Pruning promotes new healthy growth and enables lavender plants to better withstand attack from pests and diseases. Non-trimmed old stems become woody and devoid of new shoots, shortening plant life through lack of regeneration. Well-pruned hardy lavender can live for 20 years and outlive non-pruned plants by over 10 years. Timing of pruning is dependant on the lavender hardiness group, weather conditions and flowering season.
Prune Early-Flowering Hardy Lavender in Fall
Hardy lavenders grow in most northerly latitudes (zone 5) and require a period of cold for flower induction. Depending on location, Lavandula angustifolia species start flowering in June and continue for several weeks finishing by mid-August. After flowering, remove all flower spikes and prune into the new season's growth. As a guide, remove two-thirds of the entire flowering plant leaving a dome shape of about 8 inches in spread and height. New growth will flush and has time to harden before winter. Be cautious of cutting old straggly plants with woody growth devoid of fresh shoots as this may kill them.
Prune Late-Flowering Hardy Lavender in Fall and Spring
Prune L.x intermedia several weeks later than L. angustifolia as flowering is later. In areas of gradual winter onset with a long fall period prune plants back by two-thirds after flowering. In early winter areas protect plants from possible damage by leaving pruning until the following spring after severe weather is over and new growth starts to show. Remove last year's old growth down to new shoots, which will encourage strong fresh growth.
Prune Half-hardy and Tender Lavender in Spring
L. stoechas, dentata, pedunculata and viridis are half-hardy (zone 6) and flower from spring until fall. Prune to about 8 inches in height and spread during spring to establish a compact shape and also cut back any frost-damaged stems. Throughout flowering remove dead flowers as they fade to encourage new ones.
Tender lavenders (Zone 7) such as L. bipinnata, L. subnudae and L. canariensis will not withstand frost and need protection in heat overwinter. Spring prune and dead head throughout the flowering season.
Plant New and Regenerate Old Lavenders
When establishing new plants, trim all flowers off for the first year to promote new top and root growth rather than allowing energy to go into flower production. If bushes have been neglected and are woody, prune back some old growth to new shoots lower down the stems. Do this during the growing season to encourage fresh growth. Newer and lower shoots may appear later on; pruning back to these can be done the following year and gradually regenerate the plant.