Very little is known about how products derived from the hydrothermal carbonization (HTC) of municipal waste affect the availability and uptake of nitrogen in plant nutrition. This study examined the effects of 60% sewage sludge and 40% food waste HTC products, i.e., biochar (BC) and process water (PW), as nitrogen sources on garden cress growth and quality. A fertilization program using four nitrogen doses [(control), 9, 12, and 15 kg da−1 N] and BC, PW, chemical nitrogen (CN), and their combinations were used in a pot experiment conducted under greenhouse conditions. The highest nitrogen dose often produced better results in terms of plant growth and quality. Additionally, fertilization with PW+CN and BC+CN at the highest nitrogen dose significantly improved plant height, plant fresh and dry weight, and root dry weight parameters of garden cress over the previous treatments. The highest stem diameter, number of leaves, and plant area values were obtained in the 15 kg da−1 N dose PW+BC application. The vitamin C content in cress decreased with the increasing levels of CN. The highest vitamin C content was obtained with 15 kg N da−1 PW fertilization. BC+PW and CN fertilization applications improved chlorophyll a, b, and the total contents of garden cress leaves. Moreover, the nitrate (NO3) concentration of cress increased with CN doses while it decreased in all BC and PW administrations. The 9, 12, and 15 kg N da−1 doses of PW+CN and the 15 kg N da−1 dose of BC+CN yielded the highest agricultural nitrogen utilization efficiency (ANUE) values. Plant nutrient content was positively affected in all fertilization applications, except for Na and Cl. However, it was determined that BC+CN fertilizer application improved plant nutrient uptake. Surprisingly, PW+CN treatment at the lowest nitrogen dosage resulted in the highest soil organic matter and total nitrogen content. In conclusion, it has been determined that biochar and process water have a synergistic effect with CN to increase plant growth by improving nitrogen efficiency, but their application alone without CN is insufficient to meet the nitrogen requirement.