Last years root veg or brassicas. Treat sprouts as a biennual flower and you'll be amazed at the amount of bees they attract. Swedes, winter radish and parsnips did very well for flowers this spring after being buried under snow for so long we gave up on harvesting any.
I've also had an aeonium that has been flowering since the end of April and is still going and there hasn't been one day when it hasn't had several bees on it all day. Shame it will die when it's done
If you can keep your head, while those around you are losing theirs, you may not have grasped the seriousness of the situation.
Another vote for coteneaster. (So much for the theory that it's got to have flat, open flowers!). Also lavender and all the herbs: sage, thyme, lovage, oregano, and coriander that has gone to seed.
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Got buddlejas still to flower though.
Also lavender and all the herbs: sage, thyme, lovage, oregano, and coriander that has gone to seed.