We examined effects of single-layer glass and double-layer antifog polyethylene films on growth and flowering of stock (Matthiola incana L.) and snapdragon (Antirrhinum majalis L.) in a 3-year period. Stock produced more buds/spike with shorter but thicker stems under single-layer glass and under antifog 3-year polyethylene, and showed higher photosynthetic capacity (Pc) under single-layer glass than under other covers regardless of light regimes. Similarly, growth and flowering of snapdragon were significantly better under single-layer glass than in polyethylene houses. A supplemental light of 60 μmol·m-2·s-1 accelerated flowering by 20 to 25 days, improved flower quality, and eliminated differences in plant growth and quality of snapdragon between covering treatments. The Pc of stock was lower under all polyethylene covers than under single-layer glass. Among the three antifog polyethylene films, a slightly higher Pc was measured for plants under antifog 3-year polyethylene. However, there was no difference among covering treatments in the net photosynthetic rate (PN) at low light level (canopy level). Supplemental lighting reduced Pc of stock leaves, especially under single-layer glass, and diminished differences in Pc among covering treatments. Dry mass was more influenced by larger leaf area caused by higher leaf temperature than by PN. Overall, antifog 3-year polyethylene was a good covering material when both plant quality and energy saving were considered.