Gardeners can come to the aid of threatened species, says VAL BOURNE

By now you will almost certainly have seen your first bumble bee and it's likely to have been a large Bombus terrestris. These short-tongued, buff-tailed bumble bees are often seen feeding on saucer-shaped flowers like hellebores, or on crocus, or winter aconite, or early-flowering pulmonarias.

These slow-fliers have just come out of hibernation and they are looking for a nectar fix to give them energy to go off and start this year's colony.

Once they have quenched their thirst you can often see the queens zigzagging over the ground looking for new nesting sites. They may build their nests in leaf litter under shrubs, or on a sunny bank. So don't be too tidy. Leave some undisturbed areas, because all bees are important pollinators of food and seed crops. Without them we wouldn't have gooseberries, broad beans, raspberries and tree fruit.

Bombus terrestris is one of the most common of our 25 species and numbers are healthy. But some bumble bees are in decline, probably due to changes in land use.

Nowadays arable farming, which disturbs the landscape, is far more common than it used to be, leading to the loss of wildflowers and undisturbed habitat. Pesticides have also decimated wildflowers.

Farming is becoming more conservation minded. Still, so far three species of bumble bee have become extinct. Five more are precariously placed and two more species may soon be added to that list. In all 15 species have "undergone major range contractions" and, unless action is taken, more will become extinct.

The upshot is that gardens are becoming more important for bee conservation and you may be attracting something threatened already.

However, identifying different species is difficult. When the colony is young at the beginning of the year, the bees are often small in size. So always go by the markings and ignore size (see www.bumblebeeconservation.co.uk).

Gardeners can help by leaving a sunny area of grass unmowed, or by leaving leaf litter under shrubs as nesting areas. They can also offer nectar-rich flower for as much of the year as possible, starting in February and going on until late into the year. Early flowers are the most important.

The trouble is not all plants are rich in nectar so choose simply bred flowers with noticeable stamens, not petal-packed blooms. These full doubles, which look more spectacular, usually do not contain nectar.

However, most plants that have veined and spotted petals are trying to attract a pollinator, so foxgloves, hardy geraniums and penstemons flag up their presence and satisfy the bee with a sugar-rich drink. While he drinks, pollen sticks to his head. Tall flower spikes (whether its hollyhocks or aconitums) are also good value too. They take up very little ground space, yet they have lots of flowers in succession.

Annuals and biennials are usually excellent too, because most need a pollinator to set seed. The blue cornflower - Centaurea cyanus - attracts red-tailed bumble bees. Clary, cosmos, single dahlias and calendulas are also excellent annuals. Good biennials include old-fashioned varieties of wallflower and Sweet Williams.

The borage family are also laden with nectar and these include pulmonarias (for early flower), comfreys (which follow on), achusas and the annual echiums. Many have pink flowers which turn blue as they are pollinated.

So when you are thinking about what to plant this year, try to include some old-fashioned annuals, biennials, herbs and simply-shaped perennials. The bees will thank you by making your garden fecund and fertile.