• Foliar Zn content increased as much as 18-fold as a function of soil Zn concentration. • 50 g of biofortified lettuce with Zn contributed up 18% to Zn daily intake. • Oxidative damage in lettuce increased at concentrations of 20 mg Zn dm −3 and higher. • SOD activity increased at concentrations 30 and 40 mg Zn dm −3 in both growing seasons. • Milder temperatures were more favorable for biofortifying lettuce with Zn. Zinc (Zn) is an essential element for human health, but at least 1/3 of the global population suffers from Zn deficiency, causing several health problems. The agronomic biofortification of lettuce with Zn is a strategy to mitigate the low intake of this nutrient by the needy population. Here we investigated the effect of agronomic biofortification of lettuce cv. Vanda with Zn on biomass production, the Zn content in leaves and antioxidant responses in two growing seasons under tropical climatic conditions. The treatments were six concentrations of Zn (0, 5, 10, 20, 30 and 40 mg Zn dm −3 ) and two growing seasons, 7 March to 6 April 2018 (S1) and 29 May to 9 July 2018 (S2). Foliar dry matter production decreased as Zn concentration increased, regardless of season, to 11.6 and 10.3 g plant −1 at 0 and 40 mg Zn dm −3 , respectively. The concentrations increased foliar Zn contents as much as 18-fold, from 41.5 to 746.3 mg Zn kg −1 at 40 mg Zn dm −3 . The concentrations from 20 mg Zn dm −3 increased lipid peroxidation (MDA). Foliar superoxide dismutase (SOD) activity increased at the highest concentrations (30 and 40 mg Zn dm −3 ) in both seasons. Foliar catalase (CAT) activity in S2 decreased with Zn concentrations of 20 mg dm −3 and higher. Foliar ascorbate peroxidase (APX) activity did not differ between Zn concentrations in either season. The milder temperatures of S2 were more favorable for biofortifying plants. Our results suggest that limiting Zn concentrations for agronomic biofortification in each growing season is important due to the sensitivity of the plant to Zn, allowing for adequate productivity and quality.