33. Hydrangea
Hydrangea is a beautiful flowering shrub. However, this perennial is also a fun science experiment for you or your children. The color of the flowers, rather than being determined by breeding like most flowers, is entirely dependent on the pH, or acidity vs. alkalinity, of your soil!
For true blue Hydrangea blooms, you need significantly acidic soil (a pH of 5.5 or less). For pink, the dirt must be neutral to alkaline (pH of 7 or above.) Anywhere in the middle, and you will have pink flowers. Did you realize it worked that way? Nature is amazing!