Effect of Fertilizer Rate, Substrate, and Container Type on Greenhouse Production of Sandhill Milkweed

Sandhill milkweed [ Asclepias humistrata (Walter)] is important for monarch butterfly [ Danaus plexippus (L.)] conservation efforts, yet precise cultivation practices are largely not available. We tested the effects of three fertilizer rates and four substrate types and four container types on the performance of sandhill milkweed during greenhouse production. Seedlings fertilized with a high (0.90 g per 48-cell container) controlled-release fertilizer rate of 15N–3.9P–10.0K (15–9–12 Osmocote ® Plus) had reduced performance compared with low and medium fertilizer rates (0.34 and 0.56 g per 48-cell container, respectively). Seedlings grown in large containers (∼175 mL including standard 32-cell liners and tall tree-tubes) outperformed seedlings grown in small containers (∼100 mL including standard 48-cell liners and short tree tubes). A transplant ready plant can be produced for spring within 16 weeks when seeds are sown in early January. Although sandhill milkweed seedlings can be grown under various fertilizer rates and in various containers and substrates, seedlings grown in tall tree tubes in a peat-based mix (Sunshine Mix) outperformed a nursery standard substrate and two wood fiber substrates. We recommend growing plants in a peat-based substrate within tall tree tube containers and applying a medium fertilizer rate.