Nice vine, bitter grapes

Q. I have sent three pictures of our grape vines.  We have been getting a few bunches of grapes but they are small and bitter. How do we get them to get larger and sweeter?

A. Many grapes do well in the Mojave desert climate. Some perform better than others. Some perform Much better than others. A problem we have with grapes is uneven ripening of the berries in a cluster. This means that some of the berries will be ripe earlier than others. We see the ripening of the berries as a change in its color. Technically this is called veraison. 

Uneven ripening is still a problem with grapes in general but climates with high temperatures it is more of a problem. In cooler climates, particularly with cooler nights, berries mature much more evenly. 

Make sure that you have a variety of grape that performs well here. I noticed you did not mention which variety you have which makes me a little suspicious. All of the grapes that will do well here will mature, turn color (green grapes turn a light green or yellowish green while red or black grapes turn their respective colors; the yellow green grapes in the second picture look ripe to me) in the correct season. This can range from June through August or September depending on the variety. Your vines look healthy but healthy vines do not necessarily deliver a delicious grape. Make sure you wait long enough so that the fruit fully matures. In our climate grapes perform better with aced surface mulch of wood chips covering our desert soils. My suspicion is either the wrong variety or you are not waiting long enough until they mature when you're harvesting. The individual berries in the grape clusters do not all ripen at the same time. I usually have to harvest them when a few of the berries are actually overripe. Once grapes have been harvested, they will no longer improve in flavor or sweetness, unlike plums, peaches and even bananas.