I accidentally created a nice nesting place for solitary bees on the allotment. I piled up all the weeds I dug out 2 years ago and let them rot down for a year. I planted this 'hill' last year with drought-tolerant plants, it has a nice south-facing slope which warms up quickly. I now have discovered quite a number of solitary bees swarming there, all males at the moment, but the females will come out soon too 🙂