Light quality is often used in soilless horticulture to supply plants with optimum light conditions for biomass production. However, its impact belowground is largely unknown. Here, we studied how light quality impacts root exudation rates and composition of lettuce. Root exudation is an essential process in creating a beneficial microbiome around plant roots. If lettuce was grown in DFT, light quality had no impact on root exudation rates and composition. If lettuce was grown in substrate, light quality changed the total C exudation rate and the total carbohydrate exudation rate. Total phenolics exudation rate rather depended on the composition of the growing medium. All these changes are most pronounced in the early growth stages, suggesting that these are crucial moments to steer root exudation.