Yes, some of the bigger seeds can be sown straight into your border but unless you sow them fairly thinly you may find you have to pull all the surplus plants out. The reason for sowing them in trays is either to bring them on early or because often the seed is so fine that the seedlings are rather fragile and need careful handling before they're robust enough to go in the border. Also, they will have a better start in special compost grown under sheltered conditions. But bedding plants like antirhynums (snapdragons) for example can quite happily be sown direct into the soil.
Seeds such as marigolds, cosmos, love in a mist etc which don't need heat to start off can be sprinkled but those like petunias, though they would eventually come through and flower, are better sown in a greenhouse or above a radiator. Make sure they are sown thinly then you don't have to thin them out.
hi Baby Jane (me again!) as well as those suggested by lafrancaise one of the best for just throwing down on the ground in a sunny spot is candytuft, they never fail unless it's a very wet summer.
Buy a packet of candytuft 'Fairy' mixture, sprinkle some of it on the ground, & water gently. A couple of weeks later sprinkle some more & so on. That way you get a succession of plants instead of all at once.
Next year you'll have them for free as they seed....erm, everywhere lol!
(easy to pull out if you don't want them)
Just popped out and bought some candytuft, they look so pretty on the packets and just the height of flower I wanted. I will follow your advice, and as you say "seeds for free" too. Thank you sweetie, see you (so to speak)