Erythranthe cardinalis aka Scarlet Monkey Flower

Erythranthe cardinalis is known as scarlet monkey flower. Sources also list it under the name Mimulus cardinalis. It belongs to the Phrymaceae family. Its native range runs from the western United States to Mexico, including Cedros Island. This herbaceous perennial spreads through creeping rhizomes. It can stay evergreen, though gardeners also grow it as an annual.

Plants reach about 1–3 feet [0.3–0.91 metres] tall. RHS gives an upper height of 3.3 feet [1 metre]. Spread varies between the two garden sources. RHS lists 0.3–1.6 feet [0.1–0.5 metres], while Missouri Botanical Garden allows 3 feet [0.91 metres]. Upright stems branch and bear sticky hairs. They have a yellow-green tint. The pale green leaves grow in opposite pairs without stalks. Soft hairs cover their surfaces, and sharp teeth line their edges. Leaves broaden toward the tip and reach about 4.5 inches long.

Scarlet flowers appear from June to October. Yellow flowers occur, but are rare. Each tubular bloom grows from a leaf axil. Flowers reach about two inches long and resemble snapdragons. Each has two lips, with two upper lobes and three lower lobes. The lower lobes spread apart to reveal the throat. Stamens project beyond the flower, and the anthers bear hairs. The blooms attract both hummingbirds and butterflies.

Give this plant rich soil that stays moist or wet. Missouri Botanical Garden favors part shade, including dappled sun. RHS also supports full sun where the soil stays wet. Never let the roots dry out in sunny sites. Plants tolerate brief floods and standing water. RHS allows planting at pond edges in four inches of water. The usual hardiness range covers USDA zones 7–10. A sheltered site may allow survival in zone 6. Plants may return through self-seeding where winter cold kills them.

Use scarlet monkey flower beside ponds, streams, or birdbaths. It also suits bog gardens, wet meadows, and low ground. Borders work where the soil does not dry out. Patio pots offer another option where winters are too cold. Rhizomes and seed help it spread when conditions suit it. Plants tolerate deer, rabbits, wet soil, and erosion. Long, hot summers can leave them looking worn. Missouri Botanical Garden reports no major pest or disease problems. RHS notes slugs, snails, and powdery mildew on young plants. Sow seed in autumn or early spring. Divide plants in spring, and cut stems back after flowering.