As a rule of thumb your garden should provide bee-friendly flowers that are rich in pollen and nectar which bees can easily access from spring until late summer. This will ensure that there is a good supply of pollen at the crucial times:

The story of bumblebees over the past century has been one of decline. Two species became extinct in the UK during the 20th century: Cullum's bumblebee (Bombus cullumanus) last seen on the Berkshire Downs in 1941, and the Short-haired bumblebee (Bombus subterraneus), last seen at Dungeness in 1988 and officially declared extinct in 2000. A further...

Bumblebee Conservation Trust - Posted on April 30, 2018

Bumblebee Conservation Trust - Posted on December 13, 2016

29 species of bumblebee are registered in Denmark. (Wikipedia)