Dividing an overgrown ornamental grass clump is one of the most rewarding garden jobs you can do — it rejuvenates tired plants, stops them dying out in the centre, and gives you free new plants to spread around the garden. Most ornamental grasses benefit from division every three to five years, and the process is straightforward with the right tools and timing.
Why Ornamental Grasses Need Dividing
Over time, ornamental grass clumps grow outward from the centre, and the middle section becomes woody, dead, or sparse — a tell-tale sign called "doughnutting". Left too long, the whole clump can decline. Dividing the plant:
- Restores vigorous new growth
- Improves the plant's overall appearance
- Prevents the clump from swamping neighbouring plants
- Gives you extra divisions to replant or share
Common ornamental grasses in NZ gardens that benefit from regular division include Miscanthus, Pennisetum, Carex, Libertia, Phormium (harakeke/flax), and Cortaderia (pampas — though note that pampas grass is an invasive weed in many NZ regions and should not be replanted or spread).
Best Time to Divide in New Zealand
Timing is important. The general rule is:
- Warm-season grasses (e.g. Miscanthus, Pennisetum): Divide in late winter to early spring (August–September in NZ) just as new growth begins to emerge.
- Cool-season grasses (e.g. Carex, Libertia): Divide in autumn or early spring when temperatures are mild.
Avoid dividing in the heat of summer or during drought, as the divisions will struggle to establish. NZ's variable climate means timing can shift — in colder southern regions, wait until the risk of heavy frost has passed.
Tools You'll Need
- Sharp spade or garden fork
- Pruning saw or old handsaw (for large, tough clumps)
- Secateurs or hedge shears
- Wheelbarrow
- Compost or slow-release fertiliser
- Watering can or hose
Step-by-Step: How to Divide an Ornamental Grass
Step 1 — Cut Back the Foliage
Before you dig, cut the clump back to about 15–20 cm above ground level. This makes the plant much easier to handle and reduces moisture loss from the divisions. Use hedge shears or secateurs. Tie the foliage into a bundle first if it's very large — this keeps the mess manageable.
Step 2 — Dig Out the Clump
Using a sharp spade, dig around the outside of the clump at a generous distance (30–40 cm from the centre) to avoid cutting too many roots. Work the spade underneath and lever the root ball free. For large clumps, a garden fork helps loosen the soil first. This can be hard work — don't be afraid to use your body weight on the spade.
Step 3 — Divide the Root Ball
Once the clump is out, you need to split it into sections. Depending on size:
- Small to medium clumps: Drive two garden forks back-to-back into the centre and lever apart — the classic fork-and-fork method.
- Large or very tough clumps: Use a sharp spade, an old handsaw, or even a pruning saw to cut straight through the root ball. Don't be squeamish — a clean cut is better than tearing.
Aim for divisions that are roughly 15–20 cm across with healthy roots and several growing shoots. Discard the dead, woody centre sections — these won't regenerate well.
Step 4 — Prepare the Planting Site
Whether you're replanting in the same spot or a new location, prepare the soil well:
- Remove weeds thoroughly
- Dig in a generous amount of compost to improve drainage and fertility
- If your soil is heavy clay (common in many NZ regions), consider adding grit or pumice to improve drainage — ornamental grasses generally dislike waterlogged roots
- A handful of slow-release fertiliser worked into the base of the hole will give divisions a good start
Step 5 — Replant the Divisions
Plant each division at the same depth it was growing previously — planting too deep can cause rot. Firm the soil around the roots well to eliminate air pockets, then water in thoroughly. Space divisions according to the mature size of the species; as a general guide, allow at least 50–80 cm between medium-sized grasses.
Step 6 — Aftercare
- Water regularly for the first four to six weeks until established, especially during dry spells
- Mulch around (not over) the crown with bark or compost to retain moisture and suppress weeds
- Hold off on heavy fertilising until you see active new growth
- Expect the divisions to look a little sorry for themselves initially — this is normal
Dealing With Invasive Species
Before dividing and spreading any ornamental grass, check whether it is considered invasive in your area. Pampas grass (Cortaderia selloana) is a National Pest Plant Accord species in NZ and must not be sold, propagated, or distributed. Check with your local council or the relevant regional council for regional rules, and consult your regional council's pest plant lists.
When to Call a Professional
Most ornamental grass division is a satisfying DIY job, but there are times when a professional landscaper is worth calling:
- Very large established clumps (e.g. mature Miscanthus or large Phormium) can be extremely heavy and physically demanding to remove safely
- If you're unsure whether a plant is an invasive species
- When replanting involves significant earthworks, retaining walls, or drainage changes that may require a building consent — always check with your local council first
- If you'd simply like the job done efficiently and correctly
You can find qualified local landscapers on tradedirectory.co.nz who can advise on plant selection, soil preparation, and garden design.
Licensed Building Practitioner (LBP)
