The seeds of almost all maple tree varieties must be stratified--given a chill--before they'll germinate. Don't be fooled by seedlings that are sprouting up all around you in the spring. It's highly likely that they've been germinating in the ground for one to two years. Harvesting maple tree seeds is very easy, although timing can be a little tricky. Much depends upon the species of the tree that you wish to harvest seed from, as well as your USDA planting zone.
Observe the tree's seed production, usually in the fall. Mature, winged maple tree seeds are ready for harvest when they turn dark and begin falling from the tree like little helicopters. Marked increases in wildlife activity around your tree is a reliable indicator that ripe seeds are available.
Gather the maple tree seeds from the ground immediately. Work quickly because you'll have plenty of competition from birds, squirrels and chipmunks.
Clip a few seed-bearing stem tips from a ripening tree if you're unable to harvest an adequate supply of maple seeds from the ground. Strike the stems gently against the inside of a bucket. Seeds that fall readily when shaken are mature enough for collection. Toss the stems with immature seeds attached back out onto the ground for the deer. Repeat this step until you have considerably more seeds than you think you'll need to allow for failed germination.
Pre-treat, stratify or immediately plant maple seed trees according to whatever is appropriate for its particular species in your planting zone.