Lanthanum modified organo silicon quantum dots (La-SiQDs) were fabricated by a simple one-step hydrothermal method. The obtained La-SiQDs exhibited good water solubility, excellent stability and strong fluorescence. The effect of La-SiQDs on the growth of cotton sprouts was further explored. Interestingly, the La-SiQDs could promote growth obviously and the optimum concentration was measured to be 0.015 mg/mL, which was much lower than that of the control ones without lanthanum functionalization. Further analysis results showed that La-SiQDs could be adsorbed by the roots of cotton sprouts more easily than that of the control ones, which might be induced by the enhancement of endocytosis in root cells triggered by the La-SiQDs. This work implied that lanthanide modification might be an effective method to obtain silicon quantum dots with higher physiological activity, and held a positive significance for plant physiology and agricultural production.