Propagation of chestnut

The planting material of previously established chestnut orchards — due to a lack of grafted stock — consisted of seedlings grown from seed. These no longer meet the standards expected of modern fruit trees.

Chestnut trees can be grafted successfully even at an older age. However, chestnut grafting has not yet become widespread. Therefore, a detailed description of grafting is justified, even for the purpose of re-grafting existing trees.

When propagating chestnut grafts for fruit production, not only the characteristics of the selected cultivar but also those of the rootstock play an important role.

Chestnut can be grafted not only onto its own seedlings but also onto various oak species. However, Italian experience has shown incompatibility with oak rootstocks, resulting in unproductive trees that produced many empty fruits (without embryos or cotyledons). The only reliable rootstock is the chestnut seedling. The ecological conditions of domestic production sites are highly variable. To produce biologically fully valuable grafts, it is advisable to use seedlings from mother trees adapted to the local production site. Mother trees selected in forestry operations are unsuitable for fruit production because they are selected for vigorous growth and low branching.

For fruit production, rootstock should come from moderately growing trees with early and widely angled branching — the so-called “bushy” trees that are unfit for forestry purposes but highly productive.

For rootstocks in commercial orchards, seeds should be collected from mother trees adapted to the site of planting, and sown individually for each mother tree, with records maintained in the nursery register.

Establishment of a Grafting Nursery

The grafting nursery must meet the specific soil requirements of chestnut; otherwise, seedlings will fail to develop and, by two to three years of age, will die with chlorosis symptoms. It is advantageous if well-growing chestnuts are present in the vicinity of the nursery. The soil pH of the nursery should be between 4.8 and 6.5.

Soil preparation in fruit tree nurseries usually involves deep plowing. In terms of nutrient supply, potassium is particularly important. A deep, nutrient-rich soil is required; on poor soils the grafted tree grows slowly, reaching the necessary size 2–3 years later. As a result, the older root system suffers more from transplanting, and the cultivation cost also becomes disproportionately higher. During the seedling-growing year, it is beneficial to stimulate growth with acidic nitrogen fertilizers. Depending on the soil’s nutrient content, 80–160 kg/ha of active substance is applied in two doses: at bud break and during the period of rapid growth (up to the end of June). In the year of whip-growing, nitrogen can be harmful, since vigorous whips (with annual growth longer than 150 cm) may become sensitive to winter frost.

The layout of the area and the required plant spacing are the same as in the fruit tree nursery (90 × 35 cm), corresponding to the requirements of the row-cultivating equipment.

The graft nursery can be established either with germinated seeds or with seedlings. The seedlings grown in the nursery (35 × 4–5 cm spacing) are lifted in spring and planted in the same way as other young plants, heeled in, but not cut back.

Chestnut seeds are perishable and cannot be stored in sacks. Autumn sowing of chestnuts is unreliable, because in winter the seeds may start germinating due to stronger temperature fluctuations, and then be killed by subsequent frosts. It is more advisable to stratify the seeds collected in autumn in boxes layered with peat, and during winter store them in a cooled space at ±2 °C until the time of sowing.

For rootstock purposes, medium-sized chestnuts are more economical. Large chestnuts have a low germination rate, while small ones, although they germinate well, produce weak seedlings in the first year. On average, one kilogram of chestnuts contains about 100 seeds, with an expected germination rate of 70%. The sowing depth is 3–5 cm (deeper in loose soil). The radicle of the chestnut seedling emerges from the tip of the nut (where the remains of the stigma can be seen), therefore the nut should be sown with its tip downward or laid on its side. This helps the root system to penetrate the soil and prevents curvature of the seedling’s root collar.

Seeds sown at a depth of 3–5 cm should be covered with a 5 cm layer of peat, leaves, or chestnut husks.

Chestnut tends to develop a taproot, especially in deeply plowed, loose soil. Grafted plants with a strong taproot suffer more from transplanting. Transplanting wild seedlings into the graft nursery helps to reduce the formation of the taproot.

The formation of a taproot can also be prevented by planting in spring the seeds that were stratified in peat over winter, as they germinate, and pinching back the emerging radicle.

The cultivation and maintenance practices for chestnut seedlings are the same as for other fruit trees. Due to the slower juvenile growth of chestnut seedlings, they do not reach the development stage required for grafting in their first year. Even in the second year, only about 25% reach the appropriate size. About 75% of the rootstocks attain the necessary root collar thickness only by the autumn of the third year, and can therefore be grafted only in the spring of the fourth year.

Chestnut reaches the required thickness of 15–25 mm at a height of 80 cm for crown grafting only by the end of the fourth year, so grafting can be carried out only in the spring of the fifth year.

Budding methods have not proved successful in the case of chestnut, due to their unreliable success rate. Among the grafting methods, the most effective is the improved bark grafting technique.

Grafting technology

Suitable scion wood can only be collected from mother trees specifically cultivated for this purpose. Mature, fruit-bearing trees—whose fruit quality and productivity are already established—produce only short, thin shoots, and even these bear only 1–3 well-developed buds at their tips. In the middle part of the shoots, only the scars of the previous year’s male catkins are found where buds should be, making these portions unsuitable for scions.

The mother trees grown for scion production should be pruned back each year to short spurs with 2–5 buds, depending on the vigor of the tree. This practice stimulates the formation of vigorous, vegetative shoots suitable for scion wood. Even in the case of fertile varieties, a small percentage of catkins may still develop in place of leaf buds.

As rootstock, well-rooted and properly growing seedlings are suitable.

The advantage of grafting is that even older rootstocks with thick bark can be grafted successfully. The most favorable method is crown grafting chestnut whips 15–25 mm thick at the desired trunk height (80–120 cm). With good growth, a plant suitable for field planting can be obtained by the end of the grafting year. When using weaker rootstocks and grafting at the root collar, only a low whip can be grown during the year of grafting.

The optimal time for grafting coincides with the bud break of the rootstock. Under local conditions, the suitable period generally extends from mid-April to early May. The selected scions should be kept completely dormant until then. This can be safely achieved by storing them in a cooled space at around ±2 °C.

Grafting Procedure

Any type of high-quality grafting knife is suitable for improved bark grafting of chestnut. The scion should be cut with a pairing cut, strictly maintaining the razor-sharp edge of the knife and ensuring the blade is properly beveled from the back (nail) side.

The procedure is as follows:

• The rootstock is cut back perpendicular to its axis at the grafting site.
• The damaged tissue on the cut surface is smoothed with a sharp knife.
• From the scion carefully kept covered with a damp cloth, 1–3 bud scions (approximately 5–8 cm long, depending on the internode length) are cut. The remaining scion is re-covered, and each scion is prepared with a “pairing cut”—a straight, smooth cut. The length of the cut surface should be 2.5–3 times the thickness of the scion.
• The scion is fitted to the grafting site on the rootstock, and the bark of the rootstock is cut alongside the scion for two-thirds of the length of the cut surface.
• The bark of the rootstock at the cut surface is gently lifted between the two parallel incisions, and the scion is inserted with its cut surface against the wood, under the bark, deep enough so that about 2–4 mm of the scion’s cut surface remains above the rootstock stub to facilitate wound sealing.
• The graft is tightly bound to the rootstock stub.
• All cut and exposed surfaces of the completed graft are coated with melted paraffin. In the case of crown grafting, if the flexibility of the rootstock trunk allows, it is preferable to dip the graft into paraffin.
• To support the scion and the developing shoot, attach a guiding stake next to the graft.

The subsequent care of grafted seedlings (such as suckering and trunk support) does not differ from the cultivation of other nursery grafts. In commercial nurseries, chestnut grafting has a success rate of 50–70%.

With root-collar grafting, propagation material suitable for planting is obtained in a shorter time, but with crown grafting, the graft does not need to be repeated over several years in the nursery bed, depending on the vigor of the rootstocks. From an operational and economic standpoint, this justifies the advantages of crown grafting.

With root-collar grafting, special attention must be paid to carefully removing the vigorous shoots of the cut-back chestnut, so that they do not develop into whips.

Establishing a Chestnut Orchard

Based on past experience, chestnut grafted orchards can be established in four different ways.

• Selected seeds suitable for rootstock production are planted in clusters at their permanent locations, and the seedlings are grafted with the appropriate variety during cultivation.
• Planting with seedlings of different ages and sizes, followed by crown grafting after the rootstocks begin active growth.
• Rejuvenation of an existing orchard composed of trees of different ages and sizes, followed by crown grafting.
• Planting with nursery-grown grafts of appropriate quality.

The first three methods are ineffective, costly, and do not produce a uniform stand within an acceptable time frame; they are neither modern nor economical. Planting with grafted stock is considered the most appropriate approach.

In principle, establishing a modern chestnut orchard does not differ from that of other fruit crops.

The selection of cultivars suitable for planting is determined primarily not only by the site’s growing conditions but also by the availability of labor and the farm’s crop association possibilities. The site can be prepared depending on the topography. On flat areas or slopes not steeper than 16%, soil preparation can be carried out using deep mechanical plowing, as is customary for other fruit crops, depending on the thickness of the fertile layer. On slopes steeper than 16%, the soil should be prepared by plowing strips 4 m wide along the rows or by digging pits to prevent erosion.

It is advantageous to apply fertilizers determined based on soil test results at the same time as plowing. Potassium, in particular, is important for chestnut, and a larger dose applied at planting can ensure its availability over a longer period.

Planting with grafted stock requires a smaller spacing compared to seedlings, because trees grown from grafts come into bearing earlier and grow to a smaller size. On slopes gentler than 15%, the appropriate operational spacing is 10 × 8 m between rows and trees, allowing for mechanical cultivation. On steeper slopes, an 8 × 8 m row and tree spacing permits mechanized or horse-drawn cultivation. In the first case, 125 trees can be planted per hectare, while in the latter, 152 trees per hectare.

Maintaining a commercial orchard becomes less economical during the declining phase of tree productivity; therefore, chestnut trees are not worth keeping until the very end of their lifespan. The assumed productive lifespan of trees in an orchard is 60 years.

Planting chestnut grafts does not differ from the planting methods of other fruit trees. The most suitable for planting is a one-year-old whip grafted into the crown, with a well-branched root system. A trunk height of 80 cm and a whip height of at least 150 cm are desirable. Whips grown with a scion grafted at the root collar can also be used for planting, provided they are at least 130 cm tall. Shorter grafts require special care until the crown-pruning stage. Particular attention must be paid to the selection of shoots emerging from the rootstock in cases of trunk injury in root-collar grafted trees, as the rootstock grows vigorously and, if neglected, may develop into a tree itself.

Crown-grafted trees can also be planted; in this case, the crown shoots should be pruned back to 2–4 buds. The whip is pruned for the crown at a height of 130–150 cm. The trunk height should be at least 80 cm. At this height, remove a few buds to form a collar. Above the trunk height, train three shoots in a top view at 120° from each other, spaced 25–35 cm apart. In the first year, it is desirable to develop these three shoots as the foundation of the crown to a length of at least 25–35 cm. The ideal angle of the crown shoots relative to the trunk is 45°.

To promote tree establishment and growth, vigorous shoots emerging on the trunk—if they arise from the scion above the grafting site—should be allowed to develop, while only those growing more strongly than the crown shoots are limited by pinching. In subsequent years, the trunk-strengthening shoots are pruned back to 1–2 buds in spring, and they can be removed permanently only at 7–8 years of age, or once the trunk reaches a circumference of 30 cm.

In the subsequent years of crown training, the established crown foundation is developed by maintaining balance among the three scaffold branches, encouraging their branching, and inhibiting the development of a central leader.

On vigorously growing chestnut trees, excessively steep scaffold branch formation (triple trunks) should be prevented by propping and branch spreading. In weakly growing trees, on poor soils, or in cases of early fruit set, undesirable crown formation can be avoided by tying up branches, removing bound fruits, and applying increased nitrogen fertilization.

From grafted chestnuts, a loose, spherical crown should be trained, ensuring adequate light penetration even into the interior of the crown.

Trees with a central leader tend to develop dome-shaped crowns as they age; the poorly illuminated interior parts of these crowns become bare, and the large crown surface imposes unnecessary scaffold branch formation on the tree. The same considerations apply to trunk development: an extra 50 cm of trunk height delays the onset of fruiting by one year. Increasing the trunk height also reduces the overall size and productivity of the crown.

Soil cultivation in a chestnut orchard is determined by the slope of the productive area, and it must prioritize erosion control. On flat and gently sloping sites, the soil can be cultivated using conventional methods. On steeper terrain, in orchards with 8 × 8 m spacing, the soil should be kept under grass, and cultivation should be carried out over a width appropriate to the crown size until the trees start bearing fruit. On slopes suitable for mechanized cultivation, perennial legumes or grasses can be sown between the tree rows to serve as mulch. To evaluate the development of a young orchard, it is advisable to monitor chestnut growth performance year by year during the training period.

The development of the orchard can be assessed based on three factors: establishment, trunk circumference, and shoot growth.

• Establishment: In the first four years, it is important to meet the quality standards: 80% in the first year, 85% in the second year, 90% in the third year, and from the fourth year until the onset of fruiting, a uniform 95% establishment. Efforts should be made to replace missing trees to achieve these targets.
• Trunk Circumference: It is advisable to consider this from the third year after planting, because in the first two years, the varying development of the nursery stock significantly affects both the absolute trunk circumference and the annual increment. Trunk circumference should be measured at a height of 50–60 cm.
• Shoot Growth: For shoot growth, it is advisable to consider the annual cumulative length of the main shoots. Measure the total length of the three longest scaffold branches from the crown branching point to the tip of the lateral leader shoot, and the three measurements can then be averaged.