Why do the grapes I grew taste sour?

Are you sure your grapes are ripe?

Are there what seems to be too many grapes on the vine?

Are your grapes growing in shady conditions?

Have you been fertilizing your grapes for fruit production?

Unripe grape fruits will taste sour. Before harvesting, carefully examine your grapes. They should have a full, deep color all around. Green grapes will lighten and almost turn yellowish. Ripe fruits will have tan to brown seeds. When you feel the grapes, they should be plump, thick, and feel like they are full of juice.

The grape vine plant produces sugar through the photosynthesis process. If there is too much fruit on the vine, the plant won't be able to produce enough sugar and the grapes remain sour. To fix this problem, thin out the fruits on the vine. Removing about one-third of the fruits should suffice.

For grapes to produce enough sugar for the fruits, the foliage needs to receive lots of sunlight during the growing season. Try to give your grapes more light by thinning any surrounding plants that may cast a shadow on the grapes.

Like all plants, grapes need nutrients to develop ripe and healthy fruits. If the vines aren't growing satisfactorily, apply a high-nitrogen fertilizer in early spring. High-phosphorus fertilizer should be applied during the growing season for better fruit production.

Some grape varieties that are grown mainly for vine production have naturally sour fruits. There is nothing much you can do about it in this case. Find out if the grapes in your garden or vineyard belong to the varieties that are naturally sour, and, if so, all you can do is plant a new grape variety that is naturally sweet.