The winter months are an important part of the California table grape growing cycle. Growth and development stop temporarily and the vine rests. This stage is called “dormancy.” At this time, growers prune the vine and set it up for the upcoming growing season. Pruning and training of the vine are two of the most important aspects for quality grape production – growers decide how much and which parts of the previous season’s growth to remove in order to regulate vegetative growth (shoots and leaves) and crop load (grape clusters) to produce quality grapes and optimum yield.
In early spring tiny buds on the vine start to swell and green leaves appear. Appearance of the first green leaves through the bud scales is called bud break. Growth is slow at first. As the mean temperature rises, growth and shoot elongation accelerate. After three or four weeks, the period of most rapid growth begins – where shoots can grow an average of one inch or more per day. As the days warm up, flowers bloom, then shatter to make way for the tiny green grapes that will eventually ripen into clusters. Berry size increases rapidly. Sunlight and warm temperatures are vital to the physiological functions of the grapevine (such as photosynthesis).
The point in the growing season when ripening grapes begin to soften is called “veraison.” During ripening, colored varieties gradually change color from green to either red or black, while green varieties become translucent. Sugars start to accumulate in the berries. The interval from veraison to harvest is different for each variety. Unlike many fresh fruits, grapes are harvested fully ripe. After they’re picked, they do not become sweeter, so timing is everything.
Dormancy is an important stage of the grapevine annual cycle when growth and development stop temporarily and the vine rests. It is brought on by low temperatures and shortening day length.
At this time, growers prune the vine and set it up for the upcoming season. Pruning and training of the vine are two of the most important aspects for quality grape production – growers decide how much and which parts of the previous season’s growth to remove in order to regulate vegetative growth (shoots and leaves) and crop load (grape clusters) to produce quality grapes and optimum yield.
Two principal methods of pruning are spur and cane. It is important for growers to know their crop because some varieties produce a better crop (in terms of yield and quality) with cane pruning while others produce a better crop with spur pruning.
Spur Pruned Vine
Tiny buds on the vine start to swell and green leaves start to appear. Appearance of the first green leaves through the bud scales is called bud break. The first shoots start to grow powered by the energy derived from carbohydrates that were stored in permanent vine structures (roots, trunks and cardons) during dormancy.
Spur pruned vine with bud
Close up bud break
Green leaves on vine
Growth is slow at first. Leaves and shoots expand and cluster florets develop. As the mean temperature rises, growth and shoot elongation accelerate. After three or four weeks, the period of most rapid growth begins – where shoots can grow an average of one inch or more per day. Photosynthesis occurs as soon as there is a green tissue on the shoots, however, due to the high metabolic activity, there is no net production of assimilates until several leaves have fully expanded.
Vineyard in spring
Young shoots and developing flower cluster
Young shoot and flower cluster
Grapevine flowers are born in a cluster (or bunch). The main axis of the cluster is called the rachis. When spring temperatures rise to 68 F the flowers typically begin to bloom. The time between bud break and bloom is usually six to nine weeks, depending on the temperature. Individual flowers of California table grapes are small, greenish and usually perfect – which means they have both male (stamens) and female parts (carpels). The calyptra (or corolla) is a covering tissue for stamens and carpels and it is made of three to nine greenish petals firmly united at the tip.
When conditions are favorable, the flowers typically bloom for eight to ten days. To the casual observer the opening of a grape flower may seem to be very different from that of most other flowers: the calyptra becomes detached at the base instead of separating at the tip. It is shed entirely, as a cap. Immediately after blooming, the pollen sacs open and release pollen and pollination occurs.
Fertilization occurs two to three days after pollination. The fruit set stage follows flowering almost immediately, when the fertilized flower begins to develop a seed and grape berry to protect the seed. Most California table grape varieties are seedless – soon after fertilization the seed stops developing, resulting in seedless berries. Grape berries pass through several growth stages from the time they are set until they are fully ripe.
Vineyard appearance during flowering
Stems and carpels
Berry Formation to lag phase (Berry Development Stage 1)
Initial period of growth is rapid, due to cell division and cell enlargement. The grape berries are green and hard to the touch and enlarge rapidly. They have very little sugar and are high in organic acids.
Berry formation to lag
Berry formation to lag
Berry formation to lag
Lag phase to Veraison (Berry Development Stage 2)
In the lag phase overall berry growth rate has slowed. At the start of the lag phase, berries have reached at least half of their final size. The lag phase is less prominent in seedless varieties compared to seeded varieties. In this period the berries reach their highest level of acidity.
The ripening stage begins when berries begin to soften and the color begins to change. In green varieties the color begins to fade and in colored varieties the red or black color begins to appear. This sudden change in color and berry softening is also known as “veraison.”
Lag phase to veraison
Lag phase to veraison
Lag phase to veraison
Translucent berry and beginning of color development
Post Veraison Berry Ripening (Berry Development Stage 3)
During the ripening phase the grape berries begin to accumulate sugars, while acidity decreases. Berries are becoming softer and they are rapidly increasing in size, due to cell enlargement. Skin becomes translucent in green varieties, colored in red and black varieties, and the characteristic aroma develops.
The period of ripening is determined by variety and temperature.
The changes in sweetness, acidity and other constituents begin to decelerate when berries are ripe. The rate of change differs by variety. The color of the berries is fully developed. When the fruit has reached its full potential, harvest begins.
Beginning of color development
Ripening – color development
Ripe grape berries
Unlike many fresh fruits, grapes are harvested fully ripe. After they’re picked, they do not become sweeter, so timing is everything.
Determining when grapes are ripe is a real science and both the U.S. Department of Agriculture and California Department of Food and Agriculture are involved in setting and monitoring grape production standards. Sugar content, color, bunch and berry size and uniformity are all measured before harvest begins and the workers who decide which grapes to harvest are trained professionals with years of experience.
Once picked, fresh grapes are easily damaged by rough handling, warm temperatures, excessive moisture and decay-causing organisms. Consequently, grape bunches are carefully inspected and then immediately packed by hand into shipping containers – often right in the field.
Shortly after picking/packing, the field heat is removed from the fruit in cold storage facilities. Grapes are stored at temperatures between 30 F and 32 F. From this point until they reach their destination – markets throughout the world – the grapes will be maintained in a carefully regulated environment to assure they arrive in just-picked condition.
After the grapes are harvested, the vine continues the process of photosynthesis, creating carbohydrate reserves to store in the vine’s roots, trunks and cordons until an appropriate level of reserves has been stored. At that point the chlorophyll in the leaves begins to break down and the leaves change color from green to yellow. Following the first frost the leaves begin to fall as the vine starts to enter its winter dormancy period. The stored carbohydrate reserves will be used the following spring to support the initial growth.
ABOUT California TABLE GRAPES
Californians have been cultivating grapes for more than two centuries. Today, 99 percent of U.S. table grapes are produced in California's warm, dry climate that is ideal for grape growing. With more than 90 grape varieties grown, California grapes come in three colors—green, red and black—and are in season from May through January.