are produced abundantly and they are mildly fragrant.

Bright, golden yellow: like springtime daffodils, and like autumn's goldenrods...but on a summer-blooming tree. It's got thorns, plenty of them, so you won't want to mess around with this thing too much. The bees, on the other hand, do love to hang out in and around these small, bushy trees. The flowers are irresistible: brilliant yellow (there are five petals) with plenty of pollen and nectar to be had from a visit.

This woody plant is a member of the bean family. It is a resident, in nature, in the American Southwest, and adjacent Mexico, where it inhabits dry slopes and seasonally wet arroyos. It sends up one or a few trunks from each plant, and each trunk makes a spreading crown, thus, the bushy effect. The bark on these plants tends to be green, and that's the reason the Spanish name for it is "Palo verde." Plenty of leaves are produced, and these are long and skinny, with lots of smallish leaflets on both sides of the midrib. In dry seasons, the leaflets tend to fall off, leaving these spindly leaf rachises, which themselves eventually fall off, too. The flowers are produced abundantly, and they are mildly fragrant, very pleasantly so. If you study a flower carefully, you will notice that each one exhibits five petals. The uppermost petal is the largest, and usually bears something of a reddish blotch about half-way down. Because of the way that the flowers develop, especially in the way that the petals overlap, we botanists have assigned this species to the same bean family subgroup which includes our familiar redbud. (There are two other subgroups of the bean family: one with species like mimosa, those with powder-puff groups of flowers; and then the largest subgroup of the bean family, the one which includes sweet-peas, butter-beans, wisteria, and soybean.) After the blooms come the fruits, which, as in all the members of the family, are called legumes. In this case, the legumes are long and skinny, with three to five seeds inside, bulging outward as the beans get ripe.

The plants are fairly easy to grow as ornamentals, and now have a rather popular reputation as accent plants, providing unusual texture in the landscape, as well as tremendous seasonal color. Furthermore, this species, being a resident of dry places, is at home in drought-prone places. The thorns can be a nuisance for many gardeners, and so are the leaf parts, which make something of a mess on the ground as they fall away.

Despite its brilliant color, there is a dark side to this plant.

It has spread outside of cultivation in all of the Gulf Coast states, after its initial plantings. (The seeds get moved around pretty easily, sometimes by birds.) It is an aggressive weed in Hawaii and many parts of Australia, too. Botanists agree that this is an invasive species outside its normal range, and with that in mind, you might carefully consider whether you want one of these in your yard.

Answer: “Jerusalem thorn," Parkinsonia aculeata

John Nelson is the curator of the Herbarium in the Department of Biological Sciences at the University of South Carolina. As a public service, the Herbarium offers free plant identifications. For more information, visit www.herbarium.org or call 803-777-8196.

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