Grapevine pruning for canopy formation and plantation rejuvenation

Pruning is a basic surgical technique in viticulture that is performed annually. Removing and shortening annual and perennial branches allows for managing the development of the vine and obtaining a stable, high-quality harvest. Without proper pruning, it is impossible to distribute the shoots in space so that the leaf canopy effectively absorbs light, heat, and nutrients.

Pruning objectives at different life stages of the vine

The nature of pruning changes depending on the age of the plant. In the first years of life, while the grapevine is building vegetative mass and not yet bearing fruit, the main goal is to create a sturdy skeleton for the vine. To do this, pruning, desuckering, pinching, and tying are used to form the trunk and the arms of the required size.

The skeleton formation period takes 5 or more years, depending on the cultivar and growing conditions. Fruit-bearing shoots are placed on the formed arms. They are positioned so that the leaf canopy most effectively absorbs photosynthetically active radiation (PAR) and the surrounding air.

When the vine enters the period of full fruiting, pruning helps maintain a balance between shoot growth and the harvest. An agronomist regulates the vine load by eye count to maintain the canopy shape within the allotted feeding area. This allows for obtaining high yields of the required quality in specific climatic conditions.

At the aging stage, physiological processes slow down, and productivity decreases. At this time, the main objectives of pruning become the partial or complete rejuvenation of the above-ground part of the vine. Proper care allows for the restoration of the balance between vegetative and generative organs in an aging vineyard.

• Vine formation period — from 5 years
• Rejuvenation age of plants — from 25–50 years

Biological foundations and self-regulation of the grapevine

Modern pruning technology is based on the natural characteristics of the grapevine. Wild grapes are evolutionarily adapted to set an excessive number of buds — significantly more than the plant is capable of developing into shoots. The vine regulates its state itself: only the most developed buds located in optimal feeding conditions start growing.

The number of developed shoots is always limited by the reserve of nutrients in the roots and the above-ground part. The task of the agronomist is to guide this self-regulation process through pruning. This facilitates vine care and helps plants use light, heat, moisture, and nutrients as effectively as possible.

Scientific methods of pruning began to develop at the beginning of the 19th century with the study of the relationship between shoot growth and fruiting. Since the 1930s, clear patterns for regulating vine load have been established. Today, this research continues in specialized institutes and universities with a focus on the mechanization of the pruning process.

Distribution of nutrients and pruning objectives for the vine

Without regular pruning, cultivated grapes turn into a tree-like liana with chaotically arranged shoots of varying lengths and thicknesses. Wild and feral vines annually expend up to 86% of their nutrients on wood and leaf growth, directing less than 14% to the formation of generative organs. Proper pruning fundamentally changes this balance, allowing from 20% to 50% or more of organic substances to be used for harvest development.

• Nutrient expenditure on vegetation of a wild vine — up to 86%
• Nutrient expenditure on the harvest of a wild vine — less than 14%
• Nutrient expenditure on the harvest after pruning — 20–50% or more
• Optimal length of fruit-bearing shoots — 100–180 cm
• Length of unmanaged shoots due to polarity — 3–5 m or more

Heavy pruning in vineyards is necessary for two reasons. Firstly, it forms a compact canopy of the vine for ease of maintenance and mechanization of all processes in the vineyard. Secondly, it allows for the annual production of uniform shoots about 100–180 cm long for stable and high-quality fruiting. Without controlling polarity, individual shoots stretch up to 3–5 meters or more, while others remain short, which increases vegetative mass to the detriment of the harvest.

The influence of load on productivity is clearly demonstrated by the regulation of buds on vines of equal vigor:

• Underloading (20 buds left): only central buds develop, providing a high-quality but low yield. Most shoots from secondary and latent buds remain barren.
• Optimal load (40 buds left): almost all shoots develop from the central buds of the eyes and produce a full harvest.
• Overloading (60 buds left): due to nutrient deficiency, the central buds do not develop. The balance between the roots and the above-ground part is disrupted, vegetative mass grows excessively, and the harvest is of low quality.

Evaluation of vine vigor and calculation of shoot load

There is an inverse relationship between the number of shoots on a vine and the growth intensity of each of them. When the number of shoots is doubled, their average length decreases, not proportionally, but depending on the age of the vine. This relationship is expressed by the depression coefficient, which shows the degree of reduction in the length of an individual shoot when their total number is doubled.

The total length of all developed shoots increases along with their number; however, maximum leaf productivity and vine vigor are achieved only at an optimal load. Vine vigor is understood as its potential capacity for growth and fruiting, which is determined by the development of the root system, nutrient reserves, and the condition of living tissues. In practice, growth vigor is evaluated by the volume of fruit-bearing shoots (by determining their diameter and length), by their mass, or by the number of well-developed branches.

A calculation method is used to accurately determine vine vigor. It links the average shoot length, their number per vine, the depression coefficient, and a constant of 0.301 (lg 2). The final index expresses the vine vigor as the length of one conventional shoot in centimeters.

Evaluation of vine vigor using this formula, conducted by TSKhA (1965–1983) in various regions of our country and on different cultivars, made it possible to more accurately identify the rate of vegetative part (shoot) growth of the vine and proved convenient when comparing vines with different numbers and lengths of shoots. With the help of this formula, the potential growth and fruiting capabilities of vines are well identified, as there is a high correlation between vine vigor and yield (r=-0.81 – 0.95).

General patterns of changes in growth processes with different numbers of buds left during pruning (and shoots during disbudding) are expressed by specific relationships. As the number of buds or shoots on the vine increases, the total length of all shoots increases, but the average length of each of them decreases as the vine vigor changes along a curve that peaks at the optimal number of buds or shoots. With a small number of buds or shoots, the growth trend prevails over fruiting; when the number of buds or shoots exceeds the optimal amount, the vine vigor begins to decline, which negatively affects plant productivity.

Fig. 55. Correlation between the number of buds or shoots (/) and indices (//): total length of shoots on the vine (1); average length of one shoot (2); vine vigor (3).

In production, pruning is carried out by eye, determining vine vigor by the presence and development of the trunk, and the number and diameter of arms and shoots.

Based on the study of relationships between various organs and processes, complex interconnections have been established between the number of shoots (buds) and the power of the root system, assimilating surface, total shoot length, vine vigor, yield, cluster mass, and sugar content in berries. As the number of buds or shoots increases, all indicators initially increase, reaching a certain optimum (optimal bud or shoot load), and then individual indicators begin to decrease, for example, leaf area, yield, and its quality, with quality (cluster mass and sugar content in berries) decreasing even earlier than yield. These patterns are common for many regions and cultivars but are characterized by individual peculiarities depending on the cultivar, growing location, nutrient conditions, water supply, and cultivation technology: training systems, vine management systems, etc.

Fig. 56. Correlation between the number of buds or shoots (/) and growth and fruiting indices (//):

1 — power of the root system; 2 — assimilating surface; 3 — total shoot length; 4 — vine vigor; 5 — yield; 6 — cluster mass; 7 — sugar content in berries

Vine load with clusters is a good corrective factor, as the outflow of plastic substances to them stimulates the photosynthetic activity of the leaves of fruit-bearing shoots. A vine with a low cluster load is unproductive. For specific conditions, it is important to identify optimal bud, shoot, and cluster loads, which serve as indicators of the correct balance between growth and generative processes, under which the leaf apparatus ensures the highest yield of good quality in the current and subsequent years.

Methods for determining optimal bush loads. Several methods have been developed to determine optimal loads of buds, shoots, canes, and fruiting units, and correlation and regression equations have been derived to calculate optimal loads of buds, shoots, and harvest under specific conditions (Crimean Agricultural Institute, TSKhA).

For the first time, L. Ravaz (France) proposed a "weight" method of calculation using the formula: U=F/V, according to which the optimal load (U) corresponds to the ratio of harvest mass (F, kg) to shoot mass (V, kg) equal to 4–6. If this index is greater than 6, the bushes are overloaded with the harvest and the load should be reduced during pruning; if less than 4, the bushes are underloaded and the load should be increased.

A. I. Tseyko introduced corrections for berry sugar content into the calculations. In this case, the load (H) will be optimal when the ratio of harvest mass to shoot mass is equal to 1.6–1.7. The calculation is performed using the formula: where U — harvest per bush, g; C — juice sugar content of berries, %; V — shoot mass, g; 0.01 — conversion factor.

N. Shaulis (USA) recommended setting the bud load also based on shoot mass. In this case, 30 buds are left per 450 g of shoots, and for every subsequent 450 g, an additional 10 buds are added. The method was named proportional pruning. It is used in the USA, although the theoretical aspects of this method are not sufficiently substantiated.

In certain zones of the RSFSR and the Ukrainian SSR, the A. S. Merzhanian formula is used to calculate the optimal bush load: where G — number of buds per bush, pcs.; Q — harvest, kg/ha; N — number of bushes per 1 ha, pcs.; K — fruiting coefficient; P — average cluster weight, kg; A — percentage of dead buds, %; B — percentage of undeveloped buds, %.

Knowing the fertility indicators of the cultivar, this formula can be used to calculate the number of shoots required to obtain a planned harvest.

Based on the law of multiplicity, A. I. Tseyko proposed the "Magarach — CN" normalized load method, according to which the bud load (m) of bushes is determined by the number of vigorous shoots using the formula:

Optimal load coefficients for different grape cultivars

L o a d c o e f f i c i e n t C m u l t i p l i e r f o r o p t i m a l n u m b e r o f s h o o t s p e r c a n e s w i t h c l u s t e r s 1.1 5, 2.2 0, 3.2 0, 4.3 0, where C — multiple factor, equal to 1.5; 2; 3; 4…; N — number of vigorous shoots (length more than 80 cm and diameter >5–6 mm). where H — multiple load coefficient, equal to the ratio of the number of all shoots to the number of vigorous ones; quantity, hundredths of a percent: Ch? — barren shoots, n — fertile shoots, A — dead buds, B — undeveloped buds.

This method is used in Crimea and other regions.

In Moldova, a method for determining load by canes, named biological by the author (I. V. Mikhailyuk), is common. The calculation is performed using the formula: where K — optimal load by canes, pcs.; N — number of normally developed shoots on a bush; n — number of normal shoots per one last year's cane; p — optimal load coefficient.

If n=p, the load in the previous year was optimal, it should be maintained in the current year during pruning; if n < p — overload, and n > p — underload, and in the current year it needs to be adjusted using the formula. If bud death is more than 30%, a correction for the additional load D by canes is applied using the formula: where K — optimal load by canes; A — quantity of dead buds (%) minus 30%.

Conversion coefficients for shoots to average

Despite its wide application, this method is biologically insufficiently substantiated, since empirically established values of p are used in the calculations.

N. T. Panych (Anapa, V. I. Lenin State Farm) proposed a formula for calculating the necessary reserve of buds, taking into account shoots of all types, but using conversion coefficients to average shoots.

C o n v e r s i o n c o e f f i c i e n t S h o o t s — R i e s l i n g l e n g t h S 0 w e 5 a k 1 , 0 1 , 5 a v e r a g e , s t r o n g where NZ — necessary reserve of buds, pcs.; u — quantity of weak shoots, pcs.; n — quantity of normal shoots, pcs.; s — quantity of strong shoots, pcs.; B — quantity of barren shoots, hundredths of a percent; G — quantity of buds dead from unfavorable conditions, hundredths of a percent.

In Bulgaria, it is proposed to determine the optimal load (L, number of buds per bush) after counting the number of weak (w), medium (m), and strong (s) shoots on 70–100 bushes using the formula: L = 0.5 w + 3 m + 6 s — for cultivars with clusters weighing less than 100 g and L = 0.5 w + 2 m + 4 s — for cultivars with clusters weighing more than 100 g. The planned harvest per bush (H, kg) is established using the formula: where L is the bud load per bush, pcs; S is the number of developed shoots, pcs; I is the number of inflorescences per 1 bud, pcs; W is the cluster weight, kg. The I indicator is refined annually by microscopic analysis.

At the Crimean Agricultural Institute and the Timiryazev Agricultural Academy (TSKhA), the optimal load of buds, shoots, and harvest is calculated based on establishing correlation links between the bush vigor, leaf area, number, length, and volume of fruit-bearing shoots, and the size of the yield. Based on these, regression equations are used to draw conclusions about the necessary number of shoots and leaf area to create a specific harvest under concrete conditions for a specific cultivar.

Numerous attempts to derive formulas for calculating optimal loads of buds during pruning, or shoots or harvest during thinning, have not yet been completed. Work in this direction continues.

It should be emphasized that the proposed formulas and calculations have been adopted by production, but only in the zone where the work on determining optimal loads was conducted. For example, I. V. Mikhailuk’s formula is used to calculate the load by canes under production conditions in Moldova, A. I. Tseyko’s — in Crimea, N. T. Panych’s — on the Black Sea coast of the Caucasus, N. Shaulis’ — in the USA, A. S. Merzhanian’s — in the RSFSR, and regression equations — in the zones of activity of the TSKhA and the Crimean Agricultural Institute.

The load of bushes with shoots and harvest is established differentially for each specific plot and cultivar, taking into account its biology, soil and climatic conditions, technology applied in the past and current years, and the degree of plant damage by frosts, freezing, etc.

Insufficient load leads to vigorous vegetative growth of bushes, the accumulation of a large vegetative mass, poor ripening of shoots, and maturation of berries to the detriment of the quantity and quality of the harvest; simultaneously, the formation of generative organs in the buds is weakened.

For vigorous cultivars on fertile soil under optimal cultivation technology, the load is increased, while on less fertile soil, in mountainous conditions, and under dry farming, it is decreased. Weak development of shoots indicates bush overloading, and it must be reduced the following year; if there is excessive vegetative mass, it should be increased.

A bush is considered optimally loaded if the following do not develop on it:

• shoots from dormant buds (basal and water sprouts);
• sucker shoots;
• triplets and twins (i.e., weakly differentiated and without harvest).

At the same time, fruit-bearing shoots, as a rule, have a length of more than 100 cm and a diameter of 6–7 cm (each cultivar has its own criteria). If it is necessary to obtain grapes of better condition, the bud load is reduced compared to the optimal one.

The load of bushes with buds and shoots in various countries and republics of the USSR fluctuates within a wide range. This is due to the fact that in different regions, there is a very large set of cultivars, each of which has a specific biology of growth and fruiting, different nutritional areas and number of bushes per 1 ha, as well as their size, shape, and growing conditions.

It has been proven that changing the load of bushes with buds (a more accurate criterion taking into account shoot-forming capacity — with shoots and clusters) affects the course of physiological and biochemical processes, the metabolism of carbohydrates, nitrogen compounds, and enzymes. The differentiation of buds, the mass and productivity of shoots, and the quantity and quality of the harvest depend on this. At the initial stages of bush ontogenesis, the yield size is determined by the type of form; once fully formed, it is determined by the load of shoots and, especially, clusters.

Due to the high variability of genotype expression in the phenotype, one can find bushes of different vigor in the same plot for the same cultivar. Thus, according to TSKhA data, in Moldova, on a plot where plants of the Queen of Vineyards cultivar were planted, with a 16% thinning, there were 16% weak bushes, 56% medium, and 12% strong. The harvest from weak bushes equaled 3 kg, from medium — 9.2 kg, and from strong — 14.8 kg. Systematically, year after year, by changing the vigor of bushes through the load of shoots and harvest, bringing weak bushes up to medium and then to strong, while improving nutrition and water availability, it was possible to significantly increase the productivity of plantings without large additional costs (TSKhA experiments). This contains a great reserve for increasing yields, taking into account varietal cultivation technology. It is important to consider that the stronger the bush, the more shoots of optimal length with a harvest can develop on it, and the higher the yield will be. The yield size correlates with the number of clusters per bush (r = +0.97±0.02) and the cluster weight (r = +0.85±0.15). When bushes are overloaded, especially with clusters, the harvest may be large, but the diameter of the shoots and their productivity, and the mass of berries and clusters decrease; they contain fewer sugars, although their gross amount is larger, and the growth of shoots and the harvest are not supported by plastic substances. Only with optimal loads under specific conditions and a certain state of the bushes can one obtain an optimal amount of good quality harvest.

Regulating growth and fruiting through pruning length. Pruning determines not only the optimal load of buds and canes but also the length of fruiting shoots, as the same load can be achieved through a large number of short-pruned fruiting shoots (e.g., 10 spurs with 2 buds each) or a small number of fruiting units (2 units with a renewal spur of 2–3 buds and a fruiting shoot with 6–8 buds). However, the quantity and quality of the harvest will differ in this case, as the embryonic fruitfulness of the buds along the length of the fruiting shoot is not uniform, with the biological properties of the cultivar playing a major role.

Buds in cultivars of the Western European group are most fruitful between the 3rd and 10th nodes, in the Eastern group between the 3rd and 20th, while in greenhouse cultivars, inflorescences are initiated in the first or second buds.

It is known that the initiation of embryonic generative organs in the buds is influenced by the meteorological conditions of the previous and current years, moisture reserves and the irrigation regime, and cultivation technology (fertilizers, vine shape, pruning length, etc.); therefore, the zone of maximum bud fruitfulness along the length of fruiting shoots varies by year even within the same cultivar.

Hence, the pruning length cannot be constant either, and for its precise determination, it is necessary to study the survival of buds from frost along the shoots and indicators of fruitfulness before pruning. Methods for the preliminary assessment of bud fruitfulness based on the presence of embryonic inflorescences have been developed and are widely used in Czechoslovakia and the USSR (Moldavia), and they can be used in all zones of our country. For this, the following methods can be applied:

• the method of forcing shoots cut from bushes in each plot under laboratory conditions (temperature above 20°C);
• evaluation of shoot diameter (the larger it is, the longer the shoot should be pruned);
• evaluation of growth vigor (vigorous cultivars are pruned longer, weak-growing ones — shorter).

However, it is necessary to approach pruning length scientifically, clearly knowing the zone of generative organ initiation in the central buds of specific cultivars in a particular plot over the years.

There are patterns in the change of productivity of different cultivars depending on the pruning length:

In the Sauvignon cultivar in Taman, the 7th buds were the most productive, and short pruning reduced the harvest size by 2–3 times; in Dagestan, for the Rkatsiteli cultivar, it was the 10th buds. A change in bud productivity can be observed at different pruning lengths. Numerous experiments conducted in different zones of our country have proven that as the length of the cane increased during pruning up to a certain limit, the indicators of fruitfulness, leaf area, total length of shoots, and harvest size increased, but the quality of the product decreased. Plants of different cultivars reacted to pruning length differently.

In connection with the transition to a new cultivation technology for grapes and the use of high-trunk forms, the criteria for pruning length are being revised. With the introduction of pruning mechanization, the issue of shortening shoot length has become acute. Work in this direction is being carried out in all research institutions on many cultivars.

Since pruning is a surgical technique that inflicts wounds and removes a large part of the organic mass, it is advisable to coordinate the timing of pruning with the dormancy period, carrying it out mainly before the beginning of sap flow, when the main part of plastic substances is in the storage organs: the roots, the framework of the bush, and the shoots.

Therefore, in non-covering zones, pruning is usually carried out after the maturation of shoots and leaf fall (where this occurs) and throughout the winter at an air temperature no lower than —5°C. However, recent studies have shown that it is best to carry out pruning in the winter-spring period — before the start of sap flow, as this is more likely to protect the plants from unfavorable conditions.

In the covering zone, pruning is carried out twice: preliminary — before covering the bushes and the onset of autumn frosts, when some shoots have matured and leaf fall has begun in their lower part (here this phase does not end; in most cases, leaves are damaged by autumn frosts before falling).

During preliminary pruning, the following operations are performed:

• shoots are left on replacement spurs with a 30–50% reserve of buds;
• fruiting canes (arches or semi-arches) are removed if they are not required for the formation of fruiting units;
• in the case of poor development or absence of shoots on the replacement spurs, a portion of the fruit-bearing shoots on the cane is left to form a fruiting unit.

Then the bushes are covered, and in the spring, after uncovering them and before bud break, final pruning is carried out. For this purpose, the survival of the buds is checked and the necessary load is determined, taking into account their mortality.

During final pruning, the following are removed:

• old cordons with weakly developed shoots;
• sucker and water shoots, if they are not needed for forming cordons;
• lateral shoots on canes, if there is no need to use them for forming fruiting units;
• broken or elongated cordons, which are replaced by leaving sucker or water shoots at the base of the cordons.

It is necessary to constantly maintain the shape of the bush and the optimal load of buds.