Fungus Humongous is an achievement in Plants vs. Zombies 2. It is achieved by planting 20 Puff-shrooms and using Plant Food on one of them. It is worth 10 points on Game Center for iOS Devices and 1,000 XP on Google Play Games for Android devices.
Its name is based on a species of fungus known as humongous fungus. It might also parody the phrase, "Fungus among us."
Even though this is considered an easy achievement, there are some things to bring that can help. Having the Imitater is useful to quickly get to the number of plants needed to earn you the achievement faster and having a Power Lily (especially along with Imitater because of its slow recharge) can help refresh Puff-shrooms to keep them alive as well. If neither of these plants are owned, usage of a defensive plant like the Wall-nut can help to delay the zombies long enough to be able to plant all 20.
The easiest way to obtain this achievement is to play Dark Ages - Night 12. Since you already have five Puff-shrooms that you have to protect using Plant Food as part of the winning strategy, only needing 15 more should not prove to be much of an issue. There are three other Dark Ages achievements obtainable in this level (if you bring Magnet-shroom) if you feel up to the challenge.
Another easy way is to play Ancient Egypt - Day 2. Puff-shrooms can be used as your only primary shooting plant. The extra sun may be used on instant-kill plants if you do not want any Puff-shrooms to be eaten too early from your set-up. Only use the given Plant Food right when one of the Puff-shrooms starts to fade away, if you have not planted enough of it. Another way is making a boost packet for it or buying Plant Food.
This is also useful in Endless Zones where the Puff-shroom must be boosted, so its duration will always reset. Just keep on planting Puff-shrooms as you hold off the horde for a bit, but do not be too overconfident, especially if the level is composed of zombies like Wizard Zombie, Jester Zombie, and Imp Cannon. Once enough Puff-shrooms are present, buff one of them with Plant Food to get your achievement.
- It is one of four rhyming achievements in Plants vs. Zombies 2. The others are Pod Squad, Porter Thwarter and Tile Denial.