Pruning and Trimming Herbs
Herbs respond well to regular attention and can become lank and woody or sprawl untidily unless they are cut back or deadheaded each year.
From: DK Books - Herbs
Cutting Back After Flowering
Pruning out of finished or fading blooms can help promote the development of further bursts of flower buds, but not all herbs are capable of flowering continuously. Some, such as Lavandula angustifolia, will only flower once, but others, such as cultivars of L. stoechas and L. pinnata, can flower almost continuously throughout summer if the flowering stem is cut back to a node or to lower developing buds.
Herbs such as lemon balm (Melissa officinalis) and fennel (Foeniculum) will seed themselves prodigiously if not deadheaded or cut back. Deadheading will also prevent seed development, so always leave a few to set if you wish to harvest the seed for sowing or culinary purposes.
Maintaining a neat and productive set of culinary herbs is dependent on regular usage or harvest. New growth is the most colorful and flavorsome and to keep this coming, regular rejuvenation is vital. Trimming off flowerheads will often prevent annuals and biennials from dying while perennials may go on to produce a second flush of foliage.
As the flowers fade, cut back with sharp scissors, aiming to reduce the stems by about half on the first occasion. (image 1)
The foliage will grow back by 1/2-3/4 inches within a few weeks and this new growth can be trimmed off for immediate use. (image 2)
Many herbs are shrubs and some, such as santolinas and Helichrysum italicum, can become woody and sparse at their bases. Renovate in late spring or early summer when there is little chance of the tender new growth being damaged by frosts. Some of the prunings could even be used as cutting material.
Using sharp pruners, cut out all dead and diseased wood to just above ground level; cut back all other stems to around 4 inches. (image 1)
Thin out the remaining branches to leave 6–12 stems with a number of new shoots on each—aim for an open goblet shape. (image 2)
Hedges of Lavandula angustifolia and its cultivars need pruning at least once a year to keep them compact and robust. An additional light trim in early spring delays flowering by a few weeks but helps keep the hedge in good shape. Never prune into old wood as it is unlikely to regenerate and may also cause rots to develop.
In late summer or as the flowers begin to fade, cut back most of the previous season’s growth using sharp shears or pruners. (image 1)
Aim to leave 3/4-2 inches of actively growing or budding wood and make sure no lose trimmings are left to rot in the hedge. (image 2)