Flaxes (Phormium tenax and Phormium cookianum) and cabbage trees (Cordyline australis) are two of New Zealand's toughest native plants — and both can handle clay soil better than most. The key is giving them a well-prepared planting hole and good drainage from day one, so their roots establish without sitting in waterlogged ground.
Understanding Your Clay Soil
Heavy clay holds moisture and nutrients well, but it drains slowly and can become compacted and airless — conditions that cause root rot in many plants. The good news is that both flaxes and cabbage trees are naturally found in damp, low-lying areas across Aotearoa, so they're well suited to clay as long as water isn't permanently pooled around the crown.
Before you plant, check your drainage by digging a hole about 30 cm deep, filling it with water, and watching how quickly it drains. If water is still sitting there after an hour or two, you'll need to take extra steps to improve drainage.
What You'll Need
- Spade and garden fork
- Compost (well-rotted)
- Coarse grit or pumice (available at most garden centres)
- Slow-release native plant fertiliser
- Mulch (bark chip or pea straw)
- Watering can or hose
Step-by-Step Planting Guide
1. Choose the Right Spot
Both plants prefer full sun to partial shade. Avoid spots where water visibly pools after rain for extended periods. A gentle slope or raised area is ideal in a clay-heavy garden.
2. Dig a Generous Hole
Dig a hole at least twice the width and the same depth as the root ball. This is more important in clay than in any other soil type — the loosened soil around the edges gives roots somewhere to push into without hitting a wall of compacted clay.
Use a garden fork to break up and roughen the sides and base of the hole. This prevents a "bathtub effect" where water collects inside a smooth-sided hole.
3. Improve the Backfill
Mix the excavated clay with:
- Up to 30% compost to improve structure and microbial activity
- A handful or two of coarse grit or pumice to open up the soil and aid drainage
Avoid adding too much compost — a very rich mix in a clay hole can actually trap moisture and encourage the roots to stay put rather than spreading outward.
4. Plant at the Right Height
This is critical in clay. Plant slightly proud — position the root ball so the top sits 2–3 cm above the surrounding soil level. This small mound helps water run away from the crown rather than pooling around it. Backfill with your improved mix, firming gently as you go to remove air pockets.
5. Water In Well
Give the plant a thorough soak immediately after planting. This settles the soil around the roots and makes good contact. In clay, you don't need to water as frequently as in sandy soils, but do keep an eye on new plantings through their first summer — clay can crack and dry out surprisingly fast in a hot Kiwi summer, pulling away from roots.
6. Mulch Generously
Apply a 5–8 cm layer of bark chip or pea straw around the plant, keeping it clear of the crown by about 10 cm. Mulch moderates soil temperature, retains moisture, and gradually improves clay structure as it breaks down.
Ongoing Care
| Task | Frequency | Notes |
|---|---|---|
| Watering | Weekly (first summer) | Reduce once established |
| Fertilising | Once a year (spring) | Use a slow-release native formula |
| Mulch top-up | Annually | Keep away from the crown |
| Remove dead leaves | As needed | Flaxes: pull downward; cabbage trees: trim |
Both plants are largely self-sufficient once established — typically after 12–18 months. Flaxes may need dividing every few years if they become very large or congested.
Dealing with Persistent Waterlogging
If your drainage test showed very slow drainage, consider these options before planting:
- Raise the bed by mounding improved soil 20–30 cm above the existing ground level
- Install a simple French drain — a gravel-filled trench that redirects water away from the planting area
- Choose Phormium tenax (harakeke) over Phormium cookianum if the site is genuinely wet — it's the more water-tolerant of the two flax species
For advice on native plants suited to your specific region and soil, BRANZ and local council resources are useful starting points, as conditions vary from Northland to Southland.
When to Call a Professional
Most homeowners can handle planting natives themselves, but there are times when a qualified landscaper is worth every cent:
- If your property has serious drainage issues affecting a large area, a landscaper or drainage specialist can design a proper solution
- If you're planning a large-scale native planting or garden redesign, a professional can advise on plant spacing, species selection, and long-term maintenance
- If you're working near retaining walls, boundary fences, or underground services, get professional advice before digging
You can find experienced local landscapers on tradedirectory.co.nz who know your region's soil conditions and can recommend the best approach for your section.
Licensed Building Practitioner (LBP)
