Dahlias flowering before they have well developed roots and foliage often form flowers with hard green centres, instead of perfect blooms. When they are fully established and growing well, flowering will return to normal.
Pinching out the shoot tips to encourage bushy growth before plants are properly established after planting can make the condition worse. Stress factors, due to planting out too early, or insufficient soil nutrients, may also be involved.
In an emergency, water the plants thoroughly and consider applying a foliar feed such as Miracle-Gro or Phostrogen applied to the foliage according to the instructions on the packet. Ideally do this in dull weather to limit the chances of leaf scorch. Foliar feeding gets nutrients into the plant even though the roots are not fully developed.
Delay dahlia planting until the soil is warm, which usually means the end of May in the south of England and early June in more northerly areas. This helps plants produce plenty of roots and leaves before they begin flowering in earnest and avoids balling. When planting dahlias 'puddle in' with plenty of water and, if the weather is cold, protect with a layer of fleece until they are growing strongly.
Adding garden compost or other organic matter at about one bucketful per sq m/ydd and a general-purpose fertiliser at 70g per sq m (2oz per sq yd) to the ground before planting also enhances leaf and root growth. Dahlias will also benefit from an 7.5cm (3in) thick mulch of organic matter.