undermathn.
Etymology: under- prefix1 2b(c): compare aftermath.Previous versions of the OED give the stress as: ˈundermath.
the world > plants > plants collectively > [noun] > growing beneath trees or undergrowth
1881 G. Allen xii. 119 Ferns..grow in the tangled shady undermath of the banks and thickets.
This entry has not yet been fully updated (first published 1921; most recently modified version published online June 2021).