Peat extraction contributes to habitat loss, lowers water tables in peatlands, and releases stored carbon into the atmosphere. Rising awareness about the unsustainable use of peat as a growing medium has led to the search for more environmentally friendly alternatives. Leaf mould (also known as leaf compost), derived from deciduous leaves and characterised by a soft, porous structure, represents a potentially renewable peat-free substrate for seedling production. This study evaluated the suitability of mature leaf mould for vegetable seedling production using a citizen science approach. Hobby and market gardeners from Hungary cultivated lettuce (Lactuca sativa L.), kale (Brassica oleracea var. sabellica L.), kohlrabi (Brassica oleracea var. gongylodes L.), and pak choi (Brassica rapa subsp. chinensis (L.) Hanelt) in leaf mould and in commercially available or homemade control substrates. Seed emergence and seedling development were monitored for up to six weeks. Data collected included seed emergence, visual seedling appearance, and the number of true leaves, supported by photographic records. Overall, leaf mould supported seed germination to a similar extent as the control substrates. Seedling appearance was significantly improved in leaf mould compared with the control media for kale (p = 0.016) and pak choi (p = 0.007). Of the four crops, only pak choi developed significantly fewer true leaves in leaf mould than in the control substrates. The comparable performance of seedlings grown in leaf mould and control substrates, together with the observed improvements in seedling appearance for some crops, indicates that mature leaf mould can effectively support the early development of the tested vegetable seedlings. It may therefore represent a viable peat-free substrate for seedling production. The findings also demonstrate the potential of citizen science for evaluating horticultural growing media under practical cultivation conditions.