Nursery preconditioning of Arundo donax L. plantlets determines biomass harvest in the first two years

•Hardening conditions of micropropagated A. donax determine the crop success.•Cell size and substrate type determine plant initial size and crop biomass production.•AMF facilitates hardening and, hence, it can reduce the dependency from specific substrate.•The increased cost of generating bigger plants could be offset by increases in biomass yield.