Lost seeds

When playing the game, the killed zombies can drop coins or sprouts. You can plant the sprouts in your Zen Garden and a random plant grows from it in a few hours (the time can be shortened by spending gems). A regular plant grown this way becomes boosted the next time (only) when you use it in the world levels, in Endless, or in Penny's Pursuit (not in the Arena, though). If the plant is Marigold, once it grows up it is converted to 500 coins.

Since 500 coins is a pitiful amount (you can get 500 just by watching a 30-second ad instead of waiting for several hours), many people don't bother farming the Zen Garden for coins and using boosted plants is rarely necessary (with the possible exception of the last level of the Wild West Wipeout quest, where having a boosted Hypno-shroom makes the level almost bearable). So, the sprouts keep accumulating. Personally, mine increase by about 100 per day.

Like all other numbers in the game, the number of sprouts you can have has an upper limit. However, while for things like coins, gems, mints, and gauntlets, the limit is huge (2^31-1, or more than 2 billion), the maximum number of sprouts that you can have is much smaller – only 2^15-1, or 32,767.

Since the limit is relatively low, it is very possible to hit it while playing the game normally – I hit it in about a year. However, there is a bug. Once you hit the limit, the number of sprouts you have doesn't just stop increasing – it resets to 0, so you essentially lose all your sprouts. Oh, well, at least it doesn't go negative.