Often grown from heat treated sets, Onions can be grown from seed, sown either indoors or outside. Seed is cheaper to buy, but slower to grow and the seedlings need more careful attention, however seed-grown plants can be less susceptible to bolting (flowering). To ensure a good crop, seed-raised plants must be growing strongly by late spring, as the lengthening days trigger the formation of bulbs – the more leaves plants have at this time, the better the bulb will be.