Granny Smith apples reach ripeness between 165 and 185 days after the tree's flowers are in full bloom. This is usually between September and November each year.
Since Granny Smith apples remain bright green all over, even after they have ripened, it is difficult to tell by appearance alone when they are ready to pick. Farmers have multiple methods for determining when their apples are ripe. They can start counting the days from a tree's full bloom. They can sample the apples periodically starting in late October until the fruits develop their signature sweet-tart taste. Farmers can also spray an apple with an iodine solution to test ripeness.
Spraying a sample Granny Smith apple with a mixture of 1 to 15 parts iodine and water helps show how much of the meat is still starch. Granny Smiths that are ripe have mostly converted their starches into sugars. Farmers cut an apple in half and spray it with the solution. If the inside of the apple remains white with only a thin black line near the outside flesh, then it is ripe. If the apple turns mostly black inside, it is not ready to harvest. Farmers repeat this test periodically during the fall until their apples have fully matured.