Great blue lobelia (Lobelia siphilitica) has deep blue flowers crowded along the upper section of 2- to 3-foot tall spikes and makes an impressive show in the perennial garden where it attracts Ruby-throated Hummingbirds, butterflies and other pollinators. Great blue lobelia plants were used medicinally by native Americans. Blue lobelia plants prefer average to moist rich soil in full sun and make a striking contrast in color when planted near red cardinal flower.