I'm in zone 8. Can't tell about elms, but for most maples species, you can collect them as soon as the wings and the enveloppe of the seed have turned from green to brown, even if the tree is still in full leaves.
But it depends on the species, the place where the seeds were collected, if the seeds are viable or not (isolated specimens are not so fertile, it's better if there are other trees of the same species around) : I've collected Montpellier maple seeds in mid-August (southern low mountains in France, hot dry summers and cold snowy winters), and other species like Acer davidii ou Acer tataricum in the third week of September (near where I live, much less contrasted seasons) with a high rate of gemination the next spring for instance.
