A good hedge looks dense from top to bottom and holds a clean shape. Getting there is mostly about timing and the shape you cut, rather than how hard you cut it. The same rules apply to hedge pruning and shrub pruning around the rest of the garden.
The main trim: late spring to early summer.
Most hedging plants put on a flush of new growth in spring. Trimming once that flush has firmed up, usually from late October to December, keeps the hedge tight and stops it getting leggy. A lighter tidy in late summer or early autumn keeps it neat through winter.
Avoid heavy trimming during a heatwave. Freshly cut leaves that were shaded can scorch in hot sun, and stressed plants recover slowly. Hard cutting back is also best left out of the coldest months in frost-prone spots like Wallan.
Common local hedges.
- Lilly pilly: trim after each growth flush. Cutting off soft new growth also removes leaves affected by psyllid, which causes pimply new leaves.
- Photinia: regular trimming encourages the red new growth it's grown for.
- Murraya: trim after the main flowering if you want to enjoy the scented flowers first.
- Conifers: trim green growth only; most don't regrow from old bare wood.
- Westringia and box: respond well to frequent light trimming.
Shape it slightly wider at the base.
Hedges that are wider at the top than the bottom shade out their own lower branches, which then thin and go bare. Cutting the sides with a slight inward slope so the base is a little wider than the top lets light reach the whole face of the hedge.
Before you start.
Check the hedge for nesting birds in spring, look for powerlines and fences along the hedge line, and plan where the clippings will go. Tall hedges that need ladders near wires, or tree work, are jobs for a qualified arborist.
Want a hand with it? See hedge trimming & shrub pruning in Wallan, Beveridge and Kalkallo.
Also worth reading: How to get an accurate garden maintenance quote, Regular gardener or one-off tidy-up: which makes more sense?.