KI just ran a double gardening event last week, so I don't think we will see it back right away. Maybe in November sometime or December? Also there is a thread here on the message boards called "Double Gardening returning soon with changes", that you may want to read through. It explains the changes that KI has made to this member benefit. They removed the double seed option.
So, any time you see Double Gardening running, you will still get double pet snacks, double garden XP, and double reagents and/or TC's - but not double seeds. Only one.
