It seems there is lots of interest currently on hybrid oak trees; including from our friends in the 3,4,5 hardiness zones who appear to enjoy fewer options because of climate. Many of the hybrids involving bur oaks are often mentioned. While I strongly favor SWOs as specimen trees in an urban/residential planting because of their shape and beauty (leaves), bur oaks are probably the king - or very close to it - with regard to overall mast production due to the size of their acorns (when grown from parents with a history of large acorns). This is probably especially true for our colleagues in the more northern zones. The white oak / bur oak hybrid - often referred to as a Bebbs oak - is probably an option for virtually all of the zones except perhaps FL. Attached are photos of Bebbs acorns from a tree that has demonstrated an outstanding performance with rewpect to the often mentioned hybrid growth vigor associateed with hybrid oaks. What makes it's outstanding growth rate most admirable is the unsavory environment in which it grows (on an extreme 5-6' slope between two asphalt parking lots). Attached are photos of acorns from this tree. Although its leaves and overall shape favor the white oak parent, the acorns reveal the bur oak influence. Acorns are elongated like the white oak with a distinct white oak acorn type cap. However, compared to white oak acorns that are more slender/tapered toward the top, this hybrid has a fuller midsection and top third of the acorn ... probably the influence of the bur oak parent. Overall, it produces a nice crop of large acorns on a pretty regular basis. Hope this info is helpful.