How often a hedge needs trimming depends on what it is, how formal you want it and how much Adelaide's weather is pushing it along. The short answer for most formal hedges in the eastern suburbs is two to three times a year. The long answer is below, species by species, with the timing I use as a horticulturist.
Why Adelaide hedges grow in bursts
Our hedges put on a big flush in spring, slow or stop in the heat of January and February, grow again in autumn when the rain returns, and sit still through winter. Trimming after each flush keeps a hedge dense; trimming during the heat stresses it and exposes the inside to sunburn. So the calendar matters more than the number of cuts.
A trimming calendar for common hedges
| Hedge | Trims per year | Best timing | Notes |
|---|---|---|---|
| Lilly Pilly (Syzygium, Acmena) | 3 to 4 | Late winter, early summer, autumn | Fast; trim after each flush. Watch for psyllid pimples on new growth. |
| Photinia 'Red Robin' | 3 to 4 | Spring, summer, autumn | Trim to encourage the red new growth. Takes hard cuts. |
| Viburnum | 2 to 3 | After flowering, then autumn | Trim too early and you lose the flowers. |
| Murraya | 2 to 3 | After each flowering flush | Fragrant flowers; trim lightly and often rather than hard. |
| Buxus (box) | 2 | Late spring, late summer | Slow. Never trim in full sun on a hot day. |
| Pittosporum | 2 to 3 | Spring, autumn | Tolerates hard pruning; keep the top narrower than the base. |
| Westringia | 2 | After flowering | Native; do not cut into bare old wood. |
| Conifers | 1 to 2 | Spring, late summer | Only trim green growth. Old wood will not reshoot. |
How hard can you cut?
Broadleaf hedges like Lilly Pilly, Photinia, Viburnum and Pittosporum can be cut back hard and will regrow from old wood, which is how an overgrown hedge is rescued. Conifers, Lavender, Rosemary and most Westringia cannot; cut them into bare wood and that section stays bare. If a hedge has got away from you, a horticulturist will stage the reduction so there is always foliage left to feed the plant.
Signs you have left it too long
- The hedge is wider at the top than the bottom and the base has gone thin and bare.
- You can see through it in winter.
- It has pushed over a path, fence line or driveway sightline.
- Flowering hedges have stopped flowering because they are cut at the wrong time.
The trick to a dense hedge
Taper. Keep the top slightly narrower than the base so sunlight reaches the lower leaves, and the hedge stays green to the ground. Most hedges in Walkerville, Prospect and Toorak Gardens that have gone bare at the bottom were cut plumb or wider at the top for years.
Trimming in a heatwave
One more timing rule that matters in Adelaide: do not trim a hedge in the days before or during a run of 35-degree days. Freshly cut leaves and the newly exposed inner foliage scorch, and a hedge that looked fine on Monday is brown on the sunny side by Friday. If the forecast is hot, the trim waits a week. Water the hedge well the day before any trim in summer and again afterwards.
I trim hedges across Norwood and Adelaide's eastern suburbs on a schedule that suits the species. See hedge trimming or book a quote.