How to tell what's wrong
Start below ground. Slide the plant out of its pot, or dig gently at the edge of an in-ground plant, and look at the roots. Healthy roots are firm and pale; rotting roots are brown or black, soft, and peel away when you tug them. Penn State Extension notes that browning tips can be a sign of root rot. Then look at the pattern above ground. Browning that starts on the inner, older needles and spreads outward points to the roots. A whole plant that browns after a hard freeze points to cold. A white, powdery film on the needles is powdery mildew.
Why it happens
Overwatering is the most common cause. University of Wisconsin–Madison Extension calls it the biggest cause of rosemary decline and death. Rosemary comes from dry Mediterranean hillsides, and its roots need air as much as water. Heavy clay, a pot without drainage holes, a saucer that holds water, or watering by the calendar instead of by the soil can keep the root zone wet long enough for rot to start. Once established, rosemary needs water only when the top 2 inches of soil are dry. Wisconsin lists root rot and gray mold as the most common rosemary diseases.
Winter is the second big cause. Most rosemary is reliably hardy only to about zone 8. Hardier varieties such as Arp may survive zone 7, or zone 6 with protection, but nothing is certain there, and plants left outside in colder areas often brown and die after the first hard freeze. Rosemary brought indoors has a different set of problems. Penn State notes that overwintering indoors is difficult because of low light and too much or too little water, and that overwatering is especially deadly in pots kept inside for winter. Dry, heated air also invites spider mites, which leave fine webbing and pale, speckled needles.
What to do now
If the roots are brown and mushy, the plant is unlikely to recover; take a few 4-inch cuttings from any healthy green tips and start over. If most roots are still firm, move the plant into fast-draining soil or a clay pot, which dries faster than plastic, and let the top 2 inches dry before each watering. Cut dead branches back to green growth. For powdery mildew, space plants so air moves between them and keep indoor pots where air circulates. For spider mites, rinse the stems and the undersides of the needles with water every few days.
Next winter
Where winters are colder than zone 8, plan the move indoors before the cold arrives. Penn State and Wisconsin both suggest bringing rosemary in 2 to 3 weeks before your first fall frost. Indoors, give it a south-facing window in full sun or a grow light, good air circulation, and water only after the soil has dried. Outdoors, plant in a raised bed or soil with coarse grit, in the most sheltered, sunny spot you have. If you lose a plant in a hard winter at the edge of its range, that's the climate, not your care.
Growing it again? When to plant rosemary for your ZIP code
- Root RotPlants wilting despite wet soil, with brown or slimy roots — wet-soil pathogens destroying the root system.
- Spider MiteStippled, bronzed leaves with fine webbing on undersides; damage intensifies in hot, dry weather.
- Powdery MildewWhite powdery spots on leaves that spread into a floury coating — common when warm, dry days follow cool nights.
- When and how should I harvest herbs for the best flavor?Harvest herbs before they flower — leaf essential oil concentration peaks right before flowering, and leaf production tends to decline once the plant shifts energy to seed production.
- There's a surprise frost forecast — what should I cover and with what?Cover warm-season transplants and seedlings with row cover fabric or old bedsheets — not clear plastic sheeting directly on leaves — and place covers before sundown to trap daytime heat.