Maple seeds (called "samaras") are best harvested when the papery wing part of the seed turns brown, but the thick fleshy part of the seed is still slightly green and not dried out. They need to be stratified, which means they must endure a period of cold. What I do is harvest the seed, break the pair of seeds in two, and then plant them in a shallow 4" garden pot - cheap plastic will do. Do not plant them deeply. You may want to cover the pot with some wire screening to keep out the squrrels and chipnumks if you have a problem with those in your area. Plant as many pots as you want and then find an area for them to spend the winter. They need to be exposed to the cold and rain. In the spring you should have new maple seedlings come up. If you don't see seedlings, don't give up. Sometimes it takes 2 years for them to sprout. Then just transplant the baby trees to whatever part of your garden you wish. Easy! P.S. They can grow for a whole year in that 4" pot if necessary.

I had both, scarified & stratified the seed. It was in the fridge for over a month. A sprout began to come out of the seed. The sprout was about....1 inch in length. Then I placed it in a soil mix (from Miracle Gro) and placed it outside. It sat outside for like a few weeks but nada. Then I finally gave up and decided I'd root around through the pot/planter and it looked like it was all dried up. The funny thing is, I watered it every 2 days so I'm not sure why this happened. Anyways, this time I purchased one of those Jiffy Peat domed planters. Hopefully I'll have better luck with that. :)

Interesting, the publication by Wright also shows hybrids with Acer platanoides (Norway maple) and Acer negundo (Boxelder maple), as well as Acer saccharinum (Silver maple) and Acer negundo (Boxelder maple). It would be interesting to see such a study replicated, and for a full growth cycle for the hybrids to take place. I live in Utah, and the native Acer grandidentatum (Bigtooth maple, close sugar maple relative) flowers at the same time as the invasive non-native Norway maple in our foothills at about 4,500 ft. Sometimes bees cross-polinate them, I'm sure of it when both trees grow near. Bees still prefer the Norway maple for its showy and nectar rich flowers. Both insects and wind pollinate them, but mostly wind because sometimes it is too cold for bees to work. I have grown some potential hybrids between the Norway and Bigtooth maple, from a Bigtooth maple 'female' tree, and the seedlings were weak, waxy/glossy, and lime green possibly from iron chlorosis. None of those made it after a week...

I am not aware of any Acer species that will germinate readily before the seeds are fully ripe. Which is typically in the fall when the samaras change color and begin to fall. The exceptions are Acer rubrum (red maple) and Acer saccharinum (silver maple) - these ripen and fall early in the year (late spring/early summer) but still need to be properly ripe for good germination. And also require stratification.