A few of my red onions flowered about 6 to 8 weeks after planting? Does this mean they will not bulb up to be nice large onions?

We have five mature oak trees on our 1/2-acre property. Last week I paid to have them trimmed, taking out any dead limbs, thinning the inner leaves, etc. They look great! We want to keep them that way. What else can we do to encourage them? I'm thinking primarily of feeding them. The only method that occurs to me are those spikes (about the size of a railroad spike) that you drive into the ground. Alas, at 12/box at the home center and not particularly cheap, it will take $$$$ to do all five trees. If that's the best choice, so be it. Are they available bulk-packed somewhere? Or are there other technologies (liquids, granules, whatever) that are better? I once saw a tree with a container leading to a hole bored in the base of the tree; feeding? Two are willow oaks, I think the other three are red oaks, if that helps.