>>I think interaction may be a better word than interference, as it has nothing to do with QRM or QRN.<<
Point taken, it's the term I should have used.
[Side Bar] In physics the term "interference" has a much broader meaning than QRM and QRN. Constructive interference is what allows a Yagi to have gain in one direction and destructive interference is what happens off the back of the Yagi. Interference is what happens to allow an antenna tuner to achieve a 50-ohm Z0-match. Interference is why a 1/2WL dipole has a 50-75 ohm feedpoint impedance. Interference is what allows one to see yellow from an RGB TV. Often, interference is deliberately designed into a system.
In a transmission line with reflections, the reflected wave interferes with the forward wave to create the standing wave pattern - but the two waves interact only at an impedance discontinuity.
Destructive interference is what keeps opposing elevated radials from radiating much RF (their fields tend to cancel). Your closely spaced radials will certainly interfere with each other. But interference can be good, bad, or not worth worrying about. Destructive interference between opposing resonant elevated radials is a good thing because that destructive interference energy tends to get radiated by the vertical element.
The book, Optics
, by Eugene Hecht has a very good chapter on interference (and superposition).