Biochar can affect plant growth and development, but how biochar affects flowering is unknown. To study the effects of biochar in soil on Arabidopsis (Arabidopsis thaliana L. (Heynh.)) flowering, rice husk biochar was prepared at three pyrolysis temperatures (350, 500, 650°C) and two residence times (1 and 2 h). Nutrient elements and organic compounds were measured in different biochars. Biochar leachates were also obtained. Arabidopsis flowering times, vegetative and reproductive growth, photoperiod pathway and the gibberellin 3 (GA₃) pathway-related gene expression, and endogenous GA₃ were measured. Different biochars had different types of nutrient elements and organic compounds. Compared without biochar, biochars prepared at higher pyrolysis temperatures (500 and 650°C) and a longer residence time (2 h) led to earlier flowering, increases in vegetative and reproductive growth, higher expression levels of photoperiod and GA₃ pathway-related genes, and higher endogenous GA₃ content. Arabidopsis treated with biochar prepared at 500°C for 2 h was the first to flower. Biochar leachates had slightly different effects from those of biochar, but those from biochar prepared at 350°C for 1 h and 500°C for 1 or 2 h also promoted flowering. The organic compounds contained in biochar can promote flowering in Arabidopsis. Biochar prepared under different conditions contained different organic compounds and therefore also had different influences on flowering. One of the reasonable mechanisms for biochar-promoted flowering in Arabidopsis was the upregulated expression of downstream gene APETALA1 (AP1) in the photoperiod and the GA₃ pathways.