A quarter wave monopole antenna doesn't have to have a ground plane... unless you want it to radiate EM energy with a certain efficiency and pattern.
EM radiation requires accelerating electrical charges, which usually implies a voltage differential between two separated areas in space. A conductive ground plane is a particularly good region of voltage differential from the voltage on portions of a (short) monopole antenna.
Otherwise the voltage differential will occur somewhere else, for instance around the hand, arm and body of a person holding a VHF HT radio with only a quarter wave whip or "ducky" antenna. And your body is not as efficient a counterpose as a conductive ground plane.
With a half wave dipole antenna, each half acts as a nicely balanced counterpose for the other half, splitting the oscillating voltage differential between the two equal length halves, which leads to a nice symmetry in the pattern of the EM field, which helps create a more predictable RF (near and far field) radiation pattern. A conductive ground plane under a quarter wave monopole leads to a similar symmetry in conjunction with the EM field's mirror image.
If there is no ground plane (or other well designed set of counterposes) under a short vertical, that leads to the RF voltage differential being between various often randomly placed conductive objects (feed lines, radio case, ground straps, power supply wiring, rain gutters, and etc.) and lossy dirt. Which leads to an unpredictable antenna radiation pattern, ground losses, and possibly shocking "RF in the shack".
The ARRL Antenna Book and Antenna Physics book have information on this subject. Also many many textbooks on electromagnetics and antennas.