In the past I’ve grown mountains of dahlia cultivars but over the last two years I’ve become increasingly interested in growing them primarily from seed. I’ll still grow some cultivars I’m sure, but this way I grow unique plants that wildlife love, because insects can only access single flowers over the heavily petaled doubles.
1) Sow seeds individually into modules
Dahlia seeds tend to germinate well and so I only put one seed, about 1cm deep, per tray module or spaced a few centimetres apart in regular seed trays. Use only peat free compost for environmental reasons, you don’t need seed compost as the plants are so strong. Keep inside in the warmth until frosts have stopped (May or June) because one cold frosty night will kill em.
2) Plant in larger pots when plants have 6 leaves
Use a 9cm pot to start with or go straight into a 2 litre pot, which I often do to save time. Grow them on somewhere warm, although if there are no frosts you can start moving them outside during the day to toughen them up to the elements.
3) Plant out when about 30cm tall
When all frosts have stopped, usually sometime in June for most of the country, the dahlias can go into their final positions in pots or borders. Keep them well fed with an organic tomato fertiliser fortnightly. I use seaweed based or homemade comfrey feed. As with all dahlias, they will form tubers that can be left in the ground over winter in most areas or dug up and stored.
4) Save seeds from your plants for next year
Toward the end of the summer, going into autumn, leave some of the flowers on the plants to form seed heads. These look like buds but are pointier rather than rounded. Eventually these will start to dry out, looking rounded again (above), at which point the seeds can be collected and stored (see the seed packets I designed for this in my shop). It’s worth saving seeds to grow more next year and to see what new colours and shapes the new plants are.