when is the best time to plant out sweet peas

Have you pinched out the growing tips? Also, have you hardened them off? Otherwise, yes, it's fine to plant them out. Make sure the soil has been well fed, they are greedy plants.

hi , spritz has answered your question , good man. like he says nip off the tips so they get bushier , i always put mine under glass and plant them out in may.
cheers
doss

I only found out the literally the other day, that when you put them next to a cane, you must lay the main stem to 1 side and train a side shoot to grow up the cane. Otherwise, the plant will remain "blind" (ie, not flower!)