Answer: There are two ways grapes can be seedless. The first is that the variety is parthenocarpic, which means that the plant sets fruit without any pollination. The second is that the flowers of the variety need pollination, get it, and then the seeds abort, leaving just a trace of their existence. 'Thompson Seedless'is an example of the latter type of variety.
Since seeds begin to develop and then naturally abort, your grapevines are somehow missing the hormonal message to stop developing seeds. This can be due to applications of growth hormone (such as herbicides) in the vicinity of your grapevines, or simply the genetic makeup of your plants.
Next spring you can treat your vines with gibberellic acid to stop seed formation or you can girdle the stems of your plants. The girdle cuts the phloem and temporarily prevents movement in the phloem from the upper part of the vine to the roots. This traps carbohydrates and hormones above the girdle and stimulates seedless berry growth.
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