Monarda fistulosa (Wild Bergamot). Photo: Wikimedia Commons, CC BY-SA 3.0.
Monarch butterflies (Danaus plexippus) are among the most widely recognized insects in North America, and their decline over the past thirty years is among the most documented cases of insect population collapse. The eastern population, which overwinters in the oyamel fir forests of central Mexico and breeds across Canada and the United States during summer, has declined by an estimated 80 percent since the mid-1990s.
The causes are multiple: loss of overwintering forest, climate disruption to migration timing, and pesticide exposure. But the factor most directly addressable by Canadian gardeners is milkweed loss. Monarchs cannot reproduce without milkweed. The connection is absolute.
The milkweed dependency
Female monarchs lay their eggs exclusively on plants in the genus Asclepias. The caterpillars that hatch eat only milkweed leaves and sequester the toxic cardenolides in milkweed sap into their own tissues, making them unpalatable to most predators. This chemical relationship between monarch and milkweed is the foundation of the insect's ecology, and it cannot be substituted.
Two milkweed species are native to most of Ontario and Quebec and are appropriate for garden planting:
- Asclepias syriaca (Common Milkweed) — The most widespread milkweed in eastern Canada. Grows to 120–150 cm, spreads aggressively by rhizome. Fragrant pink-purple flower clusters bloom in July. Tolerates poor, dry soil. The most reliable host plant for monarchs in the region.
- Asclepias tuberosa (Butterfly Milkweed) — More compact (45–70 cm), with bright orange flower clusters. Does not spread by rhizome. Requires well-drained soil; does poorly in heavy clay. Harder to establish from seed but long-lived once settled. Provides both host plant function and nectar.
A common question is whether Asclepias syriaca is too aggressive for a residential garden. The answer depends on context. In a large mixed border or a wilder area, it can be allowed to spread within defined limits. In a small urban garden, A. tuberosa is a more manageable choice, though it's slower to establish. Either plant is far more useful to monarchs than a large collection of non-native nectar plants.
Wild Bergamot as a companion plant
Monarda fistulosa (Wild Bergamot) is among the most productive native plants for midsummer pollinators in Canada. It blooms from July through August — the peak of monarch breeding season — and is visited by a wide range of native bees, including specialist bees in the tribe Anthophorini that prefer tubular flowers.
Wild Bergamot is also notable for attracting Hemaris clearwing moths, which are often mistaken for small hummingbirds. These moths are active during the day and hover while foraging, making them unusually conspicuous for moths. Their larvae feed on plants in the honeysuckle family, so they're not directly dependent on bergamot — but they return to it repeatedly as a nectar source.
For monarchs specifically, Wild Bergamot provides nectar that supports adult butterflies during the period when they're searching for milkweed and laying eggs. It also overlaps with the southward migration if the bloom extends into September in cooler years or more northern locations.
Cardinal Flower and late-summer monarchs
Lobelia cardinalis (Cardinal Flower) is worth noting in the monarch context for a different reason. Its red, tubular flowers are specifically adapted for hummingbird pollination — the flower structure makes it difficult for most insects to access nectar, and most bees ignore it. But ruby-throated hummingbirds, which are themselves long-distance migrants passing through Canada in late summer, depend heavily on Cardinal Flower along migration corridors.
In a garden designed around supporting migratory species through August and September, Cardinal Flower fills a different niche than milkweed or bergamot. It doesn't support monarchs directly but supports another migratory species whose own journey follows similar timing and routes.
Lobelia cardinalis (Cardinal Flower). Photo: Wikimedia Commons, CC BY-SA 2.5.
The migration corridor through Canada
Most monarchs that breed in Ontario and Quebec travel south through the province, cross the Great Lakes at points like Point Pelee and Presqu'ile, and then move through the United States to reach Mexico. The timing varies by year and latitude, but peak southward movement through southern Ontario typically occurs in the second and third weeks of September.
During this transit, monarchs need nectar — not milkweed, since they're not reproducing. The most valuable late-season nectar plants in the eastern Canadian garden are goldenrods and asters, as described in the companion article on fall biodiversity. A garden that has both milkweed for breeding season and goldenrod or New England Aster for the migration window covers both phases of the monarch's time in Canada.
What doesn't help
A note on tropical milkweed (Asclepias curassavica), which is sold widely at garden centres and is often labelled as suitable for monarch support. This species is native to Mexico and Central America; it is not winter-hardy in Canada and dies at the first frost. The concern is not with growing it in Canada per se — it simply won't overwinter — but with its use in the United States, where it stays evergreen in warm climates and can disrupt migration timing because monarchs stop and breed on it rather than continuing south. It is not relevant to Canadian gardens climatically, but it's worth knowing why native milkweed species are preferable wherever you're planting.
Planting notes
- Asclepias syriaca prefers full sun and average to poor, well-drained soil. It is drought-tolerant once established and will spread into adjacent areas if not physically contained.
- Asclepias tuberosa requires excellent drainage; it rots in heavy clay. Give it a south-facing position in sandy or loamy soil. Slow from seed; purchase as a young plant if possible.
- Monarda fistulosa is adaptable to most well-drained soils in full sun to part shade. It is susceptible to powdery mildew in humid, poorly ventilated locations. Space plants 45–60 cm apart to allow air circulation.
- All three species are long-lived once established and require almost no supplemental feeding or watering in their second season and beyond.