Allium cepaL.) are easily outcompeted by weeds because of slow germination and relative growth rates. Therefore, high percentage of seed germination and root vigor are important traits to improve field performance. The effects of exogenous plant growth regulators (PGRs), 2-chloroethylphosphonic acid (ethephon, Eth), indole-3-acetic acid (IAA), trans-zeatin ( tZ), and 1-aminocyclopropane-1-carboxylic acid (ACC) were evaluated on the germination and root growth of Don Victor (yellow) and Lambada (red) onion seedlings. Seeds were soaked for 10 hours in hormonal solutions and water (hydro-priming). Seed germination improved with Eth (30 and 100 m), Eth (100 m) + IAA (10 m), and IAA (3 m) treatments. Root surface area (RSA) increased in response to Eth at 30 and 100 m, Eth + IAA, and 3 mIAA. Root length (RL) and root diameter (RD) were enhanced by 1 m tZ and 100 mACC. Eth reduced RL and RD, whereas IAA showed no effects. A subsequent experiment evaluated synergistic effects of different PGRs. Treatment of seeds with ACC (250 m) + tZ (0.5 m) and ACC (250 m) + tZ (0.5 m) + Eth (20 m) enhanced RL and RD. RSA was unaffected by ACC + tZ + Eth. The results suggest that exogenous PGRs could be useful to enhance germination, RL, and RSA of onion seedlings.