← All guides · Cutting and shaping technique · Updated October 2026

How to prune a rosemary hedge in Broadstairs: timing, technique and the woody-stem rule for CT10 chalk gardens.

Rosemary is one of the most capable low hedges for chalk-garden terraces and clifftop front gardens in Broadstairs and St Peter's. It is evergreen, genuinely drought-tolerant on thin Margate Chalk, resilient to coastal salt air and presentable year-round. The trouble is that most people prune it the way they prune box or privet, and rosemary does not tolerate that. Cut into old brown wood and the plant will not regrow from it. Prune at the wrong time and the soft growth you force is vulnerable to a sharp frost before it has hardened. This guide covers the correct sequence for CT10 conditions, the one tool choice that determines whether the job works, and the test that tells you honestly whether a woody rosemary hedge is worth persisting with.

Steps

Step 1: Walk the hedge and check for nesting birds

Before you touch any rosemary hedge, walk the full length slowly. Low rosemary runs along boundary paths and garden edging are used by dunnock, robin and wren for ground-level and low nesting, particularly in dense, established specimens that have built up leaf litter at the base. If an adult bird flies out from inside the hedge repeatedly as you approach, or you see obvious entry and exit routes through the base stems, stop and come back four to five weeks later. That gap is usually enough for a single brood to fledge.

Step 2: Wait until the main flowering flush is over

The timing rule for rosemary is straightforward: prune after the main flowering is finished, never before it. In CT10, most rosemary cultivars flower from late February through to May, with the bulk of the display finished by the end of May or the first two weeks of June. The cultivar 'Miss Jessopp's Upright', which is the most widely planted hedging form in Broadstairs and St Peter's terrace gardens, typically finishes by mid-May in most years. Lower, trailing cultivars such as 'Benenden Blue' and 'Severn Sea' may carry flowers into early June.

If the plant still has open flowers on it, wait. There is no urgency: unpruned rosemary looks entirely respectable while it is in flower, and it is actively useful to pollinators. Pruning into full flower removes nectar sources and usually produces a scruffier result than simply waiting another two weeks would have done.

The upper limit on timing matters equally. Do not prune rosemary after mid-August. Any cut made after late summer forces new growth that has insufficient time to harden before the first frosts, which arrive on Thanet clifftops from late October in some years. Rosemary pruned in September in CT10 gardens shows consistently higher rates of winter dieback on the fresh-cut surfaces than the same species pruned in June. From mid-August onwards, leave it alone until the following May.

Step 3: Use sharp secateurs, not a hedge trimmer

This is the non-negotiable part of the job. A hedge trimmer is the correct tool for box, privet, hornbeam, yew and most formal evergreen hedges. It is the wrong tool for rosemary. The reason comes down to how rosemary regenerates: it produces new growth only from green-stemmed sections. The brown woody stems that make up the internal framework of the plant do not produce new shoots. Pass a hedge trimmer uniformly across the outside of a rosemary hedge and you shorten every shoot by the same amount each session. Within two or three seasons the living green zone has been pushed back against the dead wood. The next cut either goes into that dead wood, where nothing will regrow, or does not cut at all.

Use sharp bypass secateurs. Work along the plant shoot by shoot, cutting each stem back to just above a healthy pair of leaves or a side shoot. Keep the cut within the green-stemmed zone: typically the outer 100 to 150 mm of each shoot, occasionally a little more on a vigorous plant that has had a good growing season. Do not chase stems back to the point where they join the central brown framework. Leave a buffer of green growth on each stem.

The cut itself: angle slightly, just above a leaf node, in the same direction as the growth you want to encourage. Clean cuts heal cleanly; crushing cuts from blunt secateurs are an entry point for the fungal problems that can affect rosemary stems, particularly on the exposed north-east-facing positions along the Stone Bay and Kingsgate clifftop gardens.

Step 4: Trim the sides with a gentle outward taper

For a hedged rosemary run rather than a single specimen, work along the front and visible faces first. A gentle batter is appropriate: the sides angled very slightly inward from base to top, around five degrees, is sufficient. Rosemary does not need or hold a formal vertical face well, and the slight taper allows light into the lower part of the plant. Shaded lower stems on a rosemary with a vertical face lose their green zone faster than on a plant that receives light from above and the side simultaneously.

Take back to just within the previous season's growth outline. A typical annual trim removes 50 to 100 mm on each face of an established hedged run. Remove any shoots that have crossed into a neighbouring plant or grown strongly in the wrong direction, cutting back to a well-placed side shoot lower on the stem rather than leaving a stub.

As you cut, rake or shake the clippings clear of the remaining plant surface immediately. Wet rosemary clippings piled on top of living growth and left for more than a few days can cause localised dieback in the section beneath. This is less of a problem in CT10's dry climate than on heavier, damper soils, but it is still worth keeping the surface clear as you work.

Step 5: Trim the top, removing the spent flower spikes

Once the faces are done, work along the top of the hedge run. The spent flower spikes are clearly visible as dry, brown-tipped sections extending above the fresh growth. They should be removed entirely: they produce no further growth from that tip, and leaving them in place makes the hedge look unkempt for the next five months.

Take the top back to a consistent height by cutting just below the spent-flower level, into the current season's fresh growth. A string line at the target height is worth setting up for any formal low hedge run of more than three metres, where consistency of level matters for the result. On 'Miss Jessopp's Upright', which grows vertically to 1.2-1.5 m without clipping, the top growth may need to be taken back more firmly than the sides to hold the hedge at the desired height. Even so, keep the cut within the green zone on every stem. Never attempt to reduce the overall height of a rosemary hedge by cutting back to bare wood in one session.

Step 6: Assess whether any sections are past renovation

After trimming, step back and look at the base of each plant in the run. A healthy rosemary hedge has green shoots from stem level upward. If you can see 200 mm or more of bare brown leafless stems at the base before the green growth starts, those sections have passed the regeneration threshold. Scratch the surface of a brown woody stem with a fingernail: if it shows green beneath the bark, the plant is still alive, but alive is not the same as recoverable. No pruning method will push new shoots from bare woody stems on rosemary.

The practical question is whether the green zone sitting above the dead wood is enough to form a presentable hedge and whether it is sustainable over the next two or three seasons. If the green zone is less than a third of the plant's total height, or if the plant is already showing significant gaps and die-back through the previous growing season, the honest advice is to replace it. On Margate Chalk, where rosemary establishes quickly in the excellent drainage and warm chalk, a replacement plant from a reliable nursery will fill a gap within one season. Delaying replacement tends to make the adjacent plants look worse rather than improving the one being persisted with.

Why rosemary works so well on Margate Chalk

Rosemary (now classified botanically as Salvia rosmarinus, though most nurseries and garden centres still label it as Rosmarinus officinalis) is native to the Mediterranean coast: thin, alkaline, sharply draining limestone and chalk soils, full sun, salt air, summer drought and periodic frost. Margate Chalk provides almost exactly these conditions. The Upper Chalk formation underlying most of CT10 is alkaline (typically pH 7.5 to 8.0), free-draining, warm at the surface, and free from the waterlogging that affects heavier soils. Broadstairs and St Peter's receive the same sustained northeasterly coastal exposure as rosemary's native habitat receives from Mediterranean maritime winds, and the species handles it confidently once established.

The divergence from rosemary's native conditions is winter frost. Coastal CT10 reaches -6 to -8 degrees C in a severe winter, and the fully exposed clifftop positions from North Foreland to Kingsgate can be colder still in a sharp northeasterly. Most UK-sold rosemary cultivars are hardy to -10 to -15 degrees C. The exception is 'Benenden Blue', which is a degree or two less frost-hardy than the upright forms and can suffer heavier dieback on the most exposed clifftop boundaries in a severe year. On sheltered south-facing chalk terraces in St Peter's and in the more sheltered gardens behind the Viking Bay frontage, all cultivars are effectively fully hardy.

The one condition rosemary will not tolerate on Margate Chalk is impeded drainage. In gardens where chalk has been covered by a deep imported clay-based topsoil, waterlogging over winter kills rosemary more reliably than any frost. If a rosemary plant fails in a CT10 garden while the surrounding plants are healthy, check whether water is sitting against the roots in wet weather before replanting with the same species in the same position.

Cultivars for CT10 hedging and edging

Two cultivars account for most of the hedged rosemary runs in Broadstairs and St Peter's gardens:

• 'Miss Jessopp's Upright' (Salvia rosmarinus 'Miss Jessopp's Upright'): The most widely planted formal hedging cultivar in the UK. Strongly upright habit to 1.2-1.5 m without clipping; pale blue flowers from late February; AGM variety. Widely planted for low to medium boundary hedges in the Victorian terrace gardens of the Broadstairs CA and St Peter's CA, and along the cliff-edge gardens from North Foreland southward to Stone Bay. Hardy to approximately -15 degrees C. The recommended first choice for a new rosemary hedge in CT10.
• 'Benenden Blue' (Salvia rosmarinus Angustifolia Group 'Benenden Blue'): Lower, spreading habit to around 0.9 m; deep blue flowers, appearing slightly earlier than 'Miss Jessopp's Upright'; AGM. Better suited to informal low edging and rock-garden borders than to a clipped formal hedge run, since its naturally arching growth does not hold a tight formal line as consistently as the upright forms. Slightly less frost-hardy on fully exposed clifftop positions.

Also in regular use: 'Tuscan Blue' (taller and more vigorous, suitable for hedges to 1.5 m or above), 'Severn Sea' (lower habit, violet-blue flowers, AGM) and the standard species Salvia rosmarinus itself as an informal edging or specimen shrub. Prostrate forms are ground-cover plants rather than hedges and are not candidates for a hedged run.

Three things to avoid

• Pruning in autumn. October and November seem logical months for hedge maintenance because most other species are fine at this time of year. For rosemary, the timing is wrong. Any cut in autumn forces the plant to produce soft new growth as the season closes, and that growth is reliably more vulnerable to the first hard frost than growth produced in June. In CT10, first frosts can arrive from late October on the exposed clifftop positions and from November inland. Leave the rosemary entirely alone from mid-August to late April each year.
• Cutting into brown wood. The temptation to "open up" a rosemary that has become a bit too large by cutting it hard back is understandable. The outcome is almost always a dead section or a dead plant. If the only way to achieve the required size involves cutting into brown stems, the plant is already past the point where pruning can help and should be replaced. There is no recovery mechanism from bare wood on this species.
• Relying on a hedge trimmer for annual maintenance. A single pass with a hedge trimmer taken even 20 mm too deep removes the green growing zone from dozens of stems in one movement. Over two or three seasons this converts a well-established rosemary run into a row of plants with a very shallow green zone sitting on top of a woody dead framework, and the hedge will need replacing sooner than it should. Secateurs take somewhat longer and consistently produce a better result for this species.

Sources: Royal Horticultural Society, "How to grow rosemary" growing guide (rhs.org.uk/herbs/rosemary); RHS, Salvia rosmarinus 'Miss Jessopp's Upright' and 'Benenden Blue' plant profiles (rhs.org.uk); RHS, "Pruning shrubby herbs" (rhs.org.uk); hedging.co.uk, "Rosemary Hedging" (hedging.co.uk/rosemary); horticulture.co.uk, "Rosemary Pruning" (horticulture.co.uk); Gardeners' World, herb pruning guides (gardenersworld.com); British Geological Survey, Upper Chalk (Margate Chalk Formation) profile; Met Office climate averages 1991-2020, Manston station; Wildlife and Countryside Act 1981, section 1.