While we may know which garden plants attract bees and other pollinators, trees are often overlooked.
Trees vary in size, some only being suitable for large estates and parks, others small enough to fit well in a garden. When in blossom, the right tree can provide masses of precious pollen and nectar when bees need it most, especially through late winter and early spring.
Winter is the best time to plant bare-rooted trees. The Woodland Trust offers a Plant for pollinators tree pack, 210 saplings for a very reasonable cost. While this will be excessive for most gardens, the all of the species in this pack will provide good forage. Perhaps your local association or group could split the pack?
Good advice about Trees for Bees and other pollinators is also given on Urban Bees’ website