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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.