Why is my hedge bare at the bottom?
Quick answer: A hedge goes bare at the bottom because the top is shading the base. Cut the sides flat and the top out-grows the bottom every season. Trim to a taper instead, base wider than the top, and keep grass off the stems. Corokia and Photinia 'Red Robin' will both reshoot from bare old wood. Conifers like Leylandii never will.
Stand at one end of a hedge that's been cut flat-sided for ten years and you can see straight down the row. Top half thick and green. Bottom half bare sticks, and a clear view of next door's trailer.
Looks like the plants are dying from the ground up. What's happened is they've been shaded out by their own top growth, one trim at a time, for a decade.
Why does the bottom go bare?
Light.
A hedge cut with flat vertical sides shades its own base. The top sits in full sun and grows hard, and every season it pushes a little further out over the lower branches. Those branches get less light, put on less growth, drop their leaves and give up. The top keeps thickening while that happens, so the shade only gets deeper.
By the time somebody rings us about it, the bare zone is usually the bottom third.
What shape should a hedge be cut to?
Narrower at the top than at the bottom, with both sides sloped in slightly so sunlight can reach the base. The trade word for it is batter, or taper.
Two habits that make it easier to hold the shape:
Cut the bottom first and the top last. Working upwards means you're judging the line as you go, instead of finding out at the end that the top has crept out another 50mm.
Run a string line for the top and bring the sides down to it by eye. A hedge that wanders in plan looks worse than one that wanders in height.
What else strips the bottom out?
Shade is the main culprit. Four things make it worse.
Grass and weeds growing up into the stems. They take water from the exact spot where the hedge is already weakest, and they hold damp against the base. Keep a clean or mulched strip either side.
Planting too tight. Our spacing rule is half the trimmed height you want to hold. A hedge kept at 1.5m goes in at 750mm centres, one kept at 1m at 500mm. Tighter rows close over faster at the top, so the lower branches start losing light years sooner than they needed to.
A dry strip. Hedges planted hard against a fence, a wall or a building sit in a rain shadow, and the base dries out before anything else does. This is the standout cause across the Canterbury plains, where the nor'wester takes moisture out of a fence-line hedge faster than the roots can put it back.
Browsing. Rabbits, hares and stock work the bottom 600mm and leave everything above it alone. If the bare zone has a dead-level top edge to it, that's teeth, not shade.
Can you fix a hedge that's already bare?
Some species break new shoots out of bare old wood. Some never do, and no amount of feeding will change it.
Corokia is the forgiving one. Cut it hard, back into the old framework, and it shoots from the stumps. Winter is the usual time for it. Inherited a Corokia hedge that's been let go? You can reset it in one hit.
Photinia 'Red Robin' takes renovation well. Cut back to within about 300mm of the main stems in early spring, before the new growth moves, and dormant buds along the old wood break within six to eight weeks. The regrowth comes through green and colours up red as it hardens. Full recovery takes two or three seasons, and it looks rough for the first of them.
Pittosporum will regenerate from old wood, but slowly and in patches. Spread it across two or three years rather than doing the lot in one go: one side hard this year, the other side next. A Pittosporum that was already struggling before you cut it may not come back at all.
Griselinia is similar to Pittosporum, it will regenerate over time but can look a bit patchy.
Conifers won't come back. Leylandii, Lawson cypress, macrocarpa and juniper carry their growth buds on green foliage only. Once a section has browned off there are no dormant buds sitting in that old wood to push out. Cut into the brown and it stays brown for good. Yew and thuja are the exceptions, and yew is the only common hedging conifer you can truly renovate. If your bare hedge is Leylandii, the choices are to live with it, plant in front of it, or take it out and start again.
We have a Leylandii hedge at our own place, and haven’t kept up the trimming like we should have. Unfortunately this is now meaning the hedge is getting too wide, and we have no way of cutting it back hard. It’s looking like our only option will be to rip it out and start again.
When should you cut a hedge back hard?
Early spring, so the plant has a whole growing season in front of it to recover.
A hard cut in autumn pushes soft new growth straight into the first frosts, and that growth gets burnt off. Around Lumsden and the inland Southland country, and harder again through Central Otago, work to the weather rather than the calendar and let the worst of the frosts go through first. Water it through the first dry spell after the cut, and mulch the base while the new shoots are coming.
If you're a landscaper quoting a renovation like this, put the recovery time in writing before you start. A hedge that looks worse for a full season is an awkward conversation if nobody was warned.
Common questions
Will the bottom grow back on its own if I stop trimming?
No. Letting the top go makes the shade deeper, so the base gets darker still. Light only gets back down there if you reduce the top and narrow the sides.
How much can I take off in one go?
On a species that reshoots from old wood, I wouldn't take more than about a third off in a single cut. On anything I'm unsure of, I'd do one side this year and the other next year, so the plant keeps half its leaf area working while it recovers.
Should I feed it after cutting it back hard?
Yes. A hard cut asks the plant to build a lot of new leaf out of stored reserves. Feed in spring as the regrowth starts moving, and keep the base mulched through the first summer.
Can I plant in front of it instead of fixing it?
Often the better call, especially on a conifer hedge that won't recover. A low dense row along the front covers the bare zone and you keep the screening you already have. Cheaper than replacing 40m of hedge, and you keep the ten years of growth you've already paid for.
Why is only one end of my hedge bare?
Look for what changes along the row. Shade from a building or a tree at one end, a wetter corner, a change in soil, a gateway where stock gather. A bare patch with a hard edge to it nearly always has a physical cause sitting right beside it.
Most bare-bottomed hedges we get asked about were cut into that shape, season by season, by somebody doing their best with a flat-sided trim. Change the shape and the light comes back on its own.
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