Field experiments were conducted to determine whether exposure to reduced red : far-red light ratios (R : FR) typical of crop–weed environments was associated with adaptive changes in morphology, productivity, and fecundity of common lambsquarters. Plants were grown in reduced or ambient R : FR environments (both in full sunlight) until initiation of flowering, after which plants were grown in full sunlight or partial shade. At initiation of flowering, plants that had been exposed to reduced R : FR exhibited greater specific leaf area, stem elongation, main stem leaf area, specific stem length, and main stem mass compared with plants exposed to ambient R : FR. However, biomass allocation to stems, leaves, and roots did not differ between vegetative-stage R : FR treatments. At the end of flowering, morphology and productivity of plants exposed to partial shade did not differ between vegetative-stage R : FR treatments. In contrast, plants exposed to full sunlight during flowering after exposure to reduced R : FR during the vegetative stage had less total plant mass, less total leaf area, greater stem elongation, greater specific stem length, and a greater ratio of main stem to total stem mass compared with plants exposed to ambient R : FR during the vegetative stage. At physiological maturity, plants exposed to reduced R : FR during the vegetative stage and to partial shade during the reproductive stage had less total seed mass and fewer seeds compared with plants exposed to ambient R : FR during the vegetative stage and to partial shade during the reproductive stage. Fecundity of plants exposed to full sunlight during the reproductive stage did not differ between vegetative-stage R : FR treatments. These results indicate that exposure of common lambsquarters to reduced R : FR during the vegetative stage was maladaptive at later stages of growth in competitive environments, and suggest that interactions of light quality and quantity are important determinants of common lambsquarters fecundity.

Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL.