Basically, late April to mid-May here in our area is the best time to thin out, trim and fertilize seedless table grapes.
1. Thin out the bunches so that there is at least six inches of space between each remaining bunch ot table grapes. Late April to mid May is best for this.
2. Cut off the "tails" from the remaining bunches at that same time. This is the lower one-quarter to one third of the bunch, where it begins to taper down in size. This will send more energy to the remaining grapes on the bunch.
3. Fertilize...or not. The book, "The California Master Gardener Handbook" advises that a half pound of ammonium sulfate can be applied for each vine at berry set (when the berries are about a quarter-inch big, usually in May). Remember, though, that too much nitrogen can result in too much vine growth and not enough grapes for eating. Chuck Ingels, Sacramento County farm advisor, says that very little nitrogen, if any is needed. If you opt to use another commercial fertilizer, follow label directions.
4. Water. The same book advises gardeners to apply about 50 gallons of water per week per vine, during the hottest months (June through August) here in the Central Valley. Apply less (about 35 gallons a week) during May and September.
5. Girdle the Stems in late May. According to Chuck Ingels, in the Sacramento County Cooperative Extension brochure, "Grapevines: Girdling and Cluster Thinning to Increase Berry Size":
"Girdling removes the inner and outer bark from around the trunk. The inner bark, or phloem, is where carbohydrates produced by photosynthesizing leaves move to developing organs (including fruit and roots). Removal of a strip of bark prevents the translocation of carbohydrates to the root system thus making more available for fruit growth until the girdle heals in about four weeks. Girdling can make berries about 30% larger if done correctly. If is particularly effective on seedless varieties; no effect on seeded varieties.
How to Girdle Vines: Remove a strip of bark, down to the wood, that is 1/8 to 1/4 inch wide. A double bladed girdling knife makes the job easier. It is essential that all the phloem tissue is removed, so press fairly hard. Check for completeness about 20 minutes afeter the girdle is made. A proper girdle will have the appearance of an all white, fibrous ring of wood (xylem). Remove any brown portions of the ring; if there is even an 1/8" of phloem tissue left, the girdle's benefits are lost. Be sure not to cut so deep as to damage the water conducting xylem and weaken the vine. With a proper cut, the ring should just pop out."