blue cardinal flower (lobelia siphilitica), also called great blue lobelia, can be a fussy plant to start, but once it finds a favorite spot, it will return reliably. Though it’s a short-lived perennial, it tends to spread and re-seed. The blue of this tall, stately plant’s blooms is unusual, with shades of lavender and sky blue. The blooms are long-lasting—often blooming for four to eight weeks during July through September.

A clumping plant, it has a large taproot and it forms winter foliage rosettes. In spring, its stem emerge from its ground-level crown. The tubular flower spikes form on single or multiple stalks containing many flowers, which bloom from bottom to top.

Because blue cardinal flower blooms later in the season, it is an important pollen and nectar source for insects, bees and hummingbirds. It’s recognized by pollination ecologists as attracting large numbers of native bees. Often when you observe the plant on a warm summer day, you’ll see many pollinators on each flower stalk. It’s also a host plant to several beneficial predator insects, including moth species.

In addition to the benefits to pollinators, blue cardinal flowers provide tall, showy structure in floral arrangements. And they have a long vase life, often lasting for a week. In the garden, the plants form a dramatic backdrop and structure for other plants. Note: All parts of this plant are mildly toxic if consumed in large quantities, though it was once used for several pharmaceutical purposes by Indigenous Americans and early European settlers.

Lobelia in the Garden

Great companion plants include natives, such as wild columbine (Aquilegia canadensis) and wild bergamot (Monarda fistulosa), that bloom earlier and those that bloom around the same time or later, such as turtleheads (Chelone spp.) and asters (Symphyotrichum spp.).

Blue cardinal flower is happiest in medium to moist conditions; it doesn’t tolerate drought well. But if you have a spot in the garden that’s regularly watered, by nature or by humans, it isn’t fussy. It’s a great plant near ponds, streams, waterfalls or other moist garden locations. Blue cardinal flower will tolerate all types of soils, from sandy to clay, and it grows well in full sun to partial shade, or even in a woodland garden if it gets enough moisture.

There are several effective ways to propagate blue cardinal flower. It starts well from seed—either directly sown in the spring after all danger of frost, or started indoors several weeks before the last frost. The seeds need light to germinate, so they should be sown on the surface and not covered.

Established plants also can be divided in midsummer before blooming—from rooted cuttings or with roots from a large clump. Blue cardinal flower is a stunning bloomer: It can’t be ignored
when at its peak in late summer. Its color, stature, form and attractiveness to butterflies and other pollinators makes it a special plant for late in the growing season.

Blue Cardinal Flower: At a Glance

Beth Stetenfeld is an organic gardener, native-plant enthusiast and garden blogger and writer. She’s also a master naturalist volunteer and instructor.