One more added experience regarding the effects of nearby trees with our own self-interests in controlling every aspect of our gardening expectations ....
Like most natural 'wild' organisms - plant or animal - the individual specimen can & usually will go beyond it's species' norms to take advantage of sources it needs, it wants, it senses, and/or it finds.
Usually a tree's canopy indicates the extent of its root system & defines its drip-line (Look at any wind-felled tree, on it's side, after a storm, with its intimates' revealed above the soil ... However, individual trees have a way of detecting somewhat distant - beyond their normal root zone - sources of needed nutrients and/or moisture. My experiences revealed 3 examples that serious gardeners such as ourselves should keep in mind.
(1) Seeking our reliable moisture. As a young gardener I remember a tree - species now forgotten - that had sent out a feeder root probably twice - or more - its normal distance to take advantage of a long-time dripping barn-side wall water-tap. The farmer discovered just how important this water source was to that tree when he attempted to dig near the tap and discovered a significant 'arterial root' had developed to exploit this predictable water source - sending it back to the tree to use..
(2) I set up a compost box (Plastic commercially available box with lid) and over a few seasons filled it to its max as intended. When I went to remove the compost I discovered the ENTIRE interior of the box was filled with white cedar roots from a nearby cedar hedge. These cedars had found this source of nutrients and exploited it. Their thieving roots were impenetrable within the compost (It was one compact solid 'lump') Then the question came to mind, just what nutrients were still there for my daylilies to use? What I had was a 'block' of fine feeder cedar roots & depleted compost. Raised beds filler at best ....
(3) Manure piles .... I've encountered many examples of trees sending significant feeder roots considerable distances to be able to exploit the nutrients (AND moisture too) of aging, composting manure piles
BOTTOM LINE - Nearby trees, and sometimes NOT so nearby trees, will ALWAYS look out for themselves naturally (Nature - evolution, survival of the fittest et al) ) and will exploit situations to their own advantage .... Thus, perhaps, to your disadvantage.