Wait until the plants have settled in before fertilizing. Even moving from a container to the ground is stressful and produces some degree of transplant shock and an immediate fertilization only increases that. You never want to add any fertilizer to a planting hole - that's very old school and an outdated practice that has found to be counterproductive.

FWIW, there is minimal concern about a fertilizer - applied correctly - damaging roots. You certainly do not want to mix into the planting hole anyway, per the reasons above, so a dry or granular fert worked into the soil a couple of weeks after planting will be fine. And pick one with a higher second and third number......generally the formulation recommended for dahlias is a 1-2-2 or a 1-2-3 ratio so something like a 5-10-10 or a 5-10-15.

What about bone meal? Would that be useful in the planting hole?

Not really. What bone meal offers is usually present in most soils in adequate quantities already. The myth of bone meal. And it is really unnecessary (and unadvised) to add anything to the planting hole when planting something new.

OK...got it...thanks!