By Peggy Burhenn, UC Master Gardener
The beautiful orange western monarch butterfly (Danaus plexippus) was once a familiar sight in our gardens, with an estimated population of 4.5 million in the 1980s. Since that time, the population has declined dramatically, to fewer than 30,000 individuals in 2019. This decline is attributed to a combination of habitat loss, pesticide use, and possible climate factors. Understanding the monarch’s life cycle and needs can help guide efforts to restore their numbers.
Life of a Monarch
October–February: Western monarchs overwinter along the western coast from Mendocino County to Baja California, typically within 1.5 miles of the ocean. These sites provide high humidity, access to water, and protection from freezing temperatures and wind. Monarchs aggregate in trees in a reproductive diapause (a pause in reproduction), making local movements for water and nectar. Fall- and winter-blooming nectar plants are critical during this time.
February: Most monarchs leave the coast and spread across the western United States to continue their life cycle. Late-winter and early-spring blooming nectar plants provide essential energy when few plants are in bloom.
February/March–September: Breeding occurs throughout the West. Adults lay eggs only on milkweed (Asclepias spp.). Monarch caterpillars feed exclusively on milkweed, consuming leaves both for nourishment and for cardenolides—plant toxins that protect them from predators. Caterpillars usually pupate away from the milkweed, selecting protected locations such as trees, vines, fences, or other structures. Milkweed is essential for caterpillar development, while nectar plants support adult butterflies. Several generations are produced during this period, with the final generation returning to coastal overwintering sites.
October: Monarchs return to overwintering sites. Fall-blooming nectar plants are again essential for survival.
How to Help Restore Monarch Populations
- Support local and state efforts to protect overwintering habitats. Many of these sites are on public land and may be threatened by development or tree removal.
- Protect monarchs and their habitats from pesticides, including insecticides and herbicides. Avoid using or purchasing plants pretreated with neonicotinoid insecticides, which are highly toxic and persistent in the environment.
- Restore breeding habitat by planting nectar-rich flowers that bloom in late winter, early spring, and fall. Plant milkweed for caterpillars, but do not plant milkweed within five miles of the coast, where it does not naturally occur. Milkweed planted near overwintering sites may disrupt migration and the monarch life cycle.
There are 15 milkweed species native to California, four of which occur naturally in San Luis Obispo County.
Milkweed Species for San Luis Obispo County
Asclepias fascicularis (Narrowleaf milkweed)
The most common and readily available species in nurseries. Easy to grow once established, drought tolerant, and spreads by runners and seed. Blooms in summer and fall; 1.7–3.3 feet tall, about 1 foot wide; winter deciduous with white to lavender flowers.
Asclepias californica (California milkweed)
Occurs naturally at higher elevations in San Luis Obispo County. Emerges early in spring, which may benefit monarchs leaving overwintering sites. Blooms in spring and summer; about 3 feet tall and wide; semi-deciduous. Not commonly available in nurseries and may require growing from seed.
Asclepias eriocarpa (Woollypod milkweed)
Also known as Kotolo or Indian milkweed. Emerges early and may benefit early monarch migration. Blooms in summer; 1.3–3 feet tall, about 1 foot wide; deciduous with cream to pink flowers. Plants and seeds are difficult to find.
Asclepias vestita (Woolly milkweed)
Endemic to California but difficult to find as plants or seed. Blooms in spring and summer; low-growing with stems that bend along the ground; deciduous with cream to yellow flowers.
Asclepias speciosa (Showy milkweed)
Not native to San Luis Obispo County but native to much of the western United States. Widely available and grows well locally. Blooms in summer; 4–6 feet tall and up to 4 feet wide; deciduous with white, purple, and pink flowers. Emerges later in spring and spreads by rhizomes.
Avoid Tropical Milkweed
Avoid planting Asclepias curassavica (tropical milkweed). Although attractive and widely available, it is not native to the United States and can interfere with monarch migration. A protozoan parasite, Ophryocystis elektroscirrha, can build up on its leaves and harm monarchs. Replace this plant or cut it back to the ground in November and keep it trimmed during winter. Dispose of clippings in the trash, not compost.
Milkweed Propagation and Care
- Milkweed can be propagated by seed or by digging up rhizomes. A. speciosa is best grown from rhizomes.
- Seeds may be sown directly in the ground in fall or started in peat pots for transplanting. Transplants can be planted in spring or fall.
- Seeds of some species require cold stratification (refrigeration for 1–2 months before planting).
- Milkweed is drought tolerant but requires irrigation every 7–14 days during the first year, and about every 14 days during the second year until established. Mulch to retain moisture.
- Protect young plants from rabbits or deer using hardware cloth if needed.
- Plants grow slowly the first year, then spread vigorously. Choose planting sites where spreading is acceptable, or grow in containers.
- Oleander aphids are the most common pest. Control early by hand-picking or gently spraying with water. Avoid insecticides, which harm monarchs at all life stages.
References and Resources
Pelton, E., Schultz, C., et al. 2019. Western Monarch Population Plummets: Status, Probable Causes, and Recommended Conservation Actions. Frontiers in Ecology and Evolution. https://doi.org/10.3389/fevo.2019.00258
Western Monarchs Are in Trouble: This Is How You Can Help. Xerces Society. https://xerces.org/sites/default/files/publications/19-002_02.pdf
Western Monarch Biology: The Monarch Life Cycle. Monarch Milkweed Mapper. https://www.monarchmilkweedmapper.org/western-monarch-biology/
Monarch Nectar Plants. Xerces Society. https://xerces.org/sites/default/files/publications/19-046_01_MonarchNectarPlants_California_web-4pg.pdf
Monarch. Calscape, California Native Plant Society. https://calscape.org/lep/Danaus-plexippus-(Monarch)