Hydrolysable tannins, water-extracted from sweet chestnut (Castanea sativa Mill.) (CHT) and membrane concentrated, have several effects as antioxidant, antimicrobial, and metal complexing agents. Some patents described their use as nitrogen release modulators and iron complexing agent to fight plant chlorosis and to control legume seed-borne disease and nitrosamine and mycotoxin during plant and food processing. Biostimulating activity of raw CHT, placed near seed or transplant seedlings, was assessed on early plant growth (starter effect) and found related to earlier production of a larger plant fine root mass, with greater P early uptake. Increased resistance to nematodes, with CHT applications on tobacco, was investigated. Recent process stream fractioning permitted to identify some CHT fractions with antimicrobial and antioxidant effects which were tested for their potential in promoting selected aspects of plant yield, quality, and protection and maintaining and improving feed and food quality during processing. EU Life+2013 Evergreen found a method of application of a CHT fraction to protect tobacco and carrot plants in nematode-infested fields. A protective effect of CHT on some bacterial diseases of olive tree and kiwi was disclosed. Environmental and soil toxicities were also investigated finding very low impacts and the possibility to reduce Cu use in agriculture.