The Best Time, Key Technical Points and Functions of Grape Cluster Elongation (Berry Thinning) Explained

During grape cultivation, many growers have found that when grape clusters are too compact, it easily leads to uneven coloring, berries squeezing and deforming each other, fruit cracking and rotting, and other symptoms that affect the appearance of the grapes.

Some European-American grape varieties, after fruit-setting treatments, often set a large amount of fruit, making later thinning troublesome and labor costs very high, so cluster elongation treatment before flowering has gradually been applied in grape production. So what is the best time and key technical points for grape cluster elongation?

Functions of cluster elongation

Reduces the later work of flower and fruit thinning, saving labor.

Reduces cluster compactness, which is beneficial for later berry enlargement.

Can enhance the effect of light intensity on flowers, allowing “fruit setting by borrowing light.”

Increases the gaps between flower clusters, which helps reduce pest and disease problems.

Timing for grape cluster elongation

Application period: The degree of inflorescence elongation is related to the application period. If applied early, the inflorescence elongates more; if applied late, the elongation is not obvious. Generally, it is suitable when new shoots grow to 25-750px long (6-7 leaf stage), the inflorescence is 7-10 cm, that is, 20-25 days after bud break, and about 20 days before flowering.

Which grape varieties need cluster elongation?

Although elongating grape inflorescences has many benefits as mentioned above, not all grape varieties are suitable for it. For example, after cluster elongation, Ruby Seedless has longer clusters, but it produces large and small berries as well as small stunted berries, which are not easy to thin, and instead increase the labor for fruit thinning. So which grape varieties actually need cluster elongation?

Suitable varieties

1. Large-berry varieties: Red Globe, Lady Finger, etc.

2. Tight-cluster varieties: Red Globe, etc.

3. Varieties with high fruit-setting rate and smaller inflorescences: Summer Black, etc.

Varieties not suitable for inflorescence elongation

1. Varieties with poor fruit setting such as Kyoho are not suitable for inflorescence elongation.

2. Varieties with good fruit setting and vigorous new shoot growth are also not suitable for inflorescence elongation; otherwise, fruit setting will be unsatisfactory, clusters will be loose, and marketability will be affected.

In addition, in the hot and humid southern regions and under protected cultivation, inflorescences generally elongate naturally, so there is no need to deliberately elongate them at that time.

Introduction to cluster elongation methods

At present, cluster elongation methods mainly include fertilizer and water management for elongation and chemical agent elongation.

Management-based cluster elongation

Main characteristics: relatively safe, but slow and with inconspicuous effects, and greatly affected by external conditions, especially temperature.

Specific method: The fertilization time is usually chosen about 15 days before flowering, applying urea plus water-soluble fertilizer containing biostimulants (humic acid, seaweed extract, amino acids, etc.), such as Sea Elf biostimulant, as a drench.

Existing problems: It requires certain practical experience. The use of nitrogen fertilizer before flowering itself carries certain risks, and the cluster elongation method also differs with different regions, geography, temperature, light, humidity, and tree vigor. When using water and fertilizer for cluster elongation, the average daily temperature is required to be 20-25 degrees C.

Chemical agent cluster elongation

Main characteristics: fast-acting, relatively safe, and less affected by the environment.

Elongation period: Grape clusters appear at the same time as leaves. As long as the cluster emerges from the leaves (5-6 leaves), treatment can be carried out, and it should stop 10 days before flowering. During this period, the earlier the application, the more sensitive the grape is to the agent.

Commonly used agents mainly include: gibberellin, Bihu, abscisic acid, etc. Among them, Qibao (gibberellin) is used more often, with a dosage of 1 g Qibao diluted in 25-40 kg water.

Method of use: Dip the inflorescence or finely spray the inflorescence; dipping the inflorescence is preferable.

Existing problems: If the concentration of gibberellin, abscisic acid, etc. is not well controlled, over-elongation may occur; after cluster elongation, the clusters become loose and not compact, inflorescences become deformed, the rachis hardens, and later fruit setting may easily lead to berry drop, etc.

Precautions and management for cluster elongation

1. Use chemicals rationally

Pay attention to the grape variety, tree vigor, tree age, etc., and also to weather conditions. Temperature, humidity, and light all have an impact. At high temperatures, the concentration should be as low as possible; at low temperatures, the concentration can be a bit higher; for strong tree vigor, the concentration can be higher, and for weak tree vigor, the concentration should be lower.

Avoid using cluster elongation chemicals under strong sunlight; the concentration of treatment agents differs among different regions, so try first before large-scale application!

2. Prevent diseases well

After inflorescence elongation, the distance between branch stalks in the cluster increases. If gray mold, rachis blight, etc. cause damage, gaps will appear in the cluster, affecting commercial quality and sales. Prevention can be carried out before flowering, and agents can be selected from pyrazole · dimethomorph, procymidone, iprodione, etc.

3. Do a good job in fruit preservation

For some varieties, elongation must be accompanied by fruit preservation. Sea Elf + boron fertilizer should be sprayed before flowering and at petal-fall stage to increase the fruit-setting rate.

4. Cluster trimming and fruit thinning

When the inflorescence elongates, several small stalklets at the shoulder also elongate. After fruit setting, cut off several berries from the overly long stalklets so that the cluster forms a conical or cylindrical shape; for varieties with secondary clusters, cut off the secondary clusters before flowering; the tip of the cluster should be pinched off before flowering; after fruit setting, thin lightly, removing small berries, thus reducing the amount of thinning work.