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Gapping up a native hedge on chalk: before and after on the North Downs

An old field-boundary hedge with dead sections is one of the most common hedge problems I see on the chalk downland around Shepherdswell and Coldred. The answer is gapping up: bare-root infill plants worked into the dead or thin stretches during the dormant season. Here's what a gapped hedge looks like before, what it looks like two seasons after filling, and the method that gets new plants established in thin North Downs chalk without losing them in the first dry spring.

Before

The hedge I see most often is a mature mixed hawthorn-and-blackthorn field boundary with two or three dead sections, each four to ten metres long, where ash standards have failed from Hymenoscyphus fraxineus (ash dieback) and their canopy has taken hedgerow plants with them as they collapsed. What is left in the gap is bare ground, a line of dead wood, and a clear view through the boundary to the field beyond. Rabbit pressure is heavier in the gaps than anywhere else on the run, because the loss of cover lets them graze freely right up to where the living hedge begins. The rest of the boundary is serviceable, dense enough lower down and holding its line well, but the gaps are obvious and, on a working farm, a stock-management problem.

After

Two growing seasons after gapping up, the same boundary has a double row of hawthorn, blackthorn and field maple at knee height and climbing, spaced in the staggered double-row pattern that will eventually knit the new section into the existing flanking plants. Each plant went in with a rabbit guard, and the guards are doing their job: the plants inside them are undamaged and growing cleanly. The gap is not yet closed, native mixed hedging fills out over years, not months, but the boundary is already acting as a barrier again and the new plants are thick enough at the base that rabbits are no longer pressing in. By the end of the third growing season the new section will be ready for its first formative cut, which will thicken the base further and start it growing as a single continuous hedge rather than a patch of young plants beside an established one.

What happens in between

Diagnosing the gap first

Before ordering plants, I walk the full run of dead section and check why the original hedge failed. Ash dieback is the obvious cause on North Downs hedgerows at the moment, but old age, livestock damage, spray drift or prolonged waterlogging can all open gaps too. Getting the cause right matters because it tells you what to do with the dead wood (fell it first, or leave a remnant for wildlife), whether the soil in the gap has changed (compacted, shaded out, or chemically affected), and whether the same thing is going to happen to the plants you put in.

Timing: dormant season only

Bare-root stock goes in between November and the end of March. I aim for November or early December on chalk around here, for the same reason that timing matters on any free-draining chalk site: the ground is moist, the plants are fully dormant, and they have the whole winter to push roots into the soil before the chalk dries out in May and June. Planting in February or March is still fine but gives you less establishment time before the first dry spell hits.

Choosing species for a chalk infill

The rule for gapping up is to match what is already in the hedge rather than introduce something alien to the run. On the North Downs field boundaries around Shepherdswell and Coldred, that usually means:

• Hawthorn (Crataegus monogyna): the workhorse of every chalk downland hedge, and the right species to put in volume.
• Blackthorn (Prunus spinosa): thorny, suckering once established, very good at the base of a gap where you need density low down.
• Field maple (Acer campestre): genuinely happy on chalk, slower than hawthorn, but adds variety and autumn colour and will eventually reach the height of the flanking plants.
• Dogwood (Cornus sanguinea): a natural filler in North Downs hedgerows, good at suppressing grass competition between the other species once it gets going.

No single species should make up more than 70% of your total plants. In a gap in an otherwise mostly-hawthorn hedge, I typically mix two parts hawthorn to one part blackthorn with a smattering of field maple and dogwood, keeping hawthorn below that 70% threshold.

Plant specification and spacing

Use 2-year-old bare-root transplants, 450 to 600mm tall. Smaller stock is fine for open ground; in a gapping situation the plants need to be a reasonable size on day one so they can compete with the established hedge roots at either end of the gap that will be pulling moisture from the same thin chalk layer. Plant in a double staggered row: rows 40cm apart, plants 45 to 50cm apart within each row, offset so each plant sits in the gap opposite rather than in line with it. That gives you roughly six plants per metre of gap, which is the planting density specified under the government's BN7 countryside stewardship gapping-up grant (a useful benchmark even if you are not claiming the grant).

The 1.5-metre clear strip

Clear a strip at least 1.5 metres wide along the centre of the gap before planting. On chalk, grass competition is the main killer of bare-root plants in their first season: the thin soil holds very little moisture and established grass roots are extremely effective at taking what is there before the new hedge plants can reach it. A clear strip, either by hand-weeding or by laying a permeable membrane on the soil, makes the difference between plants that establish and plants that sit there and die. Remove the membrane after the second growing season once the plants are rooted deeply enough to fend for themselves.

Rabbit guards and stock fencing

On a chalk downland site, assume rabbit pressure will be heavy and guard every plant individually with a spiral guard and a cane. If the gap runs alongside a paddock or grazed field, offset any temporary stock fencing at least 1.2 metres from the centre line of the planting so livestock cannot reach the new plants. On the North Downs the chalk warrens are extensive: I have seen an entire season's gapping up browsed to stubs in a single night on an unguarded site. The guards are not expensive and they are not optional.

First-season aftercare on chalk

Water the new plants in well at planting time, even if the soil feels damp. On chalk the surface layers dry and crack far faster than heavier soils suggest they will, and the first hot week of May can stress unestablished plants badly. Check the plants after any dry period in the first growing season and water if needed. By the end of the second summer, if the plants are in spiral guards and the 1.5-metre strip has been kept clear, they should be settled enough to manage through dry spells on their own.

The BN7 countryside stewardship grant

Farmers and land managers gapping up a hedge that meets the eligibility criteria (at least 20 metres long, at least 80% native shrubs, management control of both sides of the boundary) can apply for the BN7 capital item under Countryside Stewardship. The payment covers gapping up at the current rate per metre of gap filled. Check the current rate and eligibility rules on the gov.uk countryside stewardship grants page before applying, as rates are reviewed periodically.

What can go wrong

The failure mode I see most often is planting without clearing the grass strip first, then wondering in August why the plants look stressed. Chalk soil simply does not hold enough moisture for both an established grass sward and a row of newly planted bare-root shrubs. The second failure mode is planting too late, February or March rather than November, on a site with no irrigation: the plants go in, they break bud correctly, and then a dry April takes them out before the root system is deep enough to find chalk moisture. Neither of these failures is the plants' fault. Get the timing and the strip right and native mixed hedging establishes well on North Downs chalk, which is exactly what the old field boundaries around Barfrestone and Eythorne are growing in already.

Sources: gov.uk, "BN7: Hedgerow gapping-up" (countryside stewardship grants); RHS, "Plant a bare root mixed wildlife hedge"; Wildlife and Countryside Act 1981, section 1.