Undertow is an achievement in Plants vs. Zombies 2. It requires the player to kill ten zombies with Tangle Kelps. It is worth 10 points on Game Center for iOS devices and 1,000 XP on Google Play Games for Android devices.
The word "Undertow" means the steady current, below the splash zone, moving offshore for waves approaching a shore.
- Main article: Big Wave Beach - Day 5
Day 5 practically cannot be beaten without earning this achievement. The same applies to the achievement Banana Storm. The full article about Day 5 is recommended to be read.
If you do not want to use Day 5, you can also feed Plant Food on a Tangle Kelp three times.