Plants respond to their environment through adaptations such as root proliferation in nutrient-rich patches. Through their burrows and casts production in soil, earthworms create heterogeneity which could lead to local root adaptations or systemic effects. To investigate the effect of earthworms on root system morphology and determine whether earthworm effect is local or systemic, we set up two independent split root experiments with rice or barley, (i) without earthworm (CC), (ii) with earthworms in both compartments (EE), and (iii) with earthworms in one single compartment (CE). Earthworms had an effect on belowground plant biomass. The relative length of thick roots decreased with an increasing abundance of earthworms. Some root diameter classes responded to earthworm number in a linear or curvilinear way, making simple conclusions difficult. We found no difference in root biomass or morphology between the two compartments of the split root system in the CE treatment, but a positive effect of earthworm biomass on root biomass, volume, surface area, and length at the whole plant level. Results supported a systemic effect dependent on earthworm abundance. Modification of nutrient mineralization, soil physical structure, and/or the concentration of signal molecules could all be responsible for this systemic effect.