Unwelcome Mat is an achievement in Plants vs. Zombies 2. It is achieved by planting ten Red Stingers in the two leftmost columns. It is worth 5 points on Game Center and 500 XP on Google Play Games for Android devices.
The achievement's name is a portmanteau of "unwelcome," referring to the zombies and "welcome mat," referring to a type of door mat commonly sold that has the word "welcome" imprinted on their design.
Lost City - Day 3 is the best level to achieve this, as the player will be given even more than ten Red Stingers by the end of the level. Any Last Stand level (except Wild West ones which have minecarts and rails) are also good.