On August 13, 2015, PopCap wanted the players to vote Imitater or Squash, which one should cost gems for a limited time. On August 18, 2015, Imitater won the competition; and the day after that, he became available in the shop for 159 gems until August 25, 2015. This offer came about again around August 9th, 2016 to August 17th, 2016. The price is the same.
Well that stinks that I missed the opportunity. Ever since I started playing late 2015 I save gems just for that purpose. I thought I managed to unlock all the gem plants but apparently I missed this one. I also somehow managed to miss out on buying gold leaf seed packs. As far as I know they're worthless as we cannot upgrade it, but to own one would be neat!