The most frequently used substrate for the production of covered root seedlings is peat, the extraction of which is under scrutiny due to its negative environmental impact; however, the use of Sphagnum moss in adequate mixtures constitutes a possible alternative. Lettuce seeds (Lactuca sativa L.) were sown in Polystyrene trays (240 alveoli, 24 cm³) to test four substrates, 50% Sphagnum moss mixed with 50% humus or compost and 60% Sphagnum moss with 40% humus or compost, and commercial peat was used as a control. A dose (2.5 or 3.5 gL-1) of controlled-release fertilizer (CRF) was added to all substrates except for the control mix, and the substrates were subsequently irrigated with or without a fertilizer solution. The study was conducted under shaded conditions between January and February 2008. A completely randomized statistical design with 16 treatments in a factorial structure (4 mixtures x 2 levels of controlled-release fertilizer x 2 with or without fertigation) was applied in addition to a common control with five repetitions, and the plant height, number of leaves and total dry weight were measured. In all cases, the results were unfavorable for the peat control and were significantly different from the treatments prepared with Sphagnum moss. The addition of the highest dose of CRF to the substrates renders the application of additional fertigation unnecessary; however, fertilizing irrigation is necessary with the lowest CRF dose. According to the physical and chemical analyses, the most suitable growth mixtures for lettuce seedlings were those composed of 50% Sphagnum moss and 50% humus or compost.