While many cacti are cherished and carefully protected due to concerns over extinction in recent years, the unfortunate "most unwanted cactus in the world" title goes to Opuntia stricta. This species is actually listed among the "100 of the World's Worst Invasive Alien Species" by the IUCN. Historically, dating back to the era of Columbus, these cacti were planted all over the globe for the red dye, cochineal, harvested from scale insects living on them—which led to explosive population growth. They're incredibly resilient: tough to kill, quick to break apart, and eager to multiply. Carelessly planting such a vigorous survivor can cause chaos! The situation is especially severe in Australia, but battles also continue in South Africa, the Caribbean, and Europe. There are no official reports of it going wild in Japan, but a quick online search suggests it's not entirely unheard of. If you have concerns, contact your local city office or town hall's environmental department.

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  1,800JPY