posted 10 years ago
A bit of observation will reveal that valley oaks grow right up to the banks of the Sacramento and other perennial and seasonal waterways, so I would say that some root access to supplemental water won't be harmful...in fact otherwise. What they don't want is wet right up against the base of the trunk in the leafed-out season. I have both blue and valley oaks in my yard, and while I'm not irrigating anything under the canopies, I am irrigating gardens and other plants nearby outside the dripline. I regularly find oak roots in the gardens, sometimes quite a distance from the nearest oak canopy. Some of these roots are new, so they are apparently after the moisture and nutrients I'm adding to the gardens. Between roof runoff, slowing water runoff in the drainage areas by means of check dams, and my irrigation, I think this may be responsible for the higher and more regular yields of acorns from the oaks in the yard as opposed to out of it (good for me and my animals, since we eat them!)