They are packed in dense clusters on panicle branches. By flowering time, the leaf canopy is composed of cauline leaves just below and on the branches of the inflorescence. The earlier developing bottom leaves have long petioles while the later developed upper leaves are nearly sessile. These tall, dense stands, with the surface soil packed with creeping stems, become virtual monospecific stands, free of other herbaceous plants. In very dry years, flowering will be abundant, but seed production is limited.
Some seeds persist until the following spring.
Nature of Competition Several different approaches have been tried to test for allelopathic competition by perennial pepperweed and to identify the substances involved. Perennial pepperweed appears to successfully compete with other plant species for moisture, nutrients, and light. Taxonomic Affiliations Lepidium is well represented in western North America where the genus is represented by approximately 75 species. Many are annuals, but there are several perennial species. None of the native species have the aggressive, invasive behavior or form the dense, robust colonies associated with perennial pepperweed.