To thrive, bees need a continuous supply of suitable pollen and nectar from early spring, through summer and into autumn.
Bees will collect from a variety of species but do show strong preferences for legume species. When available, legumes (birdsfoot trefoil, clovers and vetches) can make up 60% of the pollen loads of bumblebees, and red clover in particular may account for 30%. Clover alone however will not supply a continuous or reliable source of pollen and nectar and other species are also very important.
Areas with a high density of suitable flowers are most attractive to bumblebees and flower rich meadow and grassland mixtures are the most dependable resource for foraging bees, offering both a quantity and diversity of pollen and nectar with plants such as selfheal, yellow rattle, knapweeds and scabious being particularly sought after. Cornfield Annual seed mixtures containing species like cornflower and corn marigold can also provide a very rich if only temporary source of pollen and nectar.
Bumble bees also require suitable places in long grass, banks or hedgerows in which they can nest. These sites are easy to provide and studies show that it is not lack of nest sites that limits the success of bumble bees but lack of pollen and nectar.
Footnote: Clovers in seed mixtures
Red and white clover are natural components of many ancient meadows and grasslands. However simply adding clover to a wild flower seed mixture (especially cultivated forms) is not generally recommended, as their vigour and ability to fix nitrogen tends to push the sward to a more fertile grassy condition, creating management difficulties and a loss of overall diversity. Clovers (especially red clover) are also very prone to 'boom bust' cycles from year to year. Cultivated red clover will typically dominate the sward in the first two years, and then disappear completely in year three giving bee populations no long term stability. Within farmland environmental schemes 'pollen and nectar' mixtures based on clover are a popular and cheap way of providing a short term abundance of nectar. They are of some use if re-sown every 2-4 years and planned with other more diverse long term wild flower rich habitats to provide continuity. Clovers sown in other contexts (eg sown with lawn or grazing mixtures) can be of some benefit to foraging insects.