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Why Monarchs Need Early-Season Milkweeds

By Stephanie Frischie on 12. October 2023
Stephanie Frischie

This is part one of a two-part blog series about Xerces’ work with the California Bureau of Land Management and Great Valley Seed Company to provide more early-season milkweed species in the early breeding zone for western monarchs in California.


One thing you might know about monarch butterflies is that there are two mostly separate populations in North America, an eastern population and a western population. To survive the winter season, individual adult monarchs migrate in the fall to overwintering sites – oyamel fir forests in central Mexico and coastal groves in California, respectively. In both cases, overwintering sites are places with trees for roosting, where the climate is mild and the butterflies can enter a restful state with little need for food. As temperatures warm and their food plants regrow in the early spring, monarchs emerge from overwintering to breeding season, moving northward or inland.

If you knew all of that, you also likely know that monarch adults feed on the nectar of many plant species and that the caterpillars depend on milkweeds (genus Asclepias) and other closely related host plant species for their food. There are upwards of 90 native species of milkweed across North America, all perennials that resprout from underground stems at the beginning of each growing season. The diversity of milkweeds is delightful – with each species uniquely adapted to a particular suite of soil, sun and climate. Most places in the lower 48 states have at least one native milkweed species, which is part of the reason why monarch butterflies live throughout much of the continent.


The importance of early-season milkweed species

The western monarch population has declined by more than 99% since the 1980s and faces a high probability of near-term extinction. They are most vulnerable from late fall through the early spring phases of their life cycle. In coordination with other key conservation partners, The Xerces Society created a Western Monarch Call to Action to address some of the key factors in this decline: lack of coastal overwintering habitat, pesticide exposure, and the lack of nectar and larval host plants during the breeding season.

One goal is to protect and plant pesticide-free, early-season native milkweed and nectar plant species in the early breeding zone of California. These milkweed species typically resprout in the months of March and April. They are of particular interest because the leaves and stems are available to monarchs for egg laying and caterpillar feeding relatively early in the breeding season, when populations are at their smallest sizes at the end of winter. These early milkweeds are likely critical to monarch survival because they are the first suitable host for egg-laying. They are also important because overwintering monarchs may not survive long enough to breed on later-emerging species, or may have lower fecundity later in their lifecycle.


Map of western monarch call to action priority zones, with high priority placed on coastal overwintering sites and early breeding zones several hundred miles east and north of the coast.
Our project to increase the seed supply of early-season milkweed species is a fundamental piece of the recommended action to plant and protect these resources for western monarchs in the early breeding zone in California.


Xerces and partners team up to increase milkweed seed supplies

The early-season milkweed species native to California include woollypod (Asclepias eriocarpa), California (A. californica), and heartleaf (A. cordifolia). Later season native milkweed species include narrowleaf (A. fascicularis) and showy (A. speciosa). These two later-season species are native to many parts of California and are generally available to purchase as seeds or plants. In contrast, the early-season milkweed species are more limited in both their native ranges and how available they are to get as seeds or plants. Xerces works on multiple fronts to increase the availability and planting of early-season milkweed seeds and transplants for supporting western monarch populations. One of those efforts has been the focus of a three-year Xerces project.


California milkweed plant growing on a hill
California milkweed (Asclepias californica) is an early-season milkweed that provides critical resources to monarchs in early spring.  (Photo: The Xerces Society / Jessa Kay Cruz.)


Funded by the Bureau of Land Management’s Plant Conservation and Restoration Management Program in California, Xerces began a collaborative project with Great Valley Seed Company to:

  • Identify and ethically collect seeds from wild populations of early-season milkweed species; 
  • Propagate these seeds and establish commercial scale seed production fields;
  • Conduct pesticide risk assessment and practice integrated pest management to minimize harmful pesticide exposure to monarchs and other pollinators;
  • Describe and share the lessons learned throughout the project so that others may also work toward increasing the supply; and
  • Raise awareness about the importance of early-season milkweeds and their availability and use in habitat for monarchs and other wildlife.


Stay tuned for part two, where we’ll get into the technical side of seed production and more of this project’s story and outcomes in its third and final year. 



Based in northwest Indiana, Stephanie provides pollinator habitat expertise to farms in Canada and the U.S. She also works with the native seed industry and researchers to plan and develop seed supply of important plant species for restoration of insect habitat. Before joining Xerces, Stephanie conducted research on the potential of native cover crops in Spanish olive orchards at Semillas Silvestres, S.L. through the Native Seed Science, Technology and Conservation (NASSTEC) grant.

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