I am not there, I did not die
At the end of Advent Children, We are shown the place where Zack died, only now, yellow flowers are prolifically growing. Denzel asks Cloud "Is this someone's grave" to which Cloud replies: "No, this is where a hero began his journey" (and then I cried). Now, I really can't imagine Cloud with a spade and trowel planting seeds. So let's assume they are their by divine providence, after all Zack is assured his friend is alright now and he can go back with Aeris to the Lifestream without that nagging thought gee, I wonder if Cloud is holding up alright....
Here is a closeup shot of the flowers growing on Zack's gravesite. Interestingly enough, these same flowers are by the photos of Cloud and the gang. I'm no botanist, but I did grow up in the country and have a bit of a penchant for green growing things. I'm assuming this plant to be in the family Asteraceae, from aster meaning star. This includes daisies, sunflowers, asters. Flowers we are all used to seeing, whether in gardens out driving along the road. So now it's time for, yes you got it, flower analysis!
If the flowers growing over Zack's 'grave' are daisies, then daisies symbolize purity, innocence, loyal love, and patience. The term 'daisy chain' means series of connected events or experiences. I'd say that Aeris, Zack, and Cloud are interlinked. I really think the flowers are daisies: the leaves are similar and the way the capitulum (the yellow center) protrudes out higher than the petals. Daisies really don't need anyone sowing them and fretting over them, they just grow as they please.
Next supposition, the Garland Chrysantheum, which I had to google, as I didn't picture a Chrysantheum looking like...wel a daisy. From the Greek chrys meaning golden, and anthemion meaning flower. Chrysanthemums symbolize optimism and joy. If this is the flower growing over Zack's grave it could also be a clue about his birthday, as the Chrysanthemum is for the flower for the month of November.
Finally, the flowers growing are definitely not Rape Blossoms (nanohana in Japanese), but I thought they deserved honorable mention. These are pretty popular in Japan, as they are a symbol that Spring has arrived. With the word Spring, there are automatic thoughts of 'rebirth'.
Last but not least, Zack's Buster Sword, which in the beginning of AC, had been terribly rusted and used as his grave marker, has been thoroughly cleaned (a symbol of Cloud's regaining his honor perhaps?) and placed in Aeris Church. Okay my eyes are watering, Zack makes me all mushy cry baby emotional. Zack died, but somehow it still feels like a happy ending.