Milkweed (Asclepias sp.) is an important pollinator genus across North America and is a host plant for many butterfly species, notably the monarch butterfly (Danaus plexippus). Commercial production of Asclepias is limited to a few species, because most species lack commercial traits, with minimal branching habit, excessive height, and minimal color variation. This study used a commercially viable Asclepias species, butterfly weed (Asclepias tuberosa L.), as a maternal parent and trialed three different pollination methods in an attempt to create interspecific hybrids. Pollination methods included a traditional method, a pollen–solution-based method, and a novel inverted pollinia method. The inverted pollinia method increased pollination success rates 4-fold among intraspecific crosses of A. tuberosa. When pollination methods were optimized, A. tuberosa was used as the maternal parent, and one-way crosses were made to seven other Asclepias species using the inverted pollinia method. Of the seven species used as pollen donors, four developed hybrid seed successfully: green milkweed (Asclepias hirtella Woodson), purple milkweed (Asclepias purpurascens L.), showy milkweed (Asclepias speciosa Torr.), and common milkweed (Asclepias syriaca L.). As germination methods vary significantly among Asclepias species, three methods of germination were trialed on seed developed via interspecific hybridizations: direct seeded, cold–moist stratification, and embryo rescue. Of the three methods, cold–moist stratification was superior to direct seeding and embryo rescue. This research is the first documented case of a controlled interspecific hybridization event among these species.

This article describes the first published report of a commercially viable vegetative propagation protocol for butterfly milkweed (Asclepias tuberosa L. [Apocynaceae]). Milkweed (Asclepias L.) species are regarded as a superior North American native pollinator taxa. Restoration of species into disturbed sites and increasing its use in managed landscape environments are a means of sustaining and promoting many threatened and endangered pollinators that rely on Asclepias spp. for nectar in the adult stage and for forage in the juvenile stage of development. Unlike restoration efforts that predominantly rely on seedproduced plants, commercial ornamental production relies heavily on vegetative propagation. However, vegetative propagation of Asclepias spp. on a commercial scale has been limited due to the lack of published propagation protocol(s). This study employed vegetative cuttings of Asclepias tuberosa taken at 2 maturities and treated with potassium salt of indole-3-butyric acid (K-IBA) or 1-naphthaleneacetic acid (NAA) at varying concentrations to determine if a commercially viable protocol could be developed. Comparing cuttings taken from container-grown mature stock plants and juvenile seedlings, propagules originating from mature plants resulted in higher survival, likely because of enhanced carbohydrate reserves and photosynthetic capacity. Exogenous auxin (K-IBA or NAA) treatments had no effect on cutting survival or root number and length. The only growth parameter influenced by a specific hormone and concentration was cutting height, which was greater when K-IBA at 1000 or 3000 ppm concentrations were employed, compared to the control with no hormone applied. Ultimately, vegetative propagation can be achieved with commercially acceptable success using no exogenous hormone application.

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