They really need your help. Your success depends on your part of the world as well, but I'd say that bees need the most help at the end of summer and early autumn, when they are scrambling for last minute pollen and nectar to store for winter--so do some asters if you can, and mountain mint.

They could also use some very early spring help, like crocuses...but planting enough of them could be a challenge since you'll need to plant those bulbs in sept/oct.

Just FYI, honeybees don't often do one-offs of random garden flowers. It's not worth their time. The hive will fixate on a large source and mob it until they've exhausted it. We're talking vacant lots full of asters or queen anne's lace. Or acres full of trees like tulip poplar.

So if you have any vacant lots near you, go fill them with wildflower seeds.

We had a nice large vacant lot next to us with tons of wildflowers.

Sadly, the lot has been excavated and there is now 6 new condo buildings, 2 more on the way. This has the bees scrambling for areas to live and get nectar from. I know my sunflowers and lavender may not help them out that much, but if everyone on my street were planting them then there would be hope.

I know this sounds silly... but you could drop an anonymous note in a few of your neighbor's mailboxes; especially people that have yards showing a bit of interest in gardening/landscaping, and have the note compliment their yard, acknowledge the work they've done, mention the vacant lot that previously had flowers feeding bees and politely encourage that, as a group of neighbors, everyone makes a small effort to provide a source of food for the bees.

Keep fighting the good fight.

And yes, if the bees have a whole street of flowers, that would help a lot. You may get the solitary bees, but honeybees (at least wild ones) will likely move away. That said, cities can work for honeybees due to all of the well-maintained landscaping.

My beekeeping friend said that some great honey was coming out of Washington DC because of the linden trees that have been planted.