This column follows the previous column about ways to reduce the threat of extinction of Monarch butterflies. Readers’ responses to that column indicated strong interest in protecting the Monarchs that overwinter in the Monterey Bay area, enhancing our environment. Readers also called for additional details about ways to help the Monarchs.
Growing Nectar-producing Plants
Interested persons can improve the habitats of the Monarchs’ overwintering sites by planting nectar-producing plants. As reported earlier, the Xerces Society’s publication “California Coast: Monarch Nectar Plants” (https://tinyurl.com/ycsy8jp4) lists several good selections. In addition, Pacific Grove’s Nectar Plant Project has tested and recommended the following plants:
- Butterfly Bush (Buddlea x weyriana). This is a hybrid cultivar that blooms in October and produces much nectar.
- Daisy Tree (Montanoa Grandiflora). A native of Mexico, this woody shrub grows to eight feet or more, and produces a cloud of fragrant white flowers that bloom from late October to late November, just when the Monarchs need nectar.
- Mallow (Lavatera assurgentiflora). Another woody shrub, this California native produces red flowers year-round.
- Yellow Daisy (Euryops pectinatatus ‘viridis’). This South African native is a mainstay for the Monarchs, blooming year-round with bright yellow daisy flowers. It’s also not attractive to deer.
Daisy Tree (Montanao Grandiflora)
The Importance of Milkweed
As noted earlier, milkweed is essential to the wellbeing of Monarch caterpillars. This plant contains toxins known as cardenolides. Monarchs are immune to these toxins, and have evolved to store them in their bodies. The toxin makes them poisonous to birds, which avoid eating the Monarchs. Without this protection, birds would be major (but not the only) predators of the Monarchs.
The adult Monarchs always deposit their eggs on milkweed plants, so that the larvae begin immediately to eat the plants and accumulate the toxin in their bodies.
Where to Grow Milkweed Plants
The Western Monarchs mate around January, lay their eggs at a wide range of sites throughout the American west, and return to the Pacific coast to overwinter during a four-month period. In the Monterey Bay area, the familiar overwintering sites are at Natural Bridges State Beach and Pacific Grove.
(Other Monarchs have a different migration route. It begins in the northern United States and southern Canada, and continues to overwintering sites in Mexico. )
Milkweeds are needed where the Monarchs lay their eggs. Milkweed grows naturally in Monarch breeding areas, but not at the overwintering sites on California’s central or northern coastal areas. The migration cycle moves between the breeding areas to the overwintering sites. Planting milkweed close to overwintering sites could encourage Monarchs to breed and lay eggs during the winter, and thereby disrupt the migration cycle. Given this concern, the Xerces Society and other experts recommend that milkweed should not be planted with 5-to-10 miles of an overwintering site.
Which Milkweed Plants to Grow
The milkweed genus Asclepias includes over 200 species. California native species that grow naturally near the Western Monarchs’ home territory are preferred.
1. Widely Available Species
- Mexican Whorled Milkweed (Asclepias fascicularis) – dry climates and plains
- Showy Milkweed (A. speciosa) – savannahs and prairies
- Butterfly Weed (A. tuberosa) – well-drained soils; a non-native species that has naturalized in California; Perennial Plant Association named it Perennial Plant of the Year for 2018
2. Other California Native Species
- California Milkweed (A. californica) – grassy areas; native to central California
- Desert Milkweed (A. erosa) – desert regions
- Heartleaf Milkweed (A. cordifolia) – rocky slopes; early budding
- Woolly Milkweed (A. vestita) – dry deserts and plains
- Woolly Pod Milkweed (A. eriocarpa) – clay soils and dry areas; early budding
3. Avoid Tropical Milkweed (A. curassavica), which is evergreen, but which allows development of a protozoan parasite (Ophryocystis elektroscirrho) that harms or even kills Monarchs that eat the plant.
Gardeners in the Monterey Bay area who wish to improve the Monarchs’ habitat should emphasize the planting of nectar-producing plants more than milkweed. Those plants are good for bees, as well. This would be a good year to plant annual seeds in the early spring.