Sage (Salvia officinalis) is a short-lived perennial herb grown for its aromatic leaves. It forms a shrub up to 2 feet high with gray-green leaves and attractive lilac flowers. Sage thrives in well-drained, slightly acidic soil and full sun or light shade. It will survive mild winters, but can be damaged by hard frosts. Sage plants are also vulnerable to fungal infections.
Sage plants become woody with age and need to be replaced every three to five years. Regular pruning and harvesting of the leaves reinvigorates sage plants and encourages more leaf production. If your sage plant has developed thick branches and is no longer sending out fresh shoots, cut back all its branches by 50 percent after it flowers or in the late summer. If this does not result in new growth, replace your sage plant.
Sage plants need free-draining soil to thrive and are vulnerable to root rot if overwatered or grown in waterlogged soil. Grow your sage plant on a slight slope or on a mound and make sure that sage pots have large drainage holes at the bottom. Water when the top inch of the soil is dry.
Sage plants will survive minimum winter temperatures of between minus 10 and minus 20 degrees Fahrenheit, according to Maddy Perron, master gardener at the University of New Hampshire. If your sage plant is damaged by frost, protect it from further bouts of freezing weather with fleecing or a frost cloth. In cold areas, consider replanting with a hardier species of sage, such as clary sage (Salvia sclarea), or growing sage in pots and moving it indoors during the winter.
Sage is vulnerable to attack by downy mildew, especially when grown in shady or damp conditions. Downy mildew manifests as small grayish patches on the leaves which slowly spread and join together, causing leaf drop. Avoid mildew by keeping sage foliage dry during watering and by not watering in the evening. Avoid planting sage close to mint plants, as they also suffer from downy mildew and can spread it. Discard your sage plant if it becomes infected with downy mildew and don't plant sage in the same soil for at least two years.