indigiobush

Indigobush (Amorpha fruticosa L.), n indigo ative to the eastern United States, can now be found in nearly all contiguous states and is considered invasive in the Western states. It is a perennial species of the legume family that reproduces by seeds. Early settlers used pigment of this plant rather than true indigo when making blue dye, hence the common name “indigobush” and variants thereof. Indigobush is a shrub with firm woody branches that terminate in the current season’s growth of hairy, green twigs. The shrub grows 4 to 12 feet tall and is about twice as wide as it is tall. Most of the fine-textured, fragrant foliage occurs on the upper third of the plant. The leaves are alternate and pinnately compound, with an odd number of leaflets. The leaflets are elliptic or oblong, nearly opposite, pubescent to glabrous above and below, and often have a short bristle at the tip. Flowering occurs from spring to summer, when showy, slender, purple spikes appear at or near branch ends. Individual flowers appear tubular, but the “tube” is actually a single petal rolled around floral organs including protruding stamens. Kidney-shaped seed pods, about ¼ inch long, contain 1-2 seeds each.


Photos by Rich Old, rold@xidservices.com

Indigobush has an extensive root system and has been used for bank stabilization, erosion control, and windbreaks. It has also been used as an ornamental due to its showy flowers. The shrub is classified as a noxious weed in Washington because it is a non-native plant that escapes from cultivation and displaces native plant communities, particularly in wet areas. The shrub often forms dense thickets along waterways, in riparian areas, prairie draws and moist upland locations. Indigobush also tolerates poor site conditions, explained by its symbiotic association with certain nitrogen-fixing bacteria. The bacteria form nodules on the shrub’s roots to fix atmospheric nitrogen, which is then used by the indigobush and to some extent by nearby plants.

Control

Cultural: First and foremost, indigobush should not be planted in regions where it is non-native or is likely to invade. Regular monitoring of property, particularly wetlands, enables early detection of indigobush before infestations become unmanageable. Periodic burning may reduce the size of existing indigobush plants, but can also increase plant numbers.

Physical/mechanical: If an indigobush infestation is small and accessible, mechanical control methods may be effective. Persistent, repeated cutting of the plant at its base will prevent seed production and eventually exhaust the plant’s nutrient stores so that it will be unable to resprout. Because the plant can regenerate from stem fragments, all plant remains should be removed from the site. A tractor and chain can be used to remove isolated bushes.

Chemical:For indigobush control, the chemicals imazapic and triclopyr may be applied near water in the following sites: CRP, pastures, rangeland, and non-crop areas such as fencerows and rights-of-way. The chemicals triclopyr can be applied to freshly cut stems (cut-stump method). For the cut-stump method, cut the stem of the plant as close to the ground as possible and then quickly apply the herbicide to the severed stem. Remove plant remains from the site.

More information can be found in the
PNW Weed Management Handbook

Refer to the label for all harbicides
Always follow label directions prior to use

Biological: Research of potential biological insect agents for indigobush control is ongoing, but no biocontrol agent has been approved for release as of yet. Intensive grazing may reduce the abundance of indigobush. Because the plant favors riparian corridors, well-timed livestock rotations are critical in maintaining the integrity of the habitat.

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Questions: contact Steve Van Vleet or phone (509) 397 - 6290