Dahlias are simple to grow from seed. You can buy packets of dahlia seed, like these from True Leaf Market, that will grow into a specific variety, or you can harvest your own dahlia seed to grow into a new unique type of dahlia. Dahlia seeds are sown into seed compost in spring and should germinate quickly, though placing the containers on a heat mat will help speed up germination. The time to transplant seedlings out into the garden comes when the risk of frosts has passed and the young plants are large enough to handle.