Abstract: Thermochemical technology offers a promising approach for rapidly converting waste biomass into high-value products such as biochar. However, biochar produced through different methods varies considerably in its properties and potential effectiveness as an amendment during plant cultivation. In this study, tomato straw collected from a facility greenhouse after harvest was used as feedstock for biochar production. Three thermochemical techniques were employed: medium-temperature slow pyrolysis (500 ℃, 2 h), superheated steam-assisted torrefaction (350 ℃, 10 min), and hydrothermal carbonization (190 ℃, 2 h), yielding biochar labeled as pyrolyzed biochar (PYBC), torrefied biochar (TOBC), and hydrothermal carbonized biochar (HCBC), respectively. The physicochemical properties of the biochar were analyzed, and their effects on the growth of tomato plants (Micro Tom) were further evaluated following incorporation into the growing substrate. The results indicated that both PYBC and TOBC were beneficial to plant growth. Notably, TOBC at a mass ratio of 10% showed the most pronounced effects, which significantly increased plant height, stem diameter, and leaf area (P < 0.05). Compared with the control without biochar (CK), the 10% TOBC treatment increased plant biomass by 86%, and raised the number of fruits per plant and fruit yield by 39% and 110%, respectively. In addition, both PYBC and TOBC significantly improved fruit quality, elevating soluble solids by 7%～28%, lycopene by 20%～54%, and vitamin C by 12%～24%, respectively. Compared with CK, TOBC further enhanced soluble sugar content (by 8%～17%) and sugar-acid ratio to approximately 7.55 while reducing fruit hardness (by 8%～17%), demonstrating its potential to improve the nutritional value, flavor, and texture of tomatoes. A significant (P < 0.05) or highly significant (P < 0.01) positive correlation was observed between biochar electrical conductivity (EC) and plant height, stem diameter, leaf area, and yield. Among the three biochars, TOBC had the highest EC value, aligning with its ability to supply sufficient mineral nutrients for plant growth. Significant positive correlations were also found among biochar EC, plant growth parameters (height, stem diameter, and leaf area), and key fruit quality components such as soluble solids, soluble sugar, and vitamin C. These findings support the hypothesis that TOBC enhances plant growth by providing mineral nutrients, thereby facilitating the accumulation of photosynthetic assimilates and quality-related compounds in fruits. In contrast, HCBC negatively affected plant growth, reduced fruit vitamin C content, and increased nitrate concentrations, indicating potential health risks associated with dietary intake. The NH₄⁺-N content in biochar was negatively correlated with stem diameter, leaf area, and vitamin C content, but positively associated with nitrate content. This suggests that relatively high NH₄⁺-N content in HCBC may adversely affect tomato plants, leading to metabolic imbalances that compromise growth and fruit quality. Principal component analysis identified the 10% TOBC treatment as optimal for enhancing both tomato yield and fruit quality. Therefore, superheated steam-assisted torrefaction represents an ideal method for the rapid disposal of tomato straw waste, while simultaneously generating biochar suitable as an effective and sustainable substrate additive. These findings provide a theoretical foundation for the efficient utilization of tomato straw and its biochar, supporting the transition toward a circular bioeconomy in agricultural systems.