I did this with a 7x7 grid of 13 flowerz on a 'Standard' game. Here's a link to a rather crass image of the + formation I created in the center of the play area, apologies for poor quality:http://firstname.lastname@example.org...
y = Outer colour of your choosing (inner colour doesn't matter)
z = A different outer colour to 'y' but inner colour must be 'y'
x = Exactly the same inner/outer combo as 'z' but MUST be placed as the very last piece to create the cascade
You can't get 7x7 free spaces until level 2 due to the rocks on level 1. Flowerz carry over from one level to the next so level 1 is crucial in setting the sequence up (remove unwanted flowerz as you will need the empty space in level 2 and get 4 of your 'y' flowerz in position).
y = yellow outer (inner doesn't matter)
z = purple outer, yellow inner
x = purple outer, yellow inner (MUST BE PLACED LAST)
Placing 'x' would remove 5 purple flowerz in a + formation. These would all be replaced by yellow flowerz creating a much bigger + formation of 13 flowerz. Achievement unlocked!
Hope this helps some folk.