I've had some burrs start putting out small amounts of acorns after 6 years or so. Trees aren't hardly an inch in diameter. Not even sure if hybrids but just some ones from a local chapter of NWTF. Posted some pics of 'em on a forestry forum a few years back and they thought looked mostly like a regular ole burr oak and not hybrids (but had good roots and looked to be in great shape when planted as a bareroot).