Invasive Species—Best Control Practices Michigan Department of Natural Resources Michigan Natural Features Inventory 2/2012 Black locust Robinia pseudoacacia Black locust is native to the southern Appalachians and the Ozarks, where it occurs on slopes and forest edges. It has been planted in 48 states and was noted as spreading in jack pine barrens in Michigan as early as 1888. It tolerates a wide range of soil conditions, and spreads clonally as well as by seed. Its dense thickets shade out native vegetation. As a legume, black locust fixes nitrogen and soil nitrogen levels are higher under old trees. It produces more leaf litter and that litter has much higher nitrogen concentrations than most native tree species. Soils under black locust also have elevated levels of phosphorus and calcium. In low nutrient habitats, this facilitates invasion by weedy, nitrogen-loving non-natives, which slows and sometimes alters patterns of succession. Although it initially invades disturbed areas, it also poses a particular threat to prairies, savannas and open woods. Black locust contains several toxic components in its leaves, stems, bark and seeds. Ingestion results in both gastrointes- tinal and neurological effects which are particularly acute in horses and may be fatal. Chris Evans, River to River CWMA, Bugwood.org Identification Flowers: Habit: Black locust flowers are white Black locust is a deciduous, medium-sized tree ranging in and very fragrant, with five height from 12-25 m (40-82 ft) and 30-60 cm (12-24 in) in irregular petals. They are ar- diameter, although trees in Michigan have reached 1.5 m (5 ranged in a long, dangling ft) in diameter. It has a narrow crown and an open, irregular cluster of 10-25 flowers. They form with contorted branches. Black locust has an extensive are insect pollinated and network of lateral roots and forms dense clones. compete with native plants Paul Wray, Iowa State for pollinators. They bloom in Leaves: University, Bugwood.org May and June. Black locust leaves are alternate and pinnately com- Fruits/Seeds: pound with 7-21 leaflets per Seedpods form in the fall leaf. The rounded leaflets are but persist over winter, pods 2.5-5.0 cm (1-2 in) long and are smooth, dark-brown, flat, the leaves are up to 35.5 cm Paul Wray, Iowa State and contain 4-8 flat, brown (14 in) long. University, Bugwood.org seeds. The seeds have a highly impermeable seed coat and Bark/Stems: can remain viable in the seed Paul Wray, Iowa State Black locust has green stems, bank for many years. University, Bugwood.org smooth brown bark when young. It has paired spines Habitat: on the twigs at the leaf bases. Black locust is very shade intolerant. It can grow in many soil As it ages, the bark of mature types except those with a high water table. It was widely trees becomes thick, tan to planted in Michigan and is now colonizing old fields, dis- Paul Wray, Iowa State gray-brown, scaly and deeply turbed woodlands, prairies, pine barrens and oak savanna. University, Bugwood.org furrowed with flat ridges. It establishes well in early-successional communities. 1 Similar species Planning a control program Honey locust Resources for invasive species control invariably fall short of The native honey locust the actual need, so it is important to prioritize sites for treat- (Gleditsia triacanthos) has ment and plan carefully. Assessing both the scope of the leaves with smaller, more problem and any available resources is a critical first step: numerous leaflets. Leaves • Map known populations; do they consist of scattered may be doubly compound; a single specimens? Or are extensive clones present? single leaf has multi-leafletted • Note where spread is clonal or by single plants. leaflets branching off a central • Does it occur on high value sites? Important hunting or axis. Its clustered flowers are John D. Byrd, Mississippi State recreational lands? High quality natural areas? Sites with not conspicuous or showy. University, Bugwood.org high cultural value? Bristly locust • How is it distributed? Is it sparsely scattered in otherwise The southern native bristly native vegetation? Does it cover large expanses of low locust (Robinia hispida) is a quality habitat? shrub, with brushlike hairs Given this information, develop a strategy for control: on its stems and fruit. Its 1. Prioritize high value sites where success can be showy flowers are pink, not achieved for treatment. white and its leaves have 13 2. Choose appropriate control methods, given the growth Robert Vidéki, Doronicum Kft., or fewer leaflets, while black habit of the species, site conditions and available re- Bugwood.org locust may have up to 21. sources. False indigo 3. If using herbicide, be sure to read the product label be- Although the non-native false fore finalizing plans. Is there potential for harm to non- indigo (Amorpha fruitcosa) target species? Have you made adequate provisions to has compound leaves with minimize damage? rounded leaflets, it is shrublike 4. Do these control methods require any permits (i.e. her- and its leaves are smaller. It bicide application in wetlands, prescribed burning)? also has purple flowers, rather 5. Where clones are present, always treat the entire clone. than white. False indigo ap- 6. Eradicate smaller satellite populations. pears to be spreading in some Leslie J. Mehrhoff, University of 7. Treat larger core infestations of lower value as parts of Michigan. Connecticut, Bugwood.org resources permit. Reproduction/Dispersal 8. Monitor to ensure desired results are being achieved; adapt management to improve success. Black locust reproduction is primarily vegetative, although it can also reproduce by seed. It sprouts from the roots and Best survey period forms clones, particularly in sandy soils. It also sprouts easily Black locust is most readily identified in May and June while from stumps in response to damage. in flower but its bark is distinctive year-round. Black locust grows rapidly and matures early; some trees Documenting occurrences may produce seed at six years of age. Heavy seed crops oc- In order to track the spread of an invasive species on a cur at one or two year intervals. landscape scale, it is important to report populations where The seeds have a hard, impermeable coat and require scari- they occur. The Midwest Invasive Species Information Net- fication to germinate. They are heavy and fall close to the work (MISIN) has an easy-to-use interactive online mapping parent tree, although birds may move them over longer dis- system. It accepts reports of invasive species’ locations from tances. Michigan Flora notes that seeds may remain viable in users who have completed a simple, online training module the ground for up to 88 years. Seeds can accumulate in the for the species being reported. It also offers the potential for soil a density of 29,817 seeds/acre in second-growth mixed batch uploading of occurrence data for any invasive species. forest. Densities are much lower in mature forest. Herbaria also provide a valuable and authoritative record of Trees begin suckering at four or five years of age. A fibrous plant distribution. The University of Michigan Herbarium’s network of roots connects a black locust grove, with the database can be searched online for county records of oldest trees in the center and younger trees around the occurrence, for example. When black locust is first encoun- periphery, In late-successional communities, black locust tered in a county where it had not been known previously, becomes rare, as the species is shade-intolerant. specimens should be submitted to the Herbarium to docu- 2 ment its presence. Check the “Online Resources” section for tered by the Michigan Department of Agriculture and Rural links to both of these resources. Development and a link to their website is included in the “Online Resources” section. Control A permit from the Michigan Department of Environmental Black locust can be difficult to control. Because this species Quality is usually required to apply herbicide where stand- is strongly clonal, all stems in a clone must be treated. It is ing water is present—in wetlands, along streams, rivers critical to monitor the site to ensure that cut and treated or lakes, or over open water. A permit is also required for stumps do not resprout—if a single stem survives, the entire herbicide use below the ordinary high water mark along the clone may regenerate. Great Lakes or Lake St. Clair shoreline, whether or not stand- ing water is present. A link to their website is included in the Mechanical control “Online Resources” section. Control of established black locust populations generally A number of adjuvants or additives may be used with requires the use of herbicide although mechanical control herbicides to improve their performance including mixing may occasionally enhance the effectiveness of other agents, surfactants, penetrating oils and dyes . Some are methods. included in premixed products while others must be added. Hand-pulling/Digging/Bull-dozing Adjuvants do not work with all products; consult the prod- Since most black locust spread is vegetative, and saplings uct label to determine which adjuvants may be used with a are connected underground, hand-pulling is not useful. specific herbicide formulation. In general, damage to the root causes sprouting also. On Dyes are useful in keeping track of which plants have been highly disturbed sites, cutting trees and removing the roots treated and making spills on clothing or equipment appar- with a bulldozer has been suggested but does not appear ent. Some premixed herbicide include them or they can be practical in most settings. added to others. Clothing dyes such as Rit® can be added to water soluble herbicides, while other products require Cutting/Mowing oil-based dyes.