Pycnanthemum tenuifolium aka Slender Mountain Mint

Pycnanthemum tenuifolium has two common names: slender and narrow-leaf mountain mint. It belongs to the mint family, Lamiaceae. This herbaceous perennial has soft stems and lives for years. It grows wild in eastern Canada. Its native range also spans the central and eastern United States. It grows in fields, dry meadows, prairies, and open woods. It also grows in thickets and along roads and streams. The species name refers to its narrow leaves.

Plants grow in upright clumps with many branches. They spread by rhizomes, which are underground stems. Garden plants reach 2–3 feet [0.61–0.91 metres] tall. Some reach 4 feet [1.22 metres], according to NC State. Clumps grow 2–3 feet [0.61–0.91 metres] wide. The green stems are square in cross section. They have no hairs. Pairs of slim, green leaves line the stems. Each simple leaf has one blade with smooth edges. The leaves have a needle-like shape. The fine leaves give the plant a light texture. Crush the plant to release a strong mint scent.

Small flowers form rounded clusters called cymes at stem tips. They range from white to pale pink or lavender. Some have purple spots. Each flower has two lips. The lower lip has three lobes. Sources give bloom times from June through October. Missouri Botanical Garden gives a range of July to September. Bees and butterflies come to the flowers for food. Native sweat bees, bumblebees, and honey bees all visit. The plant also hosts the larvae of wavy-lined emerald moths. Small black capsules appear in September and October. The blooms suit both cutting and garden display.

Grow this mint in full sun or partial shade. It prefers acid soil that drains well. It grows in sand, loam, clay, and shallow rocky soil. Soil can be dry or have a medium level of moisture. It copes with drought and brief spells of wetter soil. Sources place it in USDA hardiness zones 4–8. It grows fast and needs little routine care. Its rhizomes can spread fast enough to crowd nearby plants. Hard landscape edges can help limit its spread. Grow new plants from seed or stem cuttings. You can also divide plants.

Use it in borders, herb gardens, rock gardens, or meadows. It also suits rain gardens and plantings near ponds. Its roots and spreading growth offer value on slopes. The flowers suit gardens that support pollinators. The scented leaves suit gardens designed for the senses. NC State reports no major pest or disease issues. Missouri Botanical Garden reports none either. Dried leaves and flowers have uses in tea and food flavoring. People have also rubbed leaves on skin to deter mosquitoes. These sources give no separate assessment of toxicity.