Are they Calabrese or Purple Sprouting Broccoli, and if the latter are they the summer or winter kind ? Calabrese is more tender than psb so you need to watch for frosts. psb is hardy and can be sown outside, although it does better protected from insect attack by being started off in modules. The summer version of psb (santee etc) needs to get going quickly, the winter one should be sown in about May. psb needs 2' (65cm) between plants, but calabrese can be planted closer, depending on the cultivar. The seed packet should tell you. Once you have planted them outside, protect from bird attack and from cabbage root fly.