Experiments were conducted in two consecutive years in a multilayered plastic-covered greenhouse, of the “raspa and amagado” (covered skeleton structure) types in the fields of Almeria (south-eastern Spain), to evaluate the production and quality of a triploid watermelon crop (Citrullus lanatus Thunb) cv. ‘Queen of Hearts’ grafted onto the (Cucurbita maxima x Cucurbita moschata) squash hybrid cv. ‘Strongtosa’. A 2.5% solution of the monosilicic acid [Si(OH)4] product was applied to this crop in the drip irrigation system at a dose of 260 mL·ha−1 in both experiments. Results showed that the product did not influence productive parameters in terms of kg of product·m−2, kg plant−1, or number of fruits plant−1. Nevertheless the monosilicic acid [Si(OH)4] product appeared to have a positive effect on some quality aspects of the fruit with an approximate 10% increase in pulp consistency and rind width.