For gardens, we recommend that you plant about 20-30 milkweed plants per 100 square feet. Milkweed plants should be spaced 1 foot apart, placed in clusters of 3-4 milkweeds. The clusters should then be interspersed with nectar sources that bloom at various times of the growing season (See our plant lists here http://monarchwatch.org/garden/plant-lists.html.) This will supply nectar for mating and migration in adults. Common milkweed will tend to spread by underground rhizomes, so 30 plants might be too many. If you like a “tidy” garden, your garden plan should include pulling or digging out common milkweed from areas that it may not be wanted. Check for monarch eggs before discarding plants, and place the leaves with eggs on or near a plant that you did not remove.
For restoration of natural areas, if planting common milkweed, we recommend 20-40 plugs per acre, maximum. Swamp milkweed, which does not reproduce by underground rhizomes, can be planted at up to 80 plants per acre, maximum. There is no hard and fast rule, so there is some flexibility. You will want to disperse clusters of 3-4 plants among nectar sources focusing on native, perennial grasses and flowers. Planting milkweed in large, continuous patches is not recommended. This allows predators and parasites to find the caterpillars more easily.