The most important thing that you can provide for monarchs is food by planting both milkweed and nectar plants.
Monarchs are milkweed specialists; they only lay their eggs on milkweed, and it is the only plant the monarch caterpillars can eat. In Wisconsin, we have 12 native milkweed species, but some of them are very rare and not readily available for planting.
The best time to plant milkweed seeds is November and December. It's best to scatter seeds when the ground isn't covered with snow, so that the seeds can have good soil contact and lie under a moist blanket of snow all winter. This is because in Wisconsin, most native plant seeds require a prolonged period of cold before they will germinate and grow (think of how the seeds naturally drop to the soil in the fall and sit through the freezing temperatures of winter before germinating in spring). Milkweed plants prefer sunny areas, so choose a location that has at least 6 hours of full sun per day. For small areas, you can also plant seedlings (small plants) in the spring. Be sure to work them into the soil.
Nectar plants are flowering plants that produces nectar. Monarch butterflies rely on nectar for food, so all monarch habitat should have native wildflowers blooming the entire time that monarchs are in the state, usually from the end of April or early May through September. Because most plants don't stay in bloom for that whole time, good monarch habitat contains a variety of native nectar plants, which bloom at different times throughout the spring and summer.
For information about where to find native plant seeds or seedlings in your area, please refer to the Wisconsin Native Plant Nurseries publication. For recommendations on plants for monarch butterflies, see the "Top 12" at the bottom of this page, and the full list from the USDA Natural Resources Conservation Service (includes growth habitat and blooming times).
You can also improve monarch habitat by reducing the use of pesticides, cutting back on mowing, and providing windbreaks.
Monarch habitat should have little to no exposure to pesticides. Widespread herbicide use can eliminate the milkweed and nectar plants the monarchs need for food, and insecticides can kill or sicken the monarchs themselves. Be sure to ask your nursery about pesticide use before purchasing plants for your garden.
Cut back on mowing, especially from June through September when monarchs are breeding and laying eggs or when larva and young are present, because it reduces breeding habitat and causes direct mortality.
Additionally, monarchs benefit from wind breaks and sheltered areas that they can use to pupate and turn from a caterpillar to an adult or use for roosting during migration. Note that not all monarchs pupate on windbreaks or sheltered areas.
Information above provided by the Urban & Greenspace Working Group
Plant list assembled by the Urban & Greenspace Working Group of the Wisconsin Monarch CollaborativeThese species are native to the Midwest, easy to grow, versatile in their habitat preferences, attractive, and have a variety of colors and blooming times. Many of the showiest species on the list are "late blooming" species, which are especially important for monarchs. Late summer is when they are fueling up on nectar for their long migration to Mexico. These species will also attract a variety of other butterflies and pollinators.