Invasive Plants in Support of Urban Farming: Fermentation-Based Organic Fertilizer from Japanese Knotweed

In this study, fermentation-based organic fertilizer (OF) was produced from the aboveground parts of Fallopia japonica (Houtt.) Ronse Decr. The quantity of N in OF (17.2 kg t−1 fresh lactic-fermented OF) was higher than average in cattle farmyard manure, but on a comparable level to solid poultry and rabbit manure. The OF was applied on a field to evaluate its effect on Chinese cabbage. The applied nutrients with OF N159 were 159, 19 and 100 kg ha−1 for N, P, and K, respectively. The applied nutrients with OF N317 were 317, 38, and 200 kg ha−1 for N, P, and K, respectively. The average mass of marketable Chinese cabbage (Brassica pekinensis Rupr.) single heads ranged from 253 g with N0 treatment to 602 g with N317 treatment. The nutrient recovery efficiency REN,P,K was 37, 20, and 50% for N317 and 55, 48, and 77% for N159. The OF was found to be a suitable alternative to farmyard manure. Additionally, OF produced from F. japonica could complement existing approaches to limit the spread of invasive species in cities. Further research should focus on perennial crop rotations and cropping patterns, different soil types, and a greater variety of crops and consider the possible retention of urban farmers using fertilizer from invasive plants.