Sow sweet peas between Christmas and New Year celebrations ( I always do mine on Boxing Day) they have a few months of cold to focus on their roots. Germinate them inside, somewhere warm and protected from mice but then move them into a cold frame or unheated greenhouse once the seedlings are 3 inches tall. When each seedling has grown 4 pairs of leaves, pinch out the growth tip, down to the third pair of leaves and this will ensure a bushy and stronger plant.
Sweet peas are the hungriest of cut flowers to grow. They need very fertile soil to do well with generous mulches of well-rotted manure to thrive. Organic chicken manure pellets are excellent, being full of nitrogen, as are comfrey pellets too. Rich feeding and watering weekly will ensure plentiful flowers and also prevent powdery mildew from taking hold of sweet peas, if they go hungry this will go through sweet peas like wildfire.
Sweet peas like a rough, supporting wigwam to climb. Bamboo canes are too slippery but if this is all you have, make cobwebs of garden twine through them so their tendrils can latch onto these. During the winter it’s a great garden job to harvest or source bundles of birch and hazel for sweet peas to climb up with ease. Once the sweet peas are in flower remember to also pick off the excess (loose) tendrils as these take energy away from the plant.
When the sweet peas have finished cut them to the ground but leave their roots to rot into the soil as it’s very good for soil structure. Harvest any seed pods and save them for next year's sowing as fresh seed will germinate better than old.