Q. A few months ago you had an article about grapes not growing to normal size. You stated the soil in Southern Nevada lacked a certain hormone that the grapes required. My Thompson vines grow like weeds but the fruit is the size of peas.
A. The hormone is not in the soil but manufactured by the plant. Purchasing and applying this hormone is one of three methods that can be used to encourage larger berries.
The application of this hormone to grapes is permitted for organic production in most cases. However, there are two other methods that will enlarge berry size without the application of this hormone.
These other two methods follow the same principle used for producing larger fruit in tree crops: remove fruit from the plant so the remaining fruit becomes larger. This totally organic practice is called “thinning”.
|Pinching the bottom third off of the bunch|
When berry clusters or bunches first emerge in March or early April, thin or remove bunches so only large ones remain spaced no closer than a foot apart. Remove small bunches totally.
Second, when grape individual berries are about the size of a BB, pinch off the bottom third of the bunch. Bunches of grapes are normally triangular in outline. Pinching the bottom third of the bunch produces a harvestable bunch that is round in outline with much larger berries.
After those practices have been followed, you can use a plant hormone called gibberellin to increase the berry size artificially. This is not a natural process but you are basically giving the berries a "kick in the pants" to get them to elongate more than they would normally.
Warning. Don’t expect gibberellin alone to do the work for you. You must also thin out your grape bunches.