Posted: 25/03/2015 at 23:12
The usual way to deal with small plug plants is to plant each one in a small pot (eg 9cm) and grow them on somewhere which is frost-free but has plenty of light until they can be planted out in the garden. Suitable small plastic pots pots can be bought very cheaply from places like Wilko and the usual garden centres. The reason for using small individual pots is to minimise root disturbance. What you have done will work, but you will find the roots have all grown together when you come to transplant them, so try to cause as little damage as possible when you do that.
Snapdragons are hardy so you could actually put the tray you have made outside somewhere sheltered (eg against south or west facing house wall) but it would be best to harden them off a bit by leaving them out during the day and bringing in at night for a week or two.