I am generally all about planting perennials for bees so that you have a guaranteed sources of nectar and pollen every year but there are a few annual crops which cannot be ignored for their bee-value. Phacelia is one of them. It provides masses of blue pollen for honey bees and nectar for lots of other types of bee (see http://www.rosybee.com/?p=345 for our observations on types of bee).
In several countries (US, Germany) it is grown as a green manure to enrich arable land. In the UK is is sometimes included in the mix for game cover but I have not seen it sold for crop rotations although I have seen it sold as a green manure in small packets in garden centers - presumably aimed at allotment holders. It doesnt seem to have any product other than its humus benefits..........and its masses of pale purple flowers.
My sister gave me a 100g packet she picked up in Germany and we had a patch of field going spare not far from our bees. So, having rotivated to break it up a bit I liberally chucked the seed around, raked it over roughly and then ignored. In spite of it being the driest spring on record and having to compete with well established weeds, it has come up thickly and now (first week June) is just beginning to flower.
1, how attracted the bees are to a clump 3mx8m?
2, the flowering timing - will it fill the 'June gap'?
3, after flowering will it self seed and continue to compete against the couch grass and thistles?
My observations are that the bees of all types love it and this is a fabulous 'June gap' filler. However, being an annual, you do need to sow it in freshly prepared ground every year. For a selection of perennial plants that will reliably flower every year and support bees, go to the rosybee plant range.