Showy milkweed and the monarch's return

In Denver's parks and prairie edges, the first broad leaves of showy milkweed (Asclepias speciosa) push through warming soil. These thick, gray-green ovals emerge in clusters, each plant sending up multiple stems that will reach waist-high by midsummer. The leaves feel substantial between your fingers, waxy and slightly fuzzy, built to withstand Colorado's intense sun and dry winds. If you crush one, white latex bleeds from the wound, bitter and sticky.

This is the plant monarch butterflies (Danaus plexippus) have been searching for as they move north from their wintering grounds. The orange and black adults arrive in Denver just as the milkweed establishes itself, timing honed across thousands of generations. Female monarchs patrol low over the emerging plants, landing to curl their abdomens beneath leaves and deposit single white eggs, each smaller than a pinhead. They choose carefully. The caterpillars that hatch will eat nothing but milkweed, and only certain species will do. Showy milkweed, with its robust leaves and high concentrations of cardiac glycosides, provides both nutrition and chemical protection.

The relationship runs deeper than food. As monarch caterpillars feed on milkweed leaves, they absorb the plant's toxic compounds into their own tissues. These cardenolides make both the caterpillars and the adult butterflies distasteful to birds and other predators. The bright orange and black warning coloration of adult monarchs advertises this chemical defense, learned by predators through unpleasant experience. The milkweed gains nothing immediate from this arrangement, but monarchs serve as pollinators when the plants bloom later in summer. The adults visit the dense clusters of pink flowers, their long tongues reaching deep for nectar while pollen grains stick to their legs and bodies.

Right now, in late spring, this ancient partnership plays out in small dramas across the city. A female monarch spirals down to a young milkweed plant, tests the leaf with her antennae, then presses her abdomen against the underside to glue an egg in place. The egg will hatch in three to five days. The tiny caterpillar will eat its eggshell first, then begin working through the milkweed leaf that will sustain it through five molts and two weeks of growth. By the time it pupates, the caterpillar will have increased its body weight three thousand fold, all on milkweed alone.

This specificity makes the relationship fragile. Monarchs cannot complete their life cycle without milkweeds, and showy milkweed populations in urban areas face pressure from development and landscaping practices that favor non-native plants. Yet here in Denver's parks and restored prairie spaces, both species persist. The milkweed sends up new shoots each spring from deep taproots that can survive decades underground. The monarchs return each year, part of a western population that overwinters along the California coast and breeds across the interior West.

Step outside now and look for the gray-green leaves pushing up in sunny spots. The milkweed plants are still small, easy to overlook among the taller grasses and emerging wildflowers. But they anchor something essential. Each plant represents a potential nursery for the next generation of monarchs, a chemical fortress where caterpillars can grow safely, and a future source of nectar for the adults that will emerge. The leaves catch the morning light, thick and purposeful, waiting.