by a script
there is this very useful script (by @carpiediem) :
Memrise Preview & Auto-Grow Buttons
It will skip planting session even if you didn't create the course yourself
In the address bar
You can alo click "continue planting" on any level and change /grow/ to /auto_grow/ in the address bar. On firefox and chrome a box pops up asking if you want to stay on the page, click leave and it plants the words automatically.