Developing management strategies for invasive plant species requires identifying effective treatment methods (e.g., physical or chemical treatments) and optimally timing their application. For invasive annual plants, effects of treatments on seed production and germinability are paramount to reduce seed banks and subsequent plant establishment. We compared effects of a range of physical and chemical treatments applied to plants containing seeds at different developmental stages at field sites in the Mojave Desert (Nevada and Arizona) for managing the exotic annual forb, African mustard. Effectiveness of physical treatments (hand pulling entire plants, breaking plants, or separating siliques from plants) for reducing African mustard seed size and germination varied by treatment and seed maturity stage at which treatments were applied. With all treated plant material remaining in the field (i.e., not bagged and transported off site), hand pulling or breaking plants resulted in 90 to 100% of developing and developed seeds still able to germinate. Separating siliques from plants, however, resulted in 0% germination of undeveloped and developing seeds. All three tested herbicides (glyphosate, 2,4-D, and metsulfuron) reduced germination to zero or near zero across all seed development stages. Results suggest that physical treatments are most effective at early stages of seed development, whereas several different herbicides are effective across seed developmental stages. Results also highlight the importance of considering the potential for continued seed development and germinability on treated plant material when choosing invasive plant treatment types and timing.
Nomenclature: African mustard, Brassica tournefortii Gouan.
Management Implications: Introduced to North America (California) by 1927, the annual African mustard is a priority invasive forb for management in southwestern North American arid lands such as the Mojave Desert. A field experiment in Lake Mead National Recreation Area in the eastern Mojave Desert revealed that three different physical treatments (hand pulling entire plants, breaking plants off at the base, or separating siliques from plants) differed in their ability to reduce African mustard seed germination across three seed development stages (ranging from undeveloped to fully developed seeds). Once seeds had fully developed, no physical treatment reduced seed germination below 50%, and at least 20% even of undeveloped seeds continued to develop and were germinable on pulled and broken plant material left in the field. Thus, the window for applying physical treatments is largely restricted to before any seed development. Otherwise, treated plant material needs to be sterilized or transported off site to avoid continued development and germination of seed even after plants are treated. A second field experiment revealed that three different herbicides (glyphosate, 2,4-D, and metsulfuron) eliminated or nearly eliminated germination across all three seed development stages. Results suggest that managers have several herbicide options effective at preventing germination even when applied late in plant development to fully developed seeds. Results highlight the importance of considering germination potential of residual seeds on treated plant material when evaluating treatments.
