I am not growing ornamental sweet peas this year but a close relative, sugar snap peas. This year I planted an extra row of sugar snap peas just to pinch back, the young stems and greens are edible and great for adding interest and flavor to cold salads. But the advice I heard was to wait til peas are close to 12" tall to start pinching back, going down about two rows of leaves. The plant will produce more stems and leaves and you can turn back again. I am not pinching back the snap peas that I'm growing for veggie crop. So I will see if this pinching stimulates branching.
