How Do Grapes Ripen?

Flowers

• Grape flowers are small and inconspicuous and are purely formed to perform the function of self-pollination. There is no need to attract pollinating insects with vivid color, sweet aromas and tasty nectar, such as an apple blossom is designed to do. There is one exception: Muscadine grapes have only female or male flowers; you will need to plant at least one male and one female vine for pollination to occur.

Pollination

• Each cluster on a vine has between two and 150 flowers, with 100 being an average. Taking a closer look at one of these flowers, you will notice that the petals are fused together into a tight little cap that covers the stamens and stigma. When the flower blooms, the cap breaks off. This jolting jiggles the thin stamens. This releases pollen on the stigma of the flower. Pollination is reliant on temperature, because the tops cannot pop off under 60 degrees F. Temperature also plays a key role in the movement of pollen down the pollen tube. Cooler temperatures slow the movement down the tubes, and they don't reach the ovule in time to fertilize the egg. At 60 degrees F, it takes five to seven days to reach the ovule; at 90 degrees F, it will reach it in a matter of hours.

The Role Grape Seeds Play in Ripening

• The number of seeds in your grape greatly influences the eventual size of the berry. Seeds produce hormones that stimulate the growth of the berry. You may wonder why seedless grapes from the store are just as big as seeded grapes; this is because commercially grown seedless varieties are sprayed with a growth hormone to induce growth. If you will be growing your own seedless varieties at home, don't expect them to get as big as store-bought seedless grapes.

Reaching Maturity

• Once the berries have set, it enters a phase of rapid cell division. Three to four weeks later, cell enlargement begins. At this stage the berries are small, firm, green and sour. The seeds will then begin to develop after five to seven weeks. The next stage is ripening. All grapes start out green, so don't worry if you planted a blue variety and it's still green. The next stage involves coloring and sweetening. Grapes will soften and accumulate sugar at a rapid pace, and should be checked consistently for ripeness.