In recent years, the growing number of transmission and distribution lines traversing forested areas have led to an increasing frequency of line tripping incidents caused by wildfires, seriously threatening the safe and stable operation of power grids. Although numerous experimental studies have been conducted on the reduction of air insulation strength in high-voltage transmission lines to ground under wildfire conditions, little attention has been paid to the potential reduction in insulation distance when flames reach the transmission tower, particularly around line insulators. To investigate the air breakdown mechanism under conditions where vegetation flames fully bridged the short gap between a conductor and a grounded plate, experiments on the breakdown characteristics of line-to-plate gaps under various vegetation combustion conditions are conducted. The phase difference between leakage current through the flame and applied breakdown voltage is analyzed, and the breakdown mechanism under vegetation flame conditions is discussed. The breakdown voltages and average breakdown field strengths of the gaps under different vegetation flame conditions are experimentally investigated. The results show that: increasing the gap distance reduces the inductive component within the flame, causing the overall line-to-plate gap to exhibit capacitive behavior; the breakdown process under vegetation flame conditions involves multiple discharge mechanisms, and the duration of the arc channel varies significantly during breakdown; the breakdown voltage increases linearly with gap distance. Due to its high ash content and loose structure, straw vegetation produces long ash particles during combustion, which strongly bridge the gap and distort the electric field. As a result, the average breakdown field strength under straw flame conditions is the lowest among the tested vegetation types.