I am frequently asked "How can I help the bees? I've planted flowers and encouraged my neighbors to stop using pesticides, but I want to do more!"
One of the best responses I have is to leave the weeds where they are! Many people consider weeds to be a pest because they spread rapidly, grow quickly, and "take over the garden." Many people think weedy flowers don't look as nice as a perfectly manicured lawn, but in reality, these weedy species are providing a lot of food for bees.
Weeds are often the earliest flowers to emerge in the spring, and provide critical food sources for newly emerged females looking to provision their nests. I know they're unsightly (unless you're like me and just love the little yellow splashes), but if you can put off mowing the dandelions in the spring until the apples and cherries start to bloom (it's just a couple of weeks! I promise!), the bees will appreciate the food. Not only are the weeds the early bloomers, but because of their abundance, they can offer food to many individuals. If more bees are getting plenty of food, they can make more bees! As an added bonus, the weeds might be the only source of food in an area at any given time (my dahlias aren't going to bloom until September), without which, the bees won't stick around.
Dialictus female collecting pollen from a dandelion.
I know that the blackberries will take over your yard, and the creeping buttercup will strangle your roses, but if you can find a nice balance of cutting back without completely removing the weeds, you'll get extra flowers, the bees will get extra food, and the world will get extra bees! Win-Win-Win!