Also Bonsai trees are intentionally allowed to become root-bound in their containers, and the roots too, are pruned.
He fertilizes with fish emulsion as much as possible without burning, and sets out plants as soon as they're gaining momentum, before they get root-bound.
Repot root-bound houseplants and prune any that are looking very ragged.
Otherwise, wait until late March: A smaller tomato plant with growth momentum is better than a large, root-bound plant.