How to Trim a Hedge Straight (Without a String Line)
By Cal — outdoor and landscaping, Perth WA.
You walk past hedges every day that look like a brick — flat top, flat sides, sharp corners. They were trimmed with a string line. Technically straight, sure. But hedges are plants, not bricks, and a brick-shaped hedge looks weird in an Aussie garden. The good ones — the ones at high-end Mosman or South Yarra houses — are gently undulating, slightly tapered, and look natural even though they’re carefully shaped.
That look comes from the eye, not from a string line. The trick is what I call the “step-back-every-3-metres” method. You trim a section, walk back, look at it from 5 metres, adjust, move on. It takes longer than running a string and slashing through it, but the result looks like a designer hedge, not a fence.
The other thing nobody tells you in Australia: when you trim matters more than how you trim. Lillypilly — the most popular hedge in Aussie gardens — has psyllid season. Get the timing wrong and you spread the leaf bumps across all the new growth. I’ve seen $2000 hedges ruined by a March trim done at the wrong moment.
What you’ll need
- Battery hedge trimmer (Ozito 18V or Ryobi One+ are fine for a home hedge; Stihl HSA 60 if you’ve got 30+ metres)
- Sharp secateurs — Felco 2 or 6 (the genuine ones, not the Bunnings copies — copies go blunt in a season)
- Drop sheet or old tarp to catch trimmings
- Step ladder (NOT an A-frame on grass — use a tripod orchard ladder if you have one, or work off something solid)
- Safety glasses and ear protection — non-negotiable
- Garden gloves
- Yates Confidor (psyllid spray) if your lillypilly is bumpy
Step 1: Identify your hedge species
Different hedges, different rules:
- Lillypilly (Syzygium): trim in October and again in late March, AFTER psyllid activity drops
- Murraya: trim after each flush of flowers (May, September, January) — they bounce back fast
- Photinia (Red Robin): trim early spring and again in summer to keep the red tips coming
- Buxus (English box): two trims a year, May and October — slow grower, light hand
- Westringia (native rosemary): never cut into bare wood — it won’t shoot back
Step 2: Time it around psyllid (lillypilly only)
Here’s the Aussie gotcha. Lillypilly psyllids cause those hard pimple-like bumps on the leaves. They lay eggs on NEW growth. If you trim during psyllid activity (typically February and August in most of Australia), all the fresh new shoots get hit and your hedge becomes one big bumpy mess. Trim AFTER psyllid season — late March and late September are the safe windows. If your hedge is already bumpy, spray with Yates Confidor 4 weeks before trimming to break the cycle.
Step 3: Set the height with a single anchor point
Don’t string-line. Pick the lowest acceptable height somewhere along the hedge — usually the lowest point of an existing top, or a level you’ve decided on. Mark it with a stake. That’s your anchor. Every other height decision references that one point with the eye, not with string.
Step 4: Trim the top first, in 2-metre sections
Hedge trimmers cut best when held flat and swept in long arcs. Start at one end. Trim a 2-metre section to the height of your anchor stake. Stop. Walk back 5 metres. Look at it square-on and from each side. Adjust if it’s not right. THEN move to the next 2-metre section.
This is the bit weekend-warriors skip and it’s why their hedge looks like a saw-tooth. The eye picks up height variations of 20 mm at distance that you cannot see from arm’s length.
Step 5: Taper the sides — wider at the bottom
This is the second secret to a good-looking hedge. The sides should taper IN slightly toward the top — say 50 mm narrower at the top than the bottom over a 1.5 m hedge. This lets light reach the bottom of the hedge so the lower leaves don’t go thin and woody. Brick-shaped hedges (parallel sides) inevitably go bald at the bottom within 3 years because the top shades the bottom.
Step 6: Cut sides in vertical sweeps
Hold the trimmer vertically (blade pointing down, then up alternating) and work bottom-to-top in sweeps. Don’t try to cut top-to-bottom in one go — gravity drags the trimmer in and you cut a divot. Bottom-up gives you control of the line.
Step 7: Step back. Step back. Step back.
Every 3 metres of progress, walk back 5 metres minimum, ideally 8. Look at the hedge in profile. Look at it from the angle people see it from when they walk up the path. The eye is the only level that matters. Mark any high spots with a peg of bamboo and come back to them after the rough cut.
Step 8: Detail with secateurs
Hedge trimmers leave torn edges on big leaves — lillypilly, murraya, photinia all show ragged cut leaves for weeks. Walk along the hedge with secateurs and cleanly snip every leaf the trimmer ripped, just back of the rip. Tedious. Worth it. The hedge will look immaculate within a week instead of looking shredded for a month.
Step 9: Clean up trimmings same day
Lillypilly leaves on a lawn become slippery when wet, and rake-resistant once they bond into the grass. Drop sheet under the hedge while you trim, then bag everything immediately. Council green-waste bin if you have one, or a trip to the tip.
Step 10: Feed and water after trimming
A heavily trimmed hedge has lost its food-making leaves. Give it a handful of slow-release Osmocote per metre and a deep water. Two weeks later it’ll throw a vigorous flush of new growth. This is also the moment to apply Confidor if you’re a lillypilly owner with a psyllid history.
Curved and shaped hedges — the harder version
If your hedge has a deliberate curve (a wave-top, a serpentine, an arched gateway), the step-back method matters even more. String lines simply can’t do curves. The trick is to mark waypoints — small bamboo stakes at each high point and low point of the curve — then trim between them, stepping back constantly to verify the curve flows smoothly. Take your time. Two stop-and-step-backs per metre on a curved section is normal. The result is a flowing line that looks designed rather than hacked.
Topiary balls and cones are even more demanding — for those, freehand with secateurs is better than a hedge trimmer. The trimmer wants to cut flat; topiary wants curves; the geometries fight each other. Use the trimmer for the rough shape, then refine with secateurs from multiple viewing angles.
How short can you go? — the hard pruning rule
If your hedge has gone way too tall and you want to drop it 500 mm or more, don’t try to do it in one go. Most Aussie hedge species (lillypilly, photinia, murraya) will recover from a hard prune but only if you go in stages. Take a third off in spring, wait for new growth, take another third the following autumn. Hard-prune in one hit and you risk killing the plant — especially westringia, which won’t shoot from old wood at all and you’ll just have a row of dead sticks.
The exception is buxus — slow but bombproof. You can hard-prune buxus right back to 200 mm of bare wood and it’ll regenerate within two seasons. Slowly. With patience.
Battery vs corded vs petrol — what to actually buy
For a home hedge under 30 linear metres, an Ozito 18V or Ryobi One+ battery trimmer is plenty. They’re cheap, they share batteries with your other tools, and they cut beautifully. Above 30 metres or for thicker stems (lillypilly stems over 8mm), you’ll appreciate a Stihl HSA 60 (battery, $400) or HS 56 (petrol, $500). Corded electric trimmers exist but the cord is genuinely a pain in the lawn — I haven’t bought a corded one in 8 years and I don’t miss it.
Sharpen the blades once a season with a Stihl chainsaw file (small round file) drawn along each tooth. Blunt trimmer blades tear leaves rather than cut them — same problem as a blunt mower blade.
The Cal rule
Step back every 3 metres. Trim lillypilly outside psyllid windows. Taper the sides slightly. Detail with secateurs after the trimmer. Do those four things and your hedge will look professional even if you’ve never trimmed one before. String lines are for fences, not for plants. The eye is the only level that counts.
Got a hedge species not on this list, or a particularly tricky shape? Send us a write-up.
