Milkweeds are the plants monarch butterflies cannot do without. Monarch caterpillars eat nothing else, taking up the cardiac glycosides in the sap and carrying that toxicity into adulthood as a defense, which is why a bird that eats one monarch does not go back for a second. Planting milkweed is the most direct thing a gardener can do for the species.

Asclepias tuberosa, butterfly weed, is the orange one, deep-rooted and drought-proof once settled, and slow to show in spring, so mark the spot before you dig there in April. Asclepias incarnata, the swamp milkweed, wants the opposite: damp ground, a ditch or a pond edge, where the plants carry dusky pink flowers on tall stems, and 'Ice Ballet' carries them white. The flowers themselves are worth a hand lens. The pollination machinery is among the strangest in the plant kingdom, with pollen bundled into saddlebags that clip onto a visiting insect's leg.