Growing Guide

When to Plant Dahlias in NZ + How to Plant Dahlia Tubers

Dahlias are among the most rewarding summer-flowering plants for New Zealand gardens. They flower for months from late spring until late autumn and there's a colour and flower form to suit every taste. This guide covers when to plant dahlias NZ, how to plant dahlia tubers, and the key care steps for strong growth and abundant flowering.

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When to Plant Dahlias NZ

Dahlias are frost tender. In New Zealand, dahlia tubers should be planted in spring once all danger of frost has passed and soil temperatures have begun to warm.

Generally speaking this is as follows, but there are a lot of variations within these broad areas - you know your area better than we do and when the last frosts are, and that is your main guide.

Warmer North Island areas: late September through October
Cooler regions and South Island: early October to end November
High frost zones: wait until the risk of late frosts has fully passed

Tech Talk - Dahlia Imports

For many years dahlias were imported into New Zealand from Holland and sold as early as June each year. Although few dahlias are imported now due to changed import requirements, the import of large quantities of dahlias over many years has created a false idea of when they should be planted.

Imported dahlias shoot quickly as they have been held back for six months in cold storage since they were harvested in Holland in November-December. When they arrive in retail stores in New Zealand in June, the temperatures in the store are much warmer than the approx 4C long-term storage temperature. They think it is spring and begin to shoot. This gives the wrong impression that they can or should be planted as early as August.

NZ Bulb�s locally-grown tubers will generally not start shooting until the normal time of mid October and should be planted at that time.

How to Plant Dahlia Tubers

Getting the basics right will make sure you have success.

Step 1 � Choose the Right Position

Dahlias require:

Full sun (minimum 6 hours daily)
Free-draining soil
Protection from strong winds (for taller varieties)

Poor drainage and a lack of strong light are common causes of failure. If your soil is heavy or drains poorly, plant in raised beds and improve the soil structure by working in generous amounts of good compost.

Step 2 � Prepare the Soil

Work compost in as you dig the soil over to a depth of at least 20cm, and incorporate a small handful of fertiliser at the same time. Dahlias are heavy feeders and respond well to fertile soil with good structure.

When fertilising, avoid high nitrogen fertilisers (including animal manure) at planting time, as this encourages rapid, weak leafy growth instead of flowers. Always use a balanced fertiliser with matching N:P:K levels, or a mix with N lower than the others. For example 10:10:10 or 6:10:10.

Step 3 � Planting Depth and Spacing

Correct depth is important

Dig a hole 10�15cm deep
Lie the tuber in the hole with the tubers horizontal and the crown of the tuber 2-3cm below the surface.
Fill in with soil and water in lightly

Plant spacing depends on how big the plant will grow, each variety is different. All our packs give the mature height of the plant and the suggested plant spacing.

Dwarf types: 30�40cm apart
Medium varieties: 40�50cm apart
Tall decorative or cactus forms: 50�60cm apart

Ongoing Care

Once established, dahlias are relatively low maintenance.

Feeding

Use a balanced fertiliser with N:P:K levels about 10:10:10 while the plants are actively growing before flowering. Once flowering starts, use a fertiliser lower in nitrogen and higher in potassium and phosphorus to encourage flowering and tuber growth, around 6:10:10.

Don't overdo the fertiliser use, it's strong stuff and a teaspoon per plant every couple of weeks is sufficient.

Watering

Water deeply during dry spells. Consistent moisture supports larger blooms, but never allow soil to remain waterlogged.

Deadheading

Removing spent flowers encourages continuous blooming and prevents energy being diverted into seed production.

Staking

Taller varieties should be staked at planting time to avoid root disturbance or tuber damage if done later.

Spraying

Spraying to control insects should only be done before flowering so bees are not affected.

Understanding Dahlia Growth Cycles

Dahlias are very clever plants (can we actually say that about a plant?) because they change their growth according to day length.

During shorter days (early and late season), plant energy is directed into tuber development and top growth, but not flower production. Small tubers can form before the end of October on cuttings taken in August or September from heated propagation beds. The main tuber production and growth however is from late March onwards as the days shorten quickly but the summer warmth remains.
In the longer days top growth and flowering are dominant. Roots continue to be formed and grow, but do not swell into storage tubers until the day length shortens again from late March.

This natural cycle ensures tubers are plump and full of energy by the end of the growing season so they're ready for dormancy and strong growth the following spring.

Tech Talk - Why are Dahlias so diverse?

Most plants and mammals are genetically diploid. That is, they have two copies of each chromosome, one inherited from each parent. During reproduction, those chromosomes pair and produce offspring with traits passed along from the parents in predictable and visible ways.

Dahlias are much more complex. Wild species of Dahlia, native to Mexico and Central America, exist with two, four and eight sets of chromosomes.

Dahlias were introduced to Europe in the late 1700s and more than two centuries of breeding and selection have taken place. As early breeders selected plants with larger flowers, stronger stems, improved colours and strong growth, they were at the same time unknowingly selecting the plants with eight sets of chromosomes because these desirable features were mainly found in those plants due to their greater genetic diversity. We call these plants with eight sets of chromosomes 'octoploid' and it is these dahlias that gradually came to be the dominant forms in cultivation.

Today, virtually all decorative, cactus, ball, pompon, waterlily, and other garden dahlias are octoploid. Having eight sets of chromosomes gives a massive diversity in flower shapes and colours because each plant carries multiple copies of every chromosome. During reproduction, chromosomes can pair in different combinations, producing seedlings with a wide range of flower colours, petal shapes, heights and flowering times, often looking nothing like the parent plants.

This can be very challenging for plant breeders as it is difficult to guarantee the passing on of the parent traits that you want, but on the other hand the chances of producing unique cultivars is high. Large numbers of seedlings need to be grown to find ones worth continuing with.

This is why commercial dahlia propagation is done by dividing tubers or taking cuttings. These methods mean that the new plants are exactly the same as the original plant, keeping the cultivar stable from one generation to the next. Seed propagation is mainly used for breeding or for producing mixed-colour bedding dahlias where genetic variation is the goal.

Lifting and Storing Dahlia Tubers

Most parts of New Zealand experience wet conditions during winter and spring and dahlia tubers can rot if left sitting in this cold, wet soil.

We recommend lifting your tubers in all areas of New Zealand. As well as preventing losses which may occur if left in the ground, it gives you the opportunity to divide them and get more dahlias for your garden.

Dahlia tubers will be ready to harvest once the first frosts knock the dahlia foliage down. Even if you live in a frost-free area, dahlias will start to yellow off and drop leaves by early June and will be ready to harvest.

Follow these easy steps for harvesting:

Cut stems back to 7-10cm above ground level.
Using a fork, lift the tuber clump carefully, the tuber necks are easily broken.
Rinse and dry the tubers at room temperature.
You can divide large clumps at this time, ensuring that each piece retains part of the crown, where the growing points will emerge in late spring. Without a piece of the crown containing growing points, the tuber will be 'blind' and will never produce top growth.
Store until spring in a cool place in a slightly damp mix to prevent them drying out.
Check every week in storage to make sure there that is no rot developing and they are not drying out

Choosing the Right Dahlia for Your Garden

Dahlias are classified primarily by flower shape. Popular forms include:

Cactus dahlias � spiky, dramatic blooms
Decorative dahlias � broad petals, varying in size from 8cm to giant 'dinner plate' size
Ball and pompon types � perfectly rounded blooms
Waterlily dahlias � flatter, elegant forms, just like a water lily.
Single ands collerette dahlias � excellent for pollinators and landscaping
Dwarf varieties � ideal for borders and containers

With modern breeding, there is a colour, flower and plant form for every location - compact patios, formal garden beds, cottage plantings and large-scale displays.

Selecting different heights and flower forms allows extended visual impact throughout summer and autumn.

Common Questions

When to plant dahlias?

Plant in spring, once frost risk has passed and soil is warming.

When do dahlias flower in NZ?

In New Zealand, early planted dahlias typically begin flowering from early summer, as early as the beginning of December. They will flower through summer and into late autumn until the first frosts. Flowering time depends on planting time, local climate, and variety, but well-established plants can bloom continuously for five months or more.

How to plant dahlia tubers?

Plant in a 10�15cm hole with the crown (top) 2-3cm below the surface. Full sun and good drainage are needed. Don't over water.

How to grow dahlias NZ?

Provide sun, drainage, feeding during growth, staking for tall types, and lift tubers in wet winter regions to reduce rot risk.

The Dahlia Gall Epidemic... Or Not

Crown gall is a disease that is caused by the bacteria Agrobacterium tumefaciens, which occurs naturally world-wide, including in New Zealand. It can affect dicotyledonous plants in more than 60 different plant families from stone fruit and pome trees to grapes, tomatoes, cane fruits and several species of ornamental plants � and dahlias.

Crown gall occurs only in the crown region at the interface between the underground tubers and the old aerial shoots from the previous season. The bacterium causes the buds that occur in that area to proliferate and become cauliflower like. Such buds are unable to make normal shoots for the new season.

If you were to believe social media you would think gall is sweeping across the nation like Covid-19, but this is not the case. We grow over 100,000 dahlia plants annually and our incidence of gall is zero.

We're perplexed by the continual discussions about gall on social media dahlia forums where judgement is passed on posted images by people with no plant pathology experience at all. This has been causing unnecessary concern about a plant disease that is in fact quite rare, and over the years has resulted in a lot of perfectly good tubers being discarded.

When unwell, it's better to go to the doctor instead of seeking a diagnosis by posting on social media. The same goes for plants. With plant diseases, expert advice is often needed as many diseases can have similar symptoms and these can mislead even the best growers.

Therefore, with the assistance of renown dahlia breeder and trained plant pathologist, Dr Keith Hammett - Plant Breeder, we drafted a whole article in 2021 to answer your questions about gall and show what gall is and what it isn't. Click here to read the full article.

Final Advice

Success with dahlias comes down to three essentials:

Warm soil at planting
Good drainage
Full sunlight

With these conditions met, you'll have months of vibrant garden colour.

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