My boyfriend's parents used to amuse themselves by telling him that the rhubarb plants in their garden were triffids (as in 'Day of the Triffids').
They weren't laughing so much when, in terror, he ran them down with his tricycle...
A childhood belief about plants

others believed things like this
- My mother convinced all of us kids in our family that the large rhubarb in the garden was a Man-Eating Plant. — H.Miller
- I thought plants only grew when people were not looking at them.Anon
- I used to think heirloom plants grew antiques— Honorary intern Hector
- I used to believe that when people died, they turned brown just like plants do when THEY die.— Joey Schwartzman
- I used to thing power plants were cloud factories. xD xD— Kaitlin
- Before I went to a planetarium for the first time, I thought it was just a room full of plants. Anon