Flower Bulbs Are Not Tubers
First of all, we would like to clear up a widespread misunderstanding: Bulbs are not Tubers!
A bulb is in fact a complete plant, lying all curled up, waiting to unfold. If you cut a bulb in half, an onion for instance, all the tunics and eyes are visible inside the bulb. These are the future stalks, leaves and flowers.
A Tuber is not a complete plant but an accumulation of nutrients with buds on the outside, like the potato. Although we call them 'spring bulbs', they should be planted in autumn. Not until after the cold winter spell and they develop into an amazing flowering plant. Bulbs can be planted from the beginning of September through to December. As long as the ground is easy to work, the bulbs can still be planted, even after the first frost.
Do keep in mind that early-planted bulbs will flower earlier too. You could make use of that by planting bulbs of one type with intervals of a few weeks. This way you can prolong the flowering period in a natural way. Some bulbs, early-flowering ones in particular, prefer a slightly warm soil at the time of planting so their root systems can develop faster.
To plant flower bulbs you first need to choose and buy or order them. This is something you should do at an early stage, as gardeners who buy first have the widest choice. The planting time for the earliest flowering bulbs begins in September.
Crocus, Galanthus (snowdrop) and Eranthis (winter aconite) are the first to buy. They bloom early so it is preferable to plant them early in order that they are given sufficient chance to develop a good root system. That will happen fairly quickly in a border that is still warm from the summer sun; a soil temperature between 5º and 10º C is ideal. All bulb crops that flower later can be planted in October and November.
Try to arrange for the bulbs to be delivered just before you intend to plant them. They are sent directly from the nursery and are in optimum condition.
If circumstances force you to put off planting them until later, remember to unpack the bulbs as soon as they arrive. Place them in a dry, dark place with a temperature under 20º C and open the bags so that sufficient air can circulate. If you have ordered bulbs that dry out quickly, keep them in a tray containing sand or peat dust. Types that are prone to drying out include: Allium ursinum (ramson), Anemone, Eranthis (winter aconite), Erythronium (dog toothed violet), Galanthus (snowdrop) and Leucojum vernum (spring snowflake).
Most other bulbs are not so sensitive, partly because of years of experimental adaptations, which reduce the chances of, for example, fungal viruses developing.