Costmary, also called Alecost or balsam weed, is a perennial herb in the Tanacetum genus. It forms upright, bushy clumps about 30–80 cm tall. Its aromatic foliage gives it interest as a fragrant foliage plant, culinary herb, and historical medicinal or household herb. Small yellow flowers appear in late summer, generally August and September. The plant is native to Eurasia and has been introduced elsewhere, including North America.

Where it grows best

Grow costmary in full sun or partial shade, although it may flower less well with insufficient sun. It tolerates sandy, loamy, or clay soils and a pH from 5.0 to 7.6, provided the soil drains well. Conditions may range from dry to moist, but waterlogged or poorly drained soil should be avoided. Keep soil moderately moist, allowing the surface to dry slightly between waterings. Costmary is listed here for hardiness Zones 4–8, making it a hardy perennial for many Canadian gardens. Outside that range, containers can provide shelter during severe cold or excessive heat.

Garden and wildlife value

Use costmary in herb gardens, fragrant plantings, or informal clumps where its aromatic foliage can be appreciated. Its flowers are yellow and late-season, but the approved information does not establish a firm pollinator value.

Everyday care

Water moderately, increasing watering during very hot, dry weather without allowing containers to stand in water. Space plants so clumps can become bushy without overcrowding. Costmary is considered easy to grow and tolerates a wide range of soil textures and pH conditions when drainage is adequate.

What to do through the season

In cold regions, expect the top growth to die back to the ground. Remove dead stems and allow new shoots to emerge in spring. Trim or cut back stems to maintain shape, reduce overcrowding, and help prevent legginess; cutting back after flowering is supported. Divide crowded rhizomatous clumps to renew them or manage their spread.

Problems and symptoms

Costmary can spread by rhizomes and may become invasive or difficult to manage if allowed to expand unchecked. Poorly drained or waterlogged soil can cause problems. Shade may reduce flowering, while very dry conditions can require additional watering.

Corrective and preventive care

Plant in well-drained soil and avoid sites that remain soggy. Give the plant adequate sun where flowering is important. Monitor the edges of clumps and divide or trim them when they become crowded. During hot, dry weather, water more often while still preventing standing water. In cold regions, remove dead top growth after dieback and wait for spring regrowth.

Before you plant

Choose a site with full sun or partial shade and reliable drainage. Allow room for clumping growth, but plan to manage rhizomatous spread. Costmary is perennial and hardy in Zones 4–8 in this profile; gardeners outside that range can consider a container that can be moved to shelter. Propagation is supported by rhizome division and seed, with division useful for renewing established clumps.