The light environment of a plant shoot can affect its root size and the shoot/root biomass ratio. Photoperiodic influence on shoot/root ratios of first‐year biennial sweetclover(Melilotus albaDesr.) plants was related to phytochrome measurement of day length and its regulation of photosynthate partitioning to favor successful completion of the life cycle. Short photoperiods alternated with long, uninterrupted nights resulted in low‐growing shoots and rapidly enlarging taproots. The ratio of far‐red (FR) relative to red (R) light and its effect on shoot/root biomass ratios were studied in controlled environments, in different field population densities, and among plants growing in full sunlight but receiving different spectral distributions of upwardly reflected light over different colored soils, plant residues or artificially colored mulches. Altered spectral distribution of reflected light can act through photomorphogenic pigments within a plant and influence photosynthate partitioning and shoot/root biomass ratio. Application of some basic photobiological principles should lead to improved plant‐soil‐water‐light management in crop production systems.