How to grow dahlia in New Zealand

When to plant dahlias in New Zealand

In warmer regions, you can plant as early as August once the threat of frosts has passed. However, a general rule of thumb for many is to plant dahlias around Labour Day each year.

Best times to plant dahlias in New Zealand:

• Plant dahlias – September to October
• Pick dahlias – December to April

Dahlia varieties to grow

There are hundreds of dahlia varieties to grow in New Zealand, with blooms that vary in diameter between 60-250mm.

The flowers come in many shapes, including anemone, ball, cactus, fimbriated, orchid, decorative, collerette, single and waterlily, pompom, and dwarf.

When choosing your dahlia, make sure to check their spacing requirements. Many varieties need a lot of room to grow, and you’ll want to ensure your garden can accommodate them. You’ll also need to stake the plants to prevent the flowers heads falling over as they grow.

Planting dahlias in New Zealand

Many dahlia varieties need plenty of space to grow – it’s best to check any instructions on spacing for the variety you have chosen.

These flowers can grow in almost any soil, but a well-draining, loamy soil is best.

To plant your dahlia in the garden:

1. Choose a well-draining spot that offers full sunlight or very little shade, and is protected from the wind.
2. Apply Scotts Sheep Pellets to the area a few weeks beforehand to improve the quality of your soil for optimum flowering. You may also want to use a Scotts Osmocote Premium Potting Mix to give your tubers the best start.
3. Planting a large variety? Apply stakes to the ground to support the plant once it starts growing.
4. For dahlia tubers, dig a hole 5-10cm deep and place your dahlia tuber inside with the main growth point or ‘eye’ facing upwards. For transplanting a small dahlia, the hole should be twice the width and depth of the pot it’s currently in.
5. If you’re planting more than one, space your dahlia plants between 50-100cm apart.
6. Water your freshly planted dahlia.

Note: To grow a bushier dahlia with more flowers, pinch back new growth tips or the main stem once it grows to around 25cm. This will encourage more branching.

Growing dahlias in a pot

Prefer to grow your flowers in pots? Dahlias do well as a potted plant – just make sure they have enough room.

To grow your dahlia in a pot:

• Choose a pot at least 30L in size that drains well and place it in an area that gets at least four hours of full sun daily.
• Fill the pot two-thirds of the way with Scotts Osmocote Premium Potting Mix or a specialised potting mix for flowers, like Scotts Osmocote Rose, Gardenia, Azalea & Camellia Mix.
• Place your dahlia tuber or small plant in the centre of the pot with the eye facing upwards and fill the rest of the pot with soil.
• Water your tuber, making sure the soil is moist but not drenched.

Dahlia care: fertiliser, pruning and protection

While dahlia plants have fairly simple needs, you can fertilise the soil to get the best results from your summer blooms. You’ll also want to remove dead flower heads during blooming to prevent disease.

What to feed your dahlia

For rich soil that supports healthy growth and flowering, apply Scotts Osmocote Controlled Release Fertiliser: Roses, Gardenias, Azaleas & Camellias. This fertiliser is particularly helpful if you don’t have time for regular plant care because it’s easy to apply, designed for flowers and doesn’t result in surge growth.

Water the roots of your dahlia thoroughly to keep them moist, but don’t let them sit in waterlogged soil. Try to avoid wetting the leaves to reduce the risk of mildew.

Common pests and diseases for dahlia

Protect your dahlia by keeping an eye out for:

• Slugs and snails are a common threat to dahlia plants, especially on new growth early in the season. Protect your dahlia by applying Scotts Quash Slug and Snail Killer around the base of the plant.
• Powdery mildew: a fungal disease that appears as a light-coloured mould on leaves.
• Insects that feed or suck on plant leaves and cells, such as aphids, spider mites and caterpillars can be a problem for dahlia plants.
• Stem rot: This can be an issue if you live in an area that gets lots of rain, or your soil doesn’t drain well. Watch carefully for darkening stems and white rings in the soil, and be careful not to over-water your dahlias.
• Botrytis cinerea or grey mould is also a risk to dahlias, as the fungus thrives in damp conditions. Look out for the fuzzy grey mould on flowers and leaves and remove affected plant parts immediately to stop the spread.

Pruning and trimming your dahlia plants

During the season, it’s a good idea to remove dead dahlia flower heads (also called ‘deadheading’ or ‘de-heading’). This will extend your flowering season.

As the dahlia is a perennial flower, the plant will die down completely after the season ends. Wait for this process to finish before trimming – usually in late autumn and winter.

During autumn, trim the dahlia back to about 15cm above ground level. This makes them easy to ‘lift’ from the ground if you decide to store your dahlia tubers over winter.

Storing your dahlia tubers for next season

While dahlia tubers can survive in the ground until the next season, they can run the risk of rot if you live in an area susceptible to frost or consistently wet weather.

If you decide to store your tubers, you should:

1. Carefully dig around the clump of the tubers and lift them gently (ideally with a fork).
2. Cut the remaining stems off the dahlia.
3. Shake off and rinse as much soil from the tubers as possible.
4. Place or wrap your dahlias in a dry media such as dry compost, newspaper, or vermiculite.
5. Store them in a dry place over winter, such as a shed or garage.

If you plan to leave your tubers in the ground and have relatively dry winters, a thick layer of mulch such as Scotts Osmocote Mulch and Feed can offer protection against frosts in winter.

Picking dahlia flowers

Get the best out of your dahlias by:

• Picking them only when the flowers are fully open – dahlias don’t continue blooming once picked.
• Cutting the stem at the node using sharp, clean tools.
• Putting the flowers in water immediately.

You can also trim lower leaves and replace the water every few days to preserve your dahlia flower for longer.

Tips for growing dahlias in New Zealand

• Dahlia tubers can be stored out of the ground to protect them for another season. Lift them gently from the ground with a fork, wash the soil off and store in a dry place over winter.
• If you want large, bushy dahlia plants, pinch out the new growth at the start of the season to encourage the plant to grow outwards.
• Removing dead flower heads after they’ve bloomed will prolong the flowering season.
• If you’re growing a tall dahlia variety, use stakes to support the stems and plant them in an area protected from wind.

Frequently asked questions

When should you plant dahlias in New Zealand?

The best time to grow dahlias in New Zealand is early to mid-spring (September and October). However, if you live in a more tropical climate, you could start as early as August after the last frost passes.

Where should you plant dahlias?

Dahlias like a lot of sun, so make sure your planting spot gets at least four hours of full sun per day. Ideally, your soil should drain well too.

How deep should you plant dahlia tubers?

Check any instructions for your chosen variety when deciding how deep to plant dahlia tubers. A general rule of thumb is 5-10cm deep, with the eye of the tuber facing up to support its growth.