  The finite size of a neutrino wavepacket at creation can affect its
oscillation probability. Here, we consider the electron antineutrino wavepacket
and decoherence in the context of the nuclear reactor based experiment JUNO.
Given JUNO's high expected statistics [$\sim$100k IBD events ($\bar{\nu}_e p
\rightarrow e^+ n$)], long baseline ($\sim$53\,km), and excellent energy
resolution [$\sim$$0.03/\sqrt{E_{\mathrm{vis}}~\mathrm{(MeV)}}$], its
sensitivity to the size of the wavepacket is expected to be quite strong.
Unfortunately, this sensitivity may weaken the experiment's ability to measure
the orientation of the neutrino mass hierarchy for currently allowed values of
the wavepacket size. Here, we report both the JUNO experiment's ability to
determine the hierarchy orientation in the presence of a finite wavepacket and
its simultaneous sensitivity to size of the wavepacket and the hierarchy. We
find that wavepacket effects are relevant for the hierarchy determination up to
nearly two orders of magnitude above the current experimental lower limit on
the size, noting that there is no theoretical consensus on the expectation of
this value. We also consider the effect in the context of other aspects of
JUNO's nominal three-neutrino oscillation measurement physics program and the
prospect of future enhancements to sensitivity, including from precise
measurements of $\Delta m^2_{3l}$ and a near detector.
