Daylilies are a perfect choice for your mailbox garden due to their low-maintenance needs and ability to thrive in the harsher roadside conditions you're likely to find around most mailboxes. They'll also do well in full sun, where many curbside mailboxes are located.
Irises are another lovely flowering perennial that won't require much maintenance in your mailbox garden. While they might not bloom all season long, their unique blossoms, and options available in almost any color you can imagine, make them an ideal choice for adding interest and charm to your yard.
Lavender blooms best in full sun and well-draining soil. The plants are drought tolerant once established and won't require much watering, making them perfect for a curbside space that might not always get as much attention as other areas in your garden.
Summer phlox (phlox paniculata), also called garden phlox, is a long-lasting flowering perennial that can provide a beautiful addition to your mailbox garden. Colors range from pink to purple to white, and the flowers grow in densely packed clumps that will provide nice, full coverage around your mailbox.
Clematis comes in a wide range of colors, from white to pink to blue to red, but the most well-known variety is Jackmanii, which has large purple blossoms. It's hardy in zones 3 through 8, and it's a great option to choose if you'd like to attract butterflies to your garden.
Coneflowers are an easy-to-grow, sun-loving perennial that's perfect for your mailbox garden. Available in an array of colors, the most common variety is the purple coneflower that blooms from early summer to the middle of fall. They are hardy in zones 3 through 8 and drought tolerant.
Sedum could be the right choice if you'd like a lower-growing perennial to contrast the taller stalks and larger blossoms of the other plants in your perennial garden. Though upright varieties are available, many sedum plants work well as a groundcover. Sedum, also known as stonecrop, thrives in full sun and does well in dry, rocky, and poorer soils.