Answer: Hardy lavenders such as "Munstead" or "Hidcote" are usually pruned in mid spring to tidy away any winter killed branches and to gently shape them. Be patient and allow ample time to see new growth and also make sure the wood is truly damaged before you cut it away. This may mean trimming away about one third to possibly a half of the stem height.
Also,immediately after blooming, trim away the faded blooms and wiry stems by lightly shearing the top. This should help promote additional blooming. Do not prune them back in the fall however as this may promote late season growth that will not have enough time to mature before the winter cold hits.
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