Commercial growers apply plant growth "hormones" -- gibberellic acid -- which develops the extra lager berries.

Not available to home gardeners.

Good to know I was just wondering the same thing. I thinned them out last year and they were slightly bigger, but not grocery store size.

You spray gibberillic acid first to thin out the berries and then later to make them bigger. Commercial growers also often girdle the vine to size up the berries. You could do all this but it's not worth the effort and if done wrong can hurt the vine or next years crop.

Hi! I am hoping that someone can help me!! I planted Thompson Seedless, Red Flame and Cabernet Vines on an east & north facing wall. I have them tied to a cedar split rail fence to keep the heat from radiating and trying the vines and for air circulation. Up until the 110 degree temps they have been flourishing. Now I have no new growth, the grapes have not grown larger and are actually shrinking. My leaves are turning yellow and drying. I inspected the leaves & found no bugs. We water in the morning and keep the soil damp in hot weather. My Thompson seem the less vigorous, the Cabernet are actually the more successful, then the Red Flame. I would like to ask what is the proper watering schedule for Casa Grande AZ zone 4? Also, I am wondering if I should be mulching the roots to keep them cooler?

LuvSonshine:

It sounds like your grapes aren't getting enough water. Watering every day may be causing you to apply too little and not wet enough soil. Grapes are very drought and heat tolerant IF they have a big root system. To get a big root system you need to wet a big area of soil. That works best with a deep watering usually no more than once a week. Mulch in your climate is a big plus.

Your plants may be competing with bigger better established plants nearby. Big trees even 40-50ft away may be robbing the water from your grapes. You must apply enough for all plants rooted in that area. Established plants have a huge advantage over newly planted ones.

Your grapes are in the wrong location... meaning they should be in California! You should be happy with what you get and enjoy them... REALLY... size is not important.