Garlic bulb yield formation depends on thermal-time availability, source–sink coordination, and field management, but their integrated effects under field-scale conditions remain unclear. This study evaluated garlic growth and final bulb weight (BW) during the 2024–2025 season across 26 field plots in Uiseong, Korea, using cumulative growing degree days (GDD), longitudinal canopy and bulb measurements, correlation analysis, PCA, and exploratory Random Forest (RF)-based variable importance. Delayed sowing reduced cumulative GDD at harvest by approximately 45 °C·d and was associated with lower final BW. Total plant weight (TPW) peaked around 692.2 °C·d, whereas BW continued to increase toward harvest, indicating temporal separation between whole-plant biomass accumulation and bulb filling. At harvest, BW was most strongly associated with bulb diameter (BD), while canopy traits showed weaker or stage-dependent relationships. RF analysis using the harvest-stage BW subset (n = 78) suggested that early canopy and structural traits, particularly LN, PH, SL, and SD, were associated with BW variance partitioning; however, importance values were interpreted as exploratory rankings rather than causal evidence. Overall, garlic bulb yield formation under the present field conditions appeared to be associated with thermal-time availability, sink morphology, early structural status, and localized management conditions; however, plot-level treatment comparisons did not detect significant independent treatment effects, indicating that these patterns should be interpreted as descriptive field-based trends.