Seedless grapes are highly prized for easy consumption. Most commercially grown varieties exhibit stenospermocarpy, where the fruit still requires pollination for the initial set. After fertilization, the embryo quickly aborts, preventing the development of a hard, mature seed and leaving behind only tiny, soft traces that are unnoticeable when eaten. Growing these high-quality seedless grapes requires a precise series of advanced cultural techniques that go far beyond standard vineyard management.
Establishing the Vine: Site Selection and Propagation
A vineyard location must receive full, direct sunlight for at least seven to eight hours daily to ensure the fruit can accumulate sufficient sugar content. Soil quality is also important, requiring deep, well-drained, loamy material since grapevines cannot tolerate standing water around their roots. A slightly acidic to neutral soil pH, ideally around 6.5, provides the best uptake of nutrients for the vine. Seedless grapevines are propagated asexually using hardwood cuttings or, more commonly, by grafting a cutting of the desired seedless variety onto a robust rootstock. This method ensures the new vine is a genetic clone of the parent, maintaining the desired fruit characteristics.
Annual Pruning and Training Systems
Once the vines are established, annual winter pruning is crucial for managing crop load and fruit quality. The choice between cane pruning and spur pruning depends on the specific seedless cultivar being grown. Varieties like ‘Thompson Seedless’ have fruitful buds located farther down the shoot, typically between the fourth and twelfth bud, making cane pruning necessary. Cane pruning involves removing about 90% of the previous season’s growth, leaving two to four long, one-year-old canes, each containing approximately 8 to 15 buds.
Spur pruning is suitable for cultivars like ‘Flame Seedless’ that produce fruit closer to the main cordon. This technique involves cutting back the previous year’s growth to small spurs, each retaining only two to three buds. Regardless of the method, the vine must be trained onto a fixed support system, such as a trellis, to elevate the canopy. Training systems like the Vertical Shoot Positioned (VSP) trellis maximize sunlight penetration and air circulation.
Applying Growth Regulators for Fruit Sizing
Achieving the large, marketable berry size expected of premium seedless grapes requires the precise application of the plant growth regulator, Gibberellic Acid (\(\text{GA}_3\)). This chemical application is necessary because the lack of developing seeds means the fruit does not produce the natural hormones required for optimal berry growth. The treatment is typically split into two or three applications timed to specific physiological stages of cluster development.
The first application targets cluster elongation, often called rachis stretching, and is applied just before bloom at the “parrot green” stage. Using a low concentration, typically 10 to 20 parts per million (ppm), this treatment lengthens the cluster stem, creating space between the individual flowers. This step prevents the final berries from becoming overly compacted, which can lead to crushing and a higher incidence of fungal diseases.
The second \(\text{GA}_3\) application occurs after fruit set, once the tiny berries have reached a diameter of about three to five millimeters. This post-bloom treatment is aimed at berry sizing and requires a much higher concentration, often ranging from 40 to 100 ppm, though specific rates depend on the cultivar. The hormone stimulates cell expansion within the berry, resulting in a commercially desirable size and shape. Growers must follow recommended application rates precisely, as incorrect timing or concentration can lead to undesirable effects, such as overly elongated berries or reduced fruit set in the following season.
Management of Clusters and Harvesting
Even with successful growth regulator application, growers must actively manage the crop load to ensure all remaining fruit reaches peak quality. Cluster thinning is performed early in the season, soon after fruit set, and involves removing excess clusters to leave only one, or occasionally two, per shoot. This removal balances the vine’s energy, allowing the leaves to focus photosynthetic sugars on ripening a smaller, higher-quality crop.
A secondary, labor-intensive process called berry thinning is often required for premium table grapes to further enhance quality. This involves manually removing individual berries or clipping off the lower third of a cluster when the berries are still pea-sized. This creates more space within the cluster, preventing fruit-to-fruit contact, which reduces the risk of bunch rot and promotes uniform sizing.
Harvesting is timed not by calendar date but by taste and chemistry. Table grapes are generally considered ripe when their sugar content, measured in Brix, reaches between 16 and 20 degrees. The fruit should also exhibit full color and balanced flavor, indicating the final stage of maturity before being carefully cut from the vine.