Which milkweed species are best for gardens?

Authors

Monarch butterflies (Danaus plexippus L.) depend on milkweed plants (Asclepias spp.) to feed their young, known as larvae. As concern grows over butterfly and bee declines, planting milkweed has emerged as a popular conservation strategy.

There are several species of milkweed plants in the U.S.A., and each species can exhibit unique growth and pollinator attractiveness characteristics. Some species, for example, may grow taller than others, attract more monarchs to lay their eggs, or vary in their tendency to spread out in a landscape.

The differences between milkweed species mean some species may be more suitable for smaller plantings than others. In wide-open spaces such as agricultural land, for example, the common milkweed species Asclepias syriaca is widely considered for habitat restoration. However, because of its tall height and propensity to spread, it is likely less horticulturally suitable for managed gardens.

This research measured the characteristics of eight different milkweed species to determine the most suitable species for managed gardens.

A sample of their results are summarized below. Since garden spaces tend to favor plants that won’t spread into neighboring garden beds or lawns, for gardens in Kentucky the milkweed species A. incarnata, A. tuberosa and A. viridis may be the best options.

Abstract

Planting milkweeds on public and private lands has emerged as a central conservation strategy for restoring declining North American migratory populations of the monarch butterfly (Danaus plexippus). Nearly all actionable science on this issue has focused on restoring common milkweed (Asclepias syriaca L.) in rural land types. The aim of this study was to develop recommendations for the best milkweeds for managed gardens intended to support both monarch butterflies and bees. Eight milkweed (Asclepias) species varying in height, form, and leaf shape were grown in a common-garden experiment at a public arboretum. We measured milkweed growth, tillering, and bloom periods, conducted bi-weekly counts of eggs and larvae to assess colonization by wild monarchs, and evaluated suitability for growth of monarch larvae. We also quantified bee visitation and compared the bee assemblages associated with six of the eight species, augmented with additional collections from other sites. Monarchs rapidly colonized the gardens, but did not equally use all of the milkweed species. More eggs and larvae were found on taller, broad-leaved milkweeds, but there was relatively little difference in larval performance, suggesting ovipositional preference for more apparent plants. Asclepias tuberosa and A. fascicularis attracted the greatest number of bees, whereas bee genus diversity was greatest on A. verticillata, A. fascicularis, and A. tuberosa. Milkweeds that do not spread extensively by tillering may be best suited for managed gardens. Combining milkweeds that are preferred by ovipositing monarchs with ones that are particularly attractive to bees may enhance conservation value of small urban gardens.