Growing an endangered desert milkvetch for container seed production

Abstract

In this project we developed a successful methodology for producing seeds of a federally listed endangered species, Holmgren’s milkvetch (Astragalus holmgreniorum Barneby [Fabaceae]), in container culture. The goal was to obtain seeds for augmentation and introduction studies aimed at species recovery. Once requirements for germination, establishment, healthy growth, and flowering were determined, this perennial, spring ephemeral species could readily be grown from seed to flowering in the greenhouse on a fall-to-spring production cycle. Plants required several weeks of cold treatment (vernalization) in midwinter to induce flowering. We also succeeded in carrying container-grown plants through their first dormant season to obtain a second year of seed production. The main obstacle to successful seed production was matching flowering time and place to coincide with the activity of the ground-nesting native bees (genus Anthophora) that are its principal pollinators. Most of the nearly 44,000 seeds produced by approximately 125 greenhouse-grown plants over 2 y resulted from hand-pollination, which was very effective but labor intensive. Yield from hand-pollination averaged 436 seeds per plant under the best scenario, which compares favorably with the yield for wild plants. We obtained fruit set without hand-pollination only when we placed flowering container-grown plants in the field where wild Holmgren’s milkvetch was flowering and Anthophora bees were present. This field method could be scaled up and would result in large seed crops without the need for hand-pollination. Scaling up seed production in the absence of the bees proved to be impractical.