Your large bumblebees are probably queens of Bombus terrestris, the 'buff-tailed bumblebee'. Like other queen bumblebees these spend a lot of time in spring feeding up after winter hibernation, searching for places to nest and then establishing nests (although some nests are now even known to be going through the winter in favoured locations and in London etc). There is no point (and in fact potentially destructive) to leaving any areas uncut now that will be cut later in the summer - this would simply encourage nests, which would then be destroyed at a time when established and working towards the next generation.
What you need are areas in the garden that are undisturbed throughout the whole spring/summer/early autumn period, and hopefully at least one or two will be deemed suitable by the bumblebees.
Once nests are established, the most important resources are suitable flowers i.e. not double varieties of any flowers, and depending on the bumblebee species specific kinds of flowers, but most important are usually those in the Fabaceae (pea family), Lamiaceae (dead-nettle family), Scrophulariaceae (figwort family), as well as some Asteraceae - the more the better!
What you need are areas in the garden that are undisturbed throughout the whole spring/summer/early autumn period, and hopefully at least one or two will be deemed suitable by the bumblebees.
Once nests are established, the most important resources are suitable flowers i.e. not double varieties of any flowers, and depending on the bumblebee species specific kinds of flowers, but most important are usually those in the Fabaceae (pea family), Lamiaceae (dead-nettle family), Scrophulariaceae (figwort family), as well as some Asteraceae - the more the better!