Lavender is a perennial herb prized for its fragrance and its antimicrobial and anti-inflammatory properties. Without regular pruning, the lavender plant tends to develop woody stems at the expense of fresh green growth and flowers. You should remove dead or woody stems from lavender plants to encourage vigorous new vegetation, but more serious steps may be needed if stems are dying due to disease.
Lavender generally grows best in the Mediterranean climate of its origin, but cold-hardy and hybrid types have greatly expanded the plant's horizons. Hardy English lavender (L. angustifolia) grows to USDA zone 5. Wherever they grow, lavender plants do best in full sun and alkaline soil without much organic matter. Good drainage is essential, though, so incorporating 6 mm (1/4-inch) size bark mulch into the soil can be an effective soil amendment. Lavender is drought tolerant, but regular irrigation is needed for good plant size and flower production.
Reasons for dead stems
If plants haven't been pruned for a number of years, overgrown vegetation may show signs of ageing and some long, woody stems may have died. Unpruned lavender plants also tend to develop woody centre sections. If you planted a lavender species not cold hardy in your area, frost may cause stem dieback. Regular pruning will rectify both problems. But if your lavender plants grow in poorly drained soil or have been seriously overwatered, dead or dying vegetation may be due to soil diseases.
Controlling and preventing disease
Dead stems may be an early sign of vascular wilt or root rots, destructive plant diseases caused by various pathogens. All are related to too much moisture, either due to standing water or poor drainage or very humid growing conditions. Symptoms include rapid wilting of nonwoody stems and leaves followed by browning and finally death of the plant. English lavenders, and especially dark-flowered types and those with grey foliage, are more susceptible than hybrid lavandin varieties. Remove and destroy infected plants -- there is no effective treatment or cure, once these diseases strike -- then correct drainage and air circulation problems before replanting with less susceptible lavenders. Allow adequate space between plants for good air circulation. Prune back lower branches as needed during the growing season and avoid thick organic mulches. Mulch instead with 2.5 to 5 cm (1 to 2 inches) of white sand.
Remove dead stems whenever you notice them, cutting them all the way back to the plant base. Annual pruning of lavender can prevent "woodiness," dead or dead-looking stems, particularly in the centre of the plant, which may cause the plant to split in two. Lavender plants flower on new growth, so regularly cutting plants back stimulates new succulent growth, ensuring more attractive ornamental plants and better commercial production of flowers or essential oils. Prune lavender in spring just as new green leaves begin to emerge at the plant's base. Cut back about one-third to one-half of the entire plant. Cornell University Extension suggests cutting all stems back to 15 cm (6 inches) every two or three years to prevent plants from getting straggly.