Production potentials of new cultivars of cultivated plants are very high. However, achieving high the yields is very difficult in practice. Environmental conditions, mainly climate and precipitation, are a serious barrier, because they are out of control. Japanese bio-stimulator Asahi application could improve the situation. When applied exogenously, Asahi strengthens the plant’s cell walls and it increases synthesis of phenols that, protecting cells and their enzymatic systems, elevate plant’s resistance to temperature and water stresses, mechanical injuries and disease infections. They also increase the rate of plant’s adaptation to variable environmental conditions. Moreover, phenols play an important role in nitrogen metabolism in a plant and cytoplasm transport helping to increase the plant productivity. Two-year-long experiments using Asahi preparation were performed in the cultivation of selected vegetable species: root celery cv. Mentor, potato cv. Ania, leek cv. Lancelot and tomato cv. Mikra. Asahi was applied in a form of spraying with 0.1% water solution at doses and developmental stages recommended by the producer. The total yield and its chemical quality were determined at harvest. The following items were analyzed: dry matter, reducing and soluble sugar, vitamin C, protein, and polyphenol contents. Asahi increased potato yields by about 14%. The increase of celery tubers was 17–35%, leek 22–34% and it depended on preparation dose and weather conditions. The increase of soluble sugars was observed, particularly reducing sugars. Asahi caused the increase of phenolic compound content in tomato fruits and potato tubers, as opposed to leek. Such differentiation was also recorded in relation to vitamin C. If fruits or tubers are meant as consumption parts, Asahi decreases its concentration; for leaves or leek plant, vitamin C level was elevated. Protein content increased after Asahi application only in the leaves of leek plant.