Lavender vs Rosemary

NC State rates English lavender for Zones 5a to 9b and rosemary for Zones 8a to 10b. That gap of three zones is the main reason the two herbs are not interchangeable in northern gardens.

Lavender's winter losses usually come from water rather than cold. Iowa State calls root rot the most common problem and ties it to poorly drained soil or overwatering, with plants most at risk when the soil stays saturated through winter freeze and thaw. It recommends compact English cultivars such as 'Hidcote', 'Munstead', 'Blue Cushion', and 'Royal Velvet', all rated for Zone 5.

Rosemary has a narrower margin. Wisconsin Horticulture describes the species as winter hardy only to Zone 8a, with 'Arp' hardy to Zone 7a and possibly 6a with protection, and 'Madeline Hill' surviving to 7a or 6a in protected sites. In a Zone 5 or 6 garden without that protection, rosemary is grown in a pot, kept outside in full sun for the summer, and brought in before the first frost.

Indoor winters are their own problem. NC State notes that rosemary can be difficult to overwinter indoors, and Wisconsin Horticulture names overwatering as the biggest cause of decline and death, so a potted rosemary needs a bright window and soil that dries between waterings.

They can share sun and drainage, and both fail in wet soil. NC State says English lavender does not like wet feet and dies out in heavy clay, and that overwatering is the common cause of rosemary decline or death. Iowa State often uses raised beds or berms for lavender in clay soil, and the same lift helps rosemary.

Soil pH is where they differ. NC State lists English lavender as preferring neutral to alkaline soil, and Iowa State adds lime where soil is too acidic. Rosemary is listed for light, slightly acidic soil. A bed limed for lavender can still grow rosemary, but it sits outside rosemary's preferred range, so a soil test before liming tells you which plant the bed favors.

Light is the other small difference. Both are full-sun plants, but NC State lists rosemary as tolerating partial shade, while lavender is listed for full sun of 6 or more hours. The shadier end of a shared bed suits the rosemary.

Size is the difference you see first. NC State describes English lavender as a mound 1 to 2 feet tall and 2 to 3 feet wide, and rosemary as a shrub reaching 4 to 5 feet tall and 3 to 4 feet wide where it is hardy.

Pruning follows from how each plant regrows. Iowa State warns that lavender does not readily produce new growth from old wood, so it prunes lightly after flowering to keep the shape and removes dead stems in spring. NC State prunes lavender yearly and removes the spent flower spikes after bloom. Cutting a leggy lavender back into bare wood usually leaves bare stems that do not refill.

Rosemary is far more forgiving. NC State lists it as tolerating heavy pruning, and notes that once flowering has passed, a severe pruning is an option to encourage dense foliage. That makes rosemary the better choice for a clipped hedge or a plant kept to a fixed size.

New plants come most reliably from rosemary cuttings. Wisconsin Horticulture takes 4 to 6 inch tip cuttings from shoots without flowers in late spring to early summer, and prefers cuttings to seed.

Lavender is grown for its flower stems. NC State rates English lavender as a good cut and dried flower, used in potpourri, dried arrangements, and herb pillows, with culinary uses such as jellies and vinegars. Iowa State harvests stems just as the flower buds begin to open, in the morning after the dew dries, and dries bundles upside down in a warm, dry, ventilated spot away from sun to keep the color. The timing details are in cutting lavender stems.

Rosemary is grown for its leaves. Wisconsin Horticulture picks small amounts as needed and removes no more than 20 percent of the growth at a time, and for drying cuts sprigs before the plant flowers and hangs them in an airy space.

The harvest also decides how many plants you need. A rosemary plant cut a little at a time supplies a kitchen through the season, while a lavender harvest for drying takes the season's flower stems at once, so drying lavender in quantity takes several plants.

The ASPCA lists English lavender as toxic to dogs, cats, and horses. It names linalool and linalyl acetate as the toxic compounds and lists nausea, vomiting (not in horses), and loss of appetite as signs. NC State rates the severity as low. The ASPCA lists rosemary as non-toxic to the same three animals. Neither listing covers concentrated essential oils, which need separate advice.

Both herbs feed pollinators, at different times. NC State lists lavender's flowers in summer and notes that they attract butterflies and other pollinators, while rosemary flowers in spring and summer and attracts butterflies, bees, and hummingbirds. Growing both spreads bloom across a longer season in a pollinator planting.