All dahlias are hybrids and can be grown from seeds collected from your own flowers. There are 2 ways to do this;
1.Allow the bees to cross pollinate your flowers. This will give you a totally random result and it is highly unlikely that the flowers you grow from the seeds you collect will resemble the flower from which you collected the seeds. This can be frustrating or fun. But whichever way you look at it it’s plants for nothing. If you want specific characteristics/classifications of dahlias it will help if you only grow plants with those characteristics to breed from, e.g. only decoratives or only cactus etc.
2.Pollinate by hand. You will need to select the flowers you wish to use as parents and make sure you protect them from bee pollination by putting a paper bag over the flowers before they open, to keep the bees out. Some classifications are more difficult to collect seeds from; giants and pompons are particularly difficult but still worth a try. You should continue to dead-head the rest of the plant to encourage repeat flowering. Allow those bagged flowers to mature then decide which will be the pollinator and which the seed-bearer. As soon as the petals have fallen from your chosen flowers, cut off the pollinator and take it to the seed-bearer and mix the pollen, preferably with a soft brush, then bag-up the seed-bearer again to prevent any further pollination.
Next Step. Collecting Seed.
Whichever method you chose you must now leave the seed-bearer on the plant to mature. Towards the middle to end of October, before the weather starts to turn, cut off the seed heads and bring indoors to finish off their maturing. I cut them off with a decent length of stem and put them in a vase of water. You might want to label them so you know which variety they came from. When the water has eventually evaporated, just leave the seed heads to dry out completely. When this has happened you can then extract the seeds and store in a dry place until it is time to sow them.
Sowing Seed.
Some raisers recommend putting the seeds in the fridge for a few weeks before sowing. You might like to try both ways to see if there is any significant difference. Sow the seeds in trays of peat-free compost any time in March and put in a heated propagator or on a heated greenhouse bench. The germination rate is usually very high and starts within a week or so. As the seedlings grow, plant on in individual pots and keep free from frost.
Planting Out
Once danger of frost is over you can plant out your seedlings. I recommend planting about 12 to 18 inches apart, then if you have ones you just don’t like you can pull them out and give the others more space without leaving your beds too gappy. You would normally remove the first bud on a plant to encourage the side buds and hence more flowers early in the season, but with seedlings I would say leave that first bud to flower so you can see early on whether you like it and are going to keep it.
Next Steps.
Treat your seedlings as you would any other dahlias; watering, feeding, disbudding if you wish and caning if necessary and they should make a tuber by the end of that season. You now have the job of keeping the tubers through the winter. If you manage to do this you can then take cuttings from them in the following spring and grow them on again. Sometimes it takes a season or two before a new seedling reaches its full potential. As they are unique to you, you can name them and if you want to promote them you can try exhibiting them at shows. If you wish to register them with the RHS there are naming rules you must abide by and a form to fill in available from the RHS website.
Good luck and if you manage to raise a true blue dahlia you will likely become a millionaire. I hasn’t happened in over 200 years.