Choosing a Location & Preparing Your Soil for Dahlias
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Dahlias have a reputation for being fussy. In reality, the opposite is true — get the location and soil right upfront and they'll reward you season after season. Here's how we approach it at Highland Dahlias on the Southern Highlands of NSW.
How Much Sun Do Dahlias Actually Need?
One of the most common questions we hear is: "Where should I plant my dahlias?" The honest answer is that dahlias are more adaptable than most people realise.
The ideal position is morning sun with afternoon shade. That said, dahlias will grow in full sun — it just means your soil preparation and watering need to be that bit more dialled in. Full shade is where they struggle. Without enough light, they grow leggy — tall and spindly — reaching for the sun and producing poor quality blooms.
So if you're choosing between a few spots in the garden, go for morning sun first. But don't write off a site just because it gets some afternoon sun. Work with what you have.
Drainage First — Always
Before anything else, check your drainage. This is non-negotiable.
Dahlias sitting in waterlogged soil will rot. Poor drainage is the number one reason tubers fail to establish. If water pools or drains slowly after rain, you need to address that before planting.
The good news is that once dahlias are actively growing and flowering, they can absorb a huge amount of moisture — they're thirsty plants. The difference is they need water moving through the soil, not sitting in it.
If you're building a raised bed to improve drainage, here's a critical tip that gets overlooked: break up the soil underneath before building on top of it. Laying a raised bed over compacted ground traps water exactly where you don't want it. Break the layer, then build up.
Gerard's Standard Soil Preparation
Every season at Highland Dahlias, regardless of what the soil test says, we apply what we call our maintenance prep. These are the ratios we use across our beds:
- Agricultural lime: 1 kg per 4 m²
- Gypsum: 1 kg per 2 m²
- Blood and bone: 1 kg per 10 m²
These are applied and worked into the soil before planting. It's a consistent baseline that keeps our soil in the right condition year on year.
Target pH: 6.8 to 7.2
Dahlias perform best in soil that sits between 6.8 and 7.2 pH — slightly alkaline. A little above 7.2 is fine. Once your soil hits this range, you'll notice a clear improvement in plant health and bloom quality.
If you don't know your soil pH, a basic home test kit from your local nursery will give you a starting point.
Getting a Proper Soil Test
For serious growers, we recommend a professional soil test through Eurofins (formerly Swep), based in South Australia. You post the sample to them — turnaround is 10 to 14 days and the cost is around $140–$150 including postage.
What makes their test worth it is that over the years we've worked with them to develop a dahlia-specific soil analysis. It doesn't just tell you what's in your soil — it tells you what to add to get the conditions right specifically for dahlias. That's a very different thing to a generic garden soil test.
If you're investing in a serious dahlia bed, it's money well spent.
What's Coming: A Fertiliser Built for Dahlias
We're currently working with a Queensland agronomist to develop a pre-plant fertiliser and a sub-surface side-dressing fertiliser designed specifically for dahlias. These will be formulated to work alongside the UGF products and seaweed feeds we already use. We'll share more on this when it's ready — it's been a long time coming.
Quick Checklist Before You Plant
- Morning sun position preferred; full shade is a problem, full sun is manageable
- Drainage confirmed — no pooling after rain
- If building a raised bed, break the subsoil underneath first
- Maintenance prep applied: lime, gypsum, blood and bone at the right ratios
- Soil pH sitting between 6.8 and 7.2
- Consider a professional soil test through Eurofins for a full picture
Get these foundations right and you'll spend far less time troubleshooting problems later in the season. Dahlias reward preparation more than almost any other plant we know.
Gerard Oldfield grows dahlias commercially at Highland Dahlias in the Southern Highlands of NSW. He is a regular contributor to the Dahlia Society of NSW and ACT monthly meetings.