Top 5 strongest Autoflower strains Zkittlez Auto Zkittles Auto is well-known for being the strongest indica autoflowering strain. It's also one of the fastest, taking about two months to grow. Zkittles has a THC potency of 23%, making it a very potent strain. Haze autoflowering feminized seeds Buy now 5. Black Jack Autoflower / Bubblegum Autoflower We've doubled down on this last one with two of the strongest autoflowering cannabis seeds in our catalog - Black Jack and Bubblegum. Both varieties deliver a whopping 23% THC concentration, but that's where the similarities end.