Sweet peas can be sown in the Autumn and the benefits are to produce sturdier plants, earlier in the spring, ready to plant out which will have better developed root systems and flower earlier.
Autumn sown sweet peas are sown and germinated the same as in the Spring. Sweet peas are best germinated into deep containers, preferably root trainers. I avoid using toilet rolls for autumn sown sweet peas as the Sweet pea plants will be in the containers for a lot longer, and there will be more chance of them getting damp and musty, or black mould and generally not holding up so well, for which reason root trainers or deep pots are best. Root trainers because they are deeper allow a greater root run and developement, you can re use year after year, and they are hinged at the base so you can lift the plant out with little root disturbance when you are ready to plant out.
Sow the sweet peas into root trainers, (as in image 1) and sow a few centimetres below the surface ensuring you have firmed down the soil to make sure there are no air pockets which are damaging to young roots. (image 2) If a root grows into an air pocket, its a bit like a vacuum, and the root may struggle and die so firm up the soil and then cover the sweet pea lightly with soil. Water a little, or use a spray, and place in propagator to germinate. (image 3) It is best to place out of direct sunlight but somewhere warm, such as floor of greenhouse, on newspaper if cold, and remove from propagator as soon as they germinate.
Over the winter you need to keep the sweet peas away from the worst of the weather. Although they are hardy to around -2 degrees sweet peas will not enjoy being exposed to the winter without some protection. If in a cold frame, ventilate well and prop open, during chilly and even cold weather, up to -2 and then close for the severe frosts, snow or heavy rain. If in a green house try and ventilate as much as possible during milder spells.
In early spring the plants will need to be hardened off carefully and then plant out in the usual way. More information about growing sweet peas