Plants & flowers that attract solitary bees in Denver
Solitary bees fall into two categories: generalists and those restricted to a few species of flowers. The generalists will collect pollen and nectar from a wide range of plants, particularly blue-flowered plants. Those solitary bees that eat only specific plants, have different flower needs, one species to the next. If you know which bees you have, you can investigate which plants they need in order to collect pollen and nectar.
Here is a link to some Western flowers that will help get you started on a bee garden in the West. If you are in another part of the country, contact your local extension agent for suggestions on bee-loving flowers.