Crisis!

[Nadrojj]

Again, google to the rescue!

http://bit.ly/1qEHSdU

Please grab a stack of 99, and fertilize the plants every earth hour. It will not let you over fertilize them. It will give you a message saying they are already sufficiently fertilized. A very quick way to do this is to put the fertilizer in the top left, or first, slot in a tab of your inventory. You then just right click the plant and hit the numberpad 0 key over and over (three times I believe) and it will select the fertilizer and use it automatically.

Found more info here
http://ffxiv.consolegameswiki.com/wiki/Gardening_Guide

Fertilizing shaves an hour off the total grow time. You can fertilize once an hour, meaning that if you fertilize the garden religiously you can shave half the grow time off.