One of the questions I'm asked most often is:

"What's wrong with my dahlia?"

The answer is... it depends.

Every garden is different and what becomes a major headache for one grower might never appear in another. Here in the Dandenong Ranges, surrounded by cool temperate rainforest, I rarely see spider mites.. but powdery mildew is almost guaranteed by the end of the season. Harlequin bugs? Last season they absolutely chose violence. 🫠

The good news is that most problems can either be prevented or managed if you catch them early.

Before we jump into individual pests and diseases though, let's start with the single biggest thing you can do to grow healthy dahlias.

Healthy soil grows healthy plants

If I could only give one piece of advice it wouldn't be about fertiliser or sprays.

It would be to invest in your soil.

Healthy, biologically active soil grows stronger plants and stronger plants are naturally better equipped to handle pests, diseases and whatever else the season throws at them.

Before planting, I like to incorporate plenty of organic matter into the soil.

Some of my favourite options include:

• Homemade compost
• Well-aged chicken manure
• Clyde Compost (a fantastic commercial option if you can source it)

Building healthy soil isn't something that happens overnight. It's something you improve season after season and your dahlias (and garden in general) will thank you for it.

Good drainage is essential

Dahlias enjoy consistent moisture.

They do not enjoy sitting in water.

If your soil remains waterlogged for long periods, tubers become much more susceptible to rotting.

A simple drainage test before planting can tell you a lot.

1. Dig a hole approximately 30cm deep.
2. Fill it completely with water.
3. Allow it to drain.
4. Fill it a second time and see how long it takes to disappear.

As a rough guide:

• Under an hour: Very free draining (you'll likely need to water more often.)
• Around 1–3 hours: Excellent drainage for dahlias.
• More than 4–6 hours: Your soil may benefit from improving drainage before planting.

If you're gardening on heavy clay, don't despair. Regular additions of compost and organic matter can dramatically improve drainage over time. Raised beds are also a great alternative if you are in a hurry.

Here, we have quite a lot of clay in the field. Each season of cultivating I'm seeing dramatic improvements in soil texture. It's definitely a long game but it's one worth playing!

Every garden has different challenges

One of the biggest mistakes I see new growers make is assuming they'll have exactly the same problems as someone on the other side of the country.

Climate makes a huge difference.

Here in the Dandenong Ranges I spend far more time worrying about powdery mildew than spider mites.

Someone growing in inland Victoria or South Australia may have the exact opposite experience.

Your garden will teach you what its biggest challenges are.

Common pests

Slugs & Snails

Fresh dahlia shoots seem to be five-star dining for slugs and snails.

They're usually at their worst early in the season while plants are first emerging, so I like to stay on top of them before they become a problem.

Personally, I use an iron-based snail bait rather than products containing metaldehyde. I've found it works well while also being a safer option around pets and wildlife.

Like most pest management, prevention is much easier than trying to recover after they've already eaten half your new shoots!

Spider Mites

Spider mites are tiny sap-sucking pests that thrive in hot, dry conditions.

Signs include:

• Fine webbing
• Tiny pale speckles across leaves
• Leaves losing colour and vigour

They're not something I personally battle here very often but growers in warmer, drier climates often consider them one of the biggest dahlia pests.

Healthy plants, regular monitoring and early intervention are your best defence.

Community Tip: Deb's Mite Spray

One homemade spray that comes highly recommended by many Australian dahlia growers is "Deb's Mite Mix." While I haven't personally needed to use it here (spider mites aren't a major issue in my climate), it's a recipe that frequently gets recommended within the dahlia community with many growers reporting good results.

The concentrate consists of:

• ⅓ cup rice bran oil (or soybean oil)
• ⅓ cup vodka or rubbing alcohol
• 1 teaspoon citric acid
• 1 tablespoon Sodium Lauryl Sulphate (SLS) or a tablespoon of dishwashing liquid as an alternative
• Top up with water to make the concentrate

To use, add approximately 25 mL of concentrate to 1 litre of water, shake well and spray both the upper and lower surfaces of affected leaves, paying particular attention to the undersides where spider mites tend to live.

For best results, apply in the early morning or evening and avoid spraying during hot weather.

As with any homemade spray, it's always a good idea to test it on a small section of your least favourite plant first and wait a few days to check for any adverse effects before treating your entire patch.

 

Beneficial insects

Whenever possible, I'm a big fan of letting nature do some of the work.

Beneficial insects can be incredibly effective at keeping pest populations under control before they become a real problem.

If you're unsure what you're dealing with or which beneficial insect is best suited to your situation, I highly recommend Bugs for Bugs. They're incredibly knowledgeable and can recommend biological controls specific to the pest you're battling.

Not every problem needs a chemical solution, sometimes it just needs the right predator 🕷️

Powdery Mildew

If you grow dahlias long enough, chances are you'll meet powdery mildew.

It's one of the most common fungal diseases and appears as a white powdery coating across leaves.

Here in the Dandenong Ranges it's probably my biggest seasonal challenge.

Things that have helped me include:

• Removing lower leaves to improve airflow.
• Giving plants plenty of space when planting (I don't always remember to adhere to this but we all have our weaknesses 🙈)
• Spraying with a diluted milk solution once symptoms first appear.

If a plant becomes heavily affected, don't be afraid to cut it back hard.

Dahlias are remarkably resilient and will often produce fresh, healthy growth rather than allowing the disease to continue spreading throughout your patch.

 

Dahlia Virus

Virus is probably the topic that causes growers the most anxiety.

The first thing to remember is:

Not every funny-looking leaf is virus.

Herbicide drift, nutrient deficiencies, environmental stress and insect damage can all produce symptoms that look surprisingly similar.

That said if I strongly suspect a plant has a virus, my approach is very straightforward.

I remove it immediately.

Personally, I pull the entire plant and burn or dispose of it rather than composting it.

The reason is simple - I grow tubers for sale, so I don't want to risk maintaining potentially infected stock within my collection.

Current overseas research, particularly from the US suggests virus may be far more common than many growers realise. I'm not aware of equivalent Australian studies, but it's reasonable to assume viruses are present here too.

For home gardeners, the decision may look a little different.

If you don't intend to sell or share tubers, you may choose to quarantine a symptomatic plant away from the rest of your collection and continue growing it while monitoring symptoms. Keeping it well-fed and healthy may allow it to continue performing reasonably well.

There's no single "correct" answer here, it really depends on your goals as a grower. However if you intend to sell, I would strongly recommend the "if in doubt, throw it out" approach.

Healthy habits make healthy dahlias

Most problems are easier to prevent than fix.

Some simple habits that make a big difference include:

• Build healthy soil with plenty of organic matter.
• Plant into well-draining soil.
• Give plants good spacing and airflow.
• Check your plants regularly rather than waiting until something looks seriously wrong.
• Remove obviously diseased material promptly.
• Keep plants well fed throughout the season.
• Stay on top of pests before they become overwhelming.

Final thoughts

One of the biggest lessons dahlias have taught me is that gardening isn't about perfection.

Every grower loses plants. Every grower battles pests...

Every grower has a season where something doesn't go to plan.

The goal isn't to eliminate every problem, it's to build healthy plants that can cope with them.

Observe your garden often, learn what challenges are common in your area and don't be afraid to adjust your approach as you gain experience.

Every season teaches you something new, that's half the fun!

 

Sam 

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