Begonias don’t handle cold temperatures and frost well, so wait until at least one week after the final frost. I would give them a bit more cover with soil free peat moss compost . They like acidic conditions.
Most plants need a bit of extra watering after first being planted, in order to avoid what is known as ‘transplant shock’. Give your begonias a heavy watering, so that the soil is moist but not drenched. The flowers will need to be watered every few days from there on. Begonias don’t tolerate heavy watering so you might have to bring them in if perlonged rain is due.
Give them a feed about once a month.