best to plant the sweet peas early spring, just as the soil out in the garden starts to warm up, look for weed seedlings starting to peep through the soil, that tells you the soil is warming and moist enough, OR start indoors Feb in pots, once the seedlings are large enough to handle by the second set of true leaves, devide the seedlings into tall pots or special root trainer pots and grow them on till the soil is ready to plant them into their finnal place, say about May/June, by then you should have prepared the soil well in advance and added lots of compost, manure even better as sweet peas are hungry feeders, like watered well and you need to pinch out the growing tips as they get climbing to bush the plants out, or all the flowers will be way up top of the plants, you need to deadhead or pick the flowers constantly or the plants will go to seed very fast, also try to nip off the tendrils as they form because this uses up a lot of energy and you want flowers rather than tendrills, also the tendrils are inclined to twiast around the flower stems and bend or stunt the stem developement so you can pick nice long stems to take indoors to enjoy the sweet pea perfume