The effect of introduction strategy for living mulches (LMs) to reduce the potential nitrate leaching was investigated when grown with vegetables with high nitrogen (N) demand and low use efficiency in four European countries: Italy, Slovenia, Germany and Denmark over 2 yr. The plant N uptake and soil nitrate N content at harvest, in the autumn, and in the following spring were measured below open-pollinated and hybrid cultivars of cauliflower and leek crops. The cultivars performed differently over the 2 yr. In Italy and Slovenia, when LM of burr medic or white clover was broad sown (addition design ADD) at the same time as crop transplanting, the N uptake of crops was hampered without increasing the total aboveground N accumulation. Delaying LM sowing by 2.5–4 weeks maintained the N uptake of crops. In Germany, delaying white clover sowing for a month in cauliflower reduced soil nitrate at the start of leaching by 17–33 kg N ha−1 in the ADD design and 25 kg N ha−1 in the substitution design (SUB), where rows of cash crops were replaced by rows of LM. An overwintering LM of grass–clover incorporated in strips and root pruned decreased soil nitrate at the start of the leaching period by 35 kg N ha−1 in cauliflower, and in leek, an LM of dyer's woad decreased the soil nitrate by 55 kg N ha−1 at harvest and 30 kg N ha−1 at the start of leaching. The effect of delayed LM sowing depended on the LM species and system design. Overall, the SUB and ADD designs showed potential to reduce nitrate leaching, whereas the ADD design had stronger competition against either cash crop or LM. The key to reduce soil nitrate N without jeopardizing crop yields may be to identify suitable growing periods, sometimes combined with root pruning, for each LM species and system design. The LM introduction strategy can be used to control competition and reduce the potential leaching, but the performance depends on the intensity of the interspecific competition and the local conditions.