Place preferred nectar sources near host plants. Meadow blazingstar (Liatris ligulistylis) starts blooming in midsummer. You dont have to place a favorite nectar source right by a caterpillar host plant for butterflies, but certainly within view makes sense. It can be trial and error discovering what plants butterflies prefer to gather nectar from in your area, but a sure place to start is regional lists of native plants, since those are what the butterflies evolved with over time.
