Plant library · Perennial Flowers
Lobelia siphilitica
Great Blue Lobelia is a native perennial for moist planting areas, bringing upright blue flower spikes to the garden from late summer into early fall. At about 3 feet tall, it gives a narrow vertical accent without taking over the bed. Use it where its late-season bloom can rise behind lower plants and remain visible from a nearby path or seating area.
Place Great Blue Lobelia in rain gardens, damp woodland margins, and other beds that hold moisture rather than dry quickly. Its 3-foot height suits the middle or back of a border, especially where shorter foreground plants will not hide the flowering stems. Group several plants to give the blue spikes more visual weight, while leaving a reachable bed edge for spring division if the clumps need renewal.[1][2]
Space plants 18 inches apart, center to center. Each clump reaches about 1 1/2 feet wide at maturity, so this spacing allows neighboring plants to meet into a continuous planting without requiring a broad gap. In a narrow border, set the row at least 12 inches back from the path edge to keep foliage and flower stems out of circulation space.
Reserve this plant for partial shade and consistently moist ground. It belongs in the lower, moisture-retentive portions of a planting rather than on a raised, fast-draining shoulder. Keep it with plants that can share high water needs, and avoid placing it where nearby thirsty roots or a dry foundation bed will compete for moisture.
Late-summer to early-fall flowering makes Great Blue Lobelia useful for carrying a moist border past the peak of early summer. Position it behind plants with earlier interest so its flowering stems can take over as the season advances. The plant remains a perennial anchor in the layout rather than leaving an annual-season gap.
Great Blue Lobelia may self-sow lightly where conditions suit it. Leave a little open soil within or just beyond the planting group if a gently expanding colony is welcome; otherwise, review seedlings before they become established. Avoid relying on it to fill dry pockets of a rain garden after the surrounding soil has drained.
Editorial content updated 2026-07-20.