Phostrogen, or miracle gro or tomorite, or comfrey feed.
Not nettle feed or anthing high nitrogen which encourages leaves and not flowers.
thanks a lot for the prompt response,the dahlias are doing well with plenty of flower.
They do say do not feed until July.
It is July. soon be August
Mine are flowering in the ground. I added blood fish and bone to the soil before I planted. They were in pots of compost, with added fertiliser.