If you can, visit the Monarch butterflies on a weekday when it isn't very crowded at the Desert Botanical Garden. The garden staff is great about not establishing a time limit for your visit, and they encourage you to take your time and even have a seat for a few minutes to enjoy these special beauties. On weekends, though, it does get crowded, and people will be waiting for you to exit so that they, too, can enter and enjoy these fascinating creatures up close.
Monarch Butterfly Factoid: Non-migratory Monarchs have a much shorter lifespan--just a few weeks.
