One of the most striking plants in your backyard is also one of the most poisonous. Every part of the plant is highly toxic, so if you accidentally ingest even a small portion, you’re out of luck. According to Mount Sinai, a single leaf may kill an entire adult. A study published in 2010 found that the oleander plant contains cardiac glycosides, which cause digestive issues and acute cardiac toxicity. This will send your heart into an erratic rhythm or send you into a coma. People who ingest the flower also suffer from low blood pressure, weakness, blurred vision, or hives. The plant is mainly found in California (National Library of Medicine).