After a period of dormancy and nutrient restoration during the winter, almond trees erupt in stunning light pink and white blooms sometime between late February and early March. Soon after, those blossoms fall and the almonds begin to form on the branches of the tree.
Throughout the spring and early summer months, almonds on the tree look very little like the nuts we purchase at the grocery store. The lightly fuzzy, oval green pods consist of the outer hull with the almond shell surrounding the nut hidden deep inside. The shell and nut inside the hull must be exposed to the environment to become the familiar brown skinned almond that is eaten and used in food processing.
The changes that occur to the hull, called hull split, don’t begin until mid-July to mid-August. The first signs of hull split appear as a crease and slight separation in the side of the green almond hull. As the weeks go by, that crease will become deeper until there is a visible split in the hull, usually about a half-inch wide, exposing the brown almond shell inside.
The hull split allows air to dry the almond. Eventually the hull opens completely. During the split process, the green hull turns to brown, peels away from the nut shell and shrivels as it dries.
Hull split is a critical time for almond farmers. When the shell of the delicate nut inside the hull is put into contact with its environment, this means that it is vulnerable to common pests like the navel orangeworm and to fungi that can cause hull rot. Estimating the timing of hull split is an important part of the growing process. Fruit does not always ripen at the same rate which means hulls can split at different times. Once 95 to 100 percent of the hulls have split the almonds are ready for harvest.
University of California Ag and Natural Resources: https://ipm.ucanr.edu/PMG/C003/m003fchullsplit.html
California Almond Board: https://www.almonds.com/why-almonds/almond-lifecycle#tc-downtime