How to Grow Dahlias: A Practical Guide from Seed to Flowers
Dahlias are wonderful plants for bringing plenty of colour into the garden from summer right through to the first frosts. They come in an enormous range of flower shapes, sizes and colours, and growing them from seed is an enjoyable and inexpensive way to fill your borders and containers.
One of the things I particularly like about seed-grown dahlias is that you don't always know exactly what you're going to get. Unlike growing from named tubers, where you know what the flowers should look like, seedlings can produce some lovely variations in colour and form.
They're also surprisingly easy to grow. Start them somewhere warm in spring, give the young plants plenty of light and wait until the danger of frost has passed before putting them outside.
Dahlias are tender plants, but once they're established and the weather warms up, they grow quickly and can flower for months. Keep them watered, feed them when needed and remove faded flowers regularly to keep the display going.
When to Sow
Dahlia seeds are best sown indoors from February to April.
I generally prefer sowing during March or early April. The days are getting longer, seedlings have better natural light and there's less chance of ending up with large plants that need looking after indoors for weeks before it's warm enough to plant them outside.
If you're sowing in February, a heated propagator and plenty of light will help get the seedlings off to a good start.
Dahlias are frost tender, so there's no advantage in rushing them outside. In many parts of the UK, planting out will be from late May or early June, depending on the weather and your local conditions.
How to Sow Dahlia Seeds
Fill a seed tray or small pots with good-quality peat-free seed compost and gently firm the surface.
Sow the seeds thinly and cover with around 0.5cm of fine compost.
Water gently and place the tray somewhere warm. A temperature around 18–21°C is suitable for germination.
A heated propagator is useful, although a warm windowsill can work perfectly well.
Keep the compost moist without leaving it soaking wet.
Dahlia seeds usually germinate within 7–14 days, although some may take a little longer.
Once seedlings appear, remove any propagator cover and give them plenty of light. This helps prevent them becoming tall and weak.
You can then grow them on somewhere slightly cooler, provided the temperature remains safely above freezing.
Growing Seedlings On
Once the seedlings have developed a couple of sets of true leaves and are large enough to handle, move them into individual pots.
Handle them carefully, holding the seedlings by their leaves rather than pulling on the delicate stems.
Use good-quality multipurpose compost and water after potting.
Keep the young plants somewhere bright and frost-free while they develop.
Dahlias grow quite quickly once established, so you may need to move them into slightly larger pots if their roots begin filling the original containers.
There's no need to feed heavily at this stage. Fresh compost should provide enough nutrients for the young plants.
The main thing is to produce sturdy plants ready for planting outside once the weather improves.