Thanks for the comments, Michael. I already went ahead and wrote up what I could, a few weeks ago, based on what I could discover.
Originally Posted by Michael Bush
Here's what I put in the handout:
♦ Honey Locust (Gleditsia triacanthos) √ Å, β May, 30-70′
Named for the supposedly sweet taste and smell of seed pods and foliage, which can be fodder for livestock. For the bees, it’s good for pollen, not much nectar. Some cultivars are thornless.
Pollen & Propolis
Some trees are basically wind-pollinated, including Oaks, Birches, Hickories, and Mulberries, and evergreens such as Pine and Spruce. These trees often flower early in the spring, at or before leaf out, as leaves would inhibit the passage of pollen from one tree to another. For many of these trees, male flowers are ‘catkins’ and female flowers are inconspicuous and odorless. Grains of pollen are generally small, hard, and low in protein. Many of these trees are nearly useless to insect foragers seeking nectar and digestible pollen.
Sycamores (Platanus spp.)
Commonly planted on Philadelphia streets, the London Planetree (P. x acerifolia) provides almost nothing for the bees.
I wrote that (and a lot more) but I'm not 100% sure about any of it. I'm not an expert, but I couldn't find much good information on the subject. So I found what I could, connected the dots, and wrote it up. Feedback and corrections are welcome. Another page has links and references for the best sources that I used for this project.