Grafting Techniques in Different Pistachio Varieties

At Agro Vivero del Mediterráneo, we have been perfecting the art and science of pistachio cultivation for years. One of the cornerstones for establishing a successful and productive pistachio plantation is, without a doubt, grafting. This ancient technique, combined with modern knowledge, allows us to optimize each tree to ensure its adaptability, vigor, and, of course, excellent production of that appreciated nut. Today, we want to share with you our experience and knowledge about grafting techniques in different pistachio varieties. 🌳

The Crucial Importance of Grafting in Pistachio

The pistachio tree (Pistacia vera L.) is a dioecious species, which means there are male trees and female trees. Only female trees produce the valuable nut, but they need to be pollinated by pollen from male trees. Grafting allows us to ensure not only that we will have the desired female variety, but also that we will intersperse the appropriate male pollinators in the correct proportion.

In addition, the use of specific rootstocks (or patterns) gives us the opportunity to adapt the future plantation to the specific conditions of our soil and climate. A good rootstock can confer resistance to soil diseases, tolerance to drought or certain salinity levels, and even influence vigor and earliness of entry into production. At Agro Vivero del Mediterráneo, we select and propagate the best rootstocks, such as UCB-1, known for its vigor and resistance, ensuring a solid foundation for your investment. Considering plantation profitability is fundamental from the start.

Understanding Grafting Components: Rootstock and Variety

Before diving into techniques, it is vital to understand the two protagonists of this process:

1. The Rootstock (or Pattern): It is the lower part of the new tree, which will form the root system and the base of the trunk. Its choice is critical, as it will determine the tree’s adaptation to the environment and its general resistance. The most common in Spain are Pistacia terebinthus (cornicabra), Pistacia atlantica and, increasingly strongly, clonal hybrids like UCB-1 (P. atlantica x P. integerrima). At Agro Vivero del Mediterráneo, we offer pistachio plants on these proven effective rootstocks.

   • Pistacia terebinthus (Cornicabra): Native to the peninsula, very rustic and adapted to poor and calcareous soils, and drought conditions. Its main drawback is its slow initial growth and lower vigor compared to other rootstocks, which can delay entry into production and result in smaller final caliber trees. However, its rusticity makes it valuable in certain extreme conditions.

   • Pistacia atlantica: Native to North Africa and the Middle East, it has greater vigor than P. terebinthus and good resistance to drought and certain nematodes. It is sensitive to verticillium wilt in some conditions and severe winter frosts. Its compatibility with commercial varieties is generally good.

   • UCB-1: This is, for us, the star rootstock and the one we most recommend from Agro Vivero del Mediterráneo. It is a clonal hybrid obtained at the University of California, Berkeley (hence its acronym), result of the cross between P. atlantica and P. integerrima. It stands out for its great vigor, which accelerates entry into production; its remarkable uniformity, being clonal; and its excellent resistance to Verticillium dahliae (verticillium wilt), one of the most problematic soil fungi for the pistachio tree. It also shows good tolerance to salinity and cold, wet soils. Its vigor requires proper plantation management to avoid excessive growth to the detriment of production.

   • Pistacia integerrima: Used mainly as a parent in hybridizations (like UCB-1), it provides resistance to Verticillium and tolerance to wet soils. It is not usually used directly as a rootstock on a large scale in Spain, but its genes are very valuable.

2. The Variety (or Graft): It is the upper part, the scion or bud that joins the rootstock. This will determine the characteristics of the nut (size, shape, flavor, percentage of open nuts), flowering and ripening time, and productivity. Variety choice is a key commercial and agronomic decision.

Selection of Varieties to Graft: Females and Males

The choice of female variety is fundamental and will depend on factors such as the area’s climate (chill hours, risk of late frosts), the market targeted by production, and farmer preferences. Some of the most widespread female varieties that we handle in our advisory services are:

• Kerman: Originally from Iran and selected in California, it is the most cultivated variety worldwide. It produces a large, rounded nut of excellent quality, highly appreciated in the market. It is late-ripening and requires a late pollinator like Peter. It needs a high number of winter chill hours.

• Larnaka: Coming from Cyprus, it is a vigorous and productive variety, with a good-sized and elongated nut. It is medium-ripening and adapts well to areas with milder winters than Kerman. It is usually pollinated with Randy or C-Especial.

• Sirora (or Bud P9): Selected in Australia, it is very productive and early in its entry into production. The nut is medium-sized, elongated, and with a high percentage of open nuts. It adapts well to different climatic conditions and is early to medium ripening. Pollinators like Randy or C-Especial work well.

• Aegina (Egina): Originally from Greece, it is a productive variety, with medium-sized and elongated nut. It is medium-ripening and has lower chill requirements than Kerman. Combines well with pollinators like C-Especial.

• Mateur: Tunisian variety, with elongated and tasty nut, although somewhat smaller than Kerman. It is interesting for its adaptation to warm areas and its good organoleptic quality.

Equally important is the choice of male pollinator. This must have a flowering coinciding with the female variety. It is useless to have an excellent female variety if the male flowers too early or too late.

• Peter: It is the classic pollinator for Kerman, as its flowering is late and synchronizes well. It is a vigorous tree.

• Randy: A pollinator with earlier flowering than Peter, suitable for varieties like Larnaka, Sirora, or Aegina. It is known for its abundant pollen production.

• C-Especial: Selected in Spain, it is an intermediate flowering pollinator, very versatile and with good pollen production, compatible with Larnaka, Aegina, and other varieties with similar flowering.

• Guerrero: Another pollinator of Spanish origin, with early to medium flowering, which is also used with varieties like Larnaka or Sirora.

At Agro Vivero del Mediterráneo we always recommend planting at least two varieties of male pollinators with slightly overlapping flowering periods to ensure effective pollination even in years with variable weather conditions during flowering. The proportion is usually 1 male for every 8-10 females.

Optimal Time for Grafting

Grafting success depends largely on doing it at the right time. Generally, there are two main windows:

1. Spring (April-May in the Northern Hemisphere): When the rootstock is in full sap movement and the bark “peels” easily. It is ideal for bud grafting (shield, patch) and some scion grafting. Temperatures should be mild, avoiding late frosts or excessive heat.

2. Late Summer – Early Autumn (August-September): Also known as “dormant bud” grafting. The bud is grafted, but does not sprout until the following spring. Sap is still active, but vegetative growth has slowed down.

We have observed that spring grafting, especially T-budding or shield budding on young rootstocks (1-2 years), usually offers excellent take results and vigorous graft development in the same season. For thicker rootstocks or re-grafting, scion techniques may be more appropriate.

Detailed Grafting Techniques

There are various grafting techniques, but we will focus on the most effective and commonly used in pistachio plantations. It is essential to have well-sharpened and disinfected tools (grafting knife, pruning shears) to ensure clean cuts and avoid disease transmission. 🧼🔪

1. Bud Grafting (or Shield Budding)

It is the most widespread technique for pistachio, especially in young UCB-1 rootstock seedlings. It consists of inserting a single bud of the desired variety under the bark of the rootstock.

• Subtype: T-Budding or Classic Shield Budding:

  1. Obtaining the bud: Select healthy and vigorous budsticks of the variety to be grafted. With the knife, extract a bud with a portion of bark and, optionally, a thin slice of wood, in the shape of a shield (hence the name). The bud must be well formed and undamaged. 🛡️

  2. Rootstock preparation: In a smooth area of the rootstock stem, about 15-20 cm from the ground, make a “T” shaped cut that penetrates only the bark. Carefully lift the edges of the bark in the vertical incision.

  3. Insertion: Slide the shield with the bud downwards, into the T-cut, until it is well seated. The top of the shield should match the cross cut of the T.

  4. Tying: Firmly hold the graft with specific grafting tape (rubber, biodegradable plastic, or raffia), starting below the bud and ending above, leaving the bud exposed so it can sprout. Tying must be firm to ensure cambial contact, but without strangling.

  5. Aftercare: If grafting is done in spring and takes, the bud will sprout in a few weeks. Once the shoot reaches about 15-20 cm, the rootstock can be “topped” a few centimeters above the graft to force all vigor into the new shoot. If it is dormant bud, this topping will be done the following spring.

• Subtype: Patch Budding:
  Similar to shield budding, but instead of a T-cut, a rectangle of bark is removed from the rootstock and replaced by a rectangle of bark of equal size containing the variety bud. Requires more precision and double-bladed tools are often used to ensure cuts are parallel and of the same size. It is very effective but perhaps less fast than T-budding for large volumes.

Advantages of bud grafting:

• High take percentage if done correctly and at the right time. ✅

• Efficient use of plant material (budsticks), as each bud is a potential tree.

• Strong and clean union.

Considerations:

• Requires rootstock bark to “peel” well, which happens when there is good sap circulation.

2. Scion Grafting (or Cleft Grafting)

This technique involves joining a piece of budstick (the scion), containing several buds, to the rootstock. It is often used on larger diameter rootstocks, to change variety in adult trees (re-grafting) or when bud grafting is not viable.

• Subtype: Cleft Graft:

  1. Rootstock preparation: Cut the rootstock trunk horizontally at the desired height. Make a vertical split in the center of the cut, about 3-5 cm deep, with a sturdy knife or specific cleft tool. 🪵

  2. Scion preparation: Select a scion of the desired variety, of similar or slightly smaller thickness than the rootstock (if possible), with 2-3 buds. Carve the base of the scion into a wedge shape, with two smooth and converging faces, of the same length as the rootstock split.

  3. Insertion: Insert the scion wedge into the rootstock split, ensuring that the cambium layers (the thin green layer under the bark) of the scion and rootstock match on at least one side. If the rootstock is much thicker, two scions can be inserted, one on each side of the split.

  4. Tying and Sealing: Firmly tie the union with grafting tape. It is crucial to seal all cut surfaces (top of the scion, end of the rootstock, and union area) with mastic or grafting wax to prevent dehydration and pathogen entry.

• Subtype: Whip and Tongue Graft:
  Ideal for when rootstock and scion have a similar diameter.

  1. Make a long oblique cut (about 2-3 times the stem diameter) at the base of the scion and on the rootstock.

  2. In the center of each oblique cut, make a second cut inwards, parallel to the wood grain, to create a “tongue”.

  3. Fit the two pieces together, sliding the tongues into each other. This provides a very strong union and excellent cambial contact.

  4. Tie firmly and, if necessary, seal.

• Subtype: Bark Graft:
  Used on thick rootstocks, often to change variety of established trees.

  1. Cut the rootstock trunk horizontally.

  2. Prepare scions with 2-3 buds, carving a long bevel on only one side of the base.

  3. Make vertical cuts in the rootstock bark, the length of the scion bevel, spaced around the trunk perimeter.

  4. Slightly lift the bark and insert each scion between the bark and wood, with the beveled side facing inwards. Several scions can be placed around the trunk.

  5. Tie firmly around the trunk to press scions against the wood and seal all wounds.

Advantages of scion grafting:

• Allows grafting thicker rootstocks or changing varieties. 💪

• Can result in very rapid initial growth if it takes well.

Considerations:

• Requires more plant material (budsticks) per graft.

• Wound sealing is even more critical to prevent desiccation.

• Some techniques, like cleft grafting, can be more prone to problems if the union is not perfect.

At Agro Vivero del Mediterráneo, for the production of young pistachio plants, we prioritize T-budding on one-year-old UCB-1 rootstocks, performed in spring. This allows us to deliver vigorous plants ready for rapid field establishment. For re-grafting situations or special cases, we apply the most appropriate scion technique.

Post-Grafting Care: Key to Success 💧☀️

Work does not end with tying the graft. Aftercare is fundamental to ensure a high take percentage and good development:

1. Checking the Tie: A few weeks after grafting, check that the tape is not strangling the stem as it thickens. Loosen or cut if necessary, especially with non-degradable tapes.

2. Removal of Rootstock Shoots: It is vital to remove any shoot arising from the rootstock below the graft point. These “suckers” compete with the graft for nutrients and water, potentially choking it. This task must be constant during the first growing season. 🌿✂️

3. Staking the Shoot: As the graft shoot grows, it can be vulnerable to wind or mechanical damage. Staking it to a cane or stake will help guide its vertical growth and protect it.

4. Irrigation and Fertilization: Maintain adequate soil moisture, without waterlogging, and a balanced fertilization plan according to the needs of the young tree. A well-nourished and hydrated tree will have more strength to develop the graft.

5. Protection against Pests and Diseases: Watch for the appearance of aphids, caterpillars, or other insects that can damage tender shoots. Also be alert to signs of fungal diseases.

6. Topping or Heading Back the Rootstock (if not done before): If bud grafting was performed and it has sprouted strongly, ensure that the top of the rootstock (above the graft) is cut to direct all energy to the new shoot. This cut should be made at a bevel, opposite the bud, so that rainwater runs off and does not affect the graft.

Specific Considerations by Variety

While the grafting techniques described are applicable to most pistachio varieties, there are some subtle considerations:

• Budstick thickness: Some varieties may naturally produce thicker or thinner budsticks. This can slightly influence the ease of obtaining buds for shield budding or the choice of scion diameter. However, an experienced grafter adapts their technique.

• Rootstock and variety vigor: A very vigorous rootstock like UCB-1 combined with an also vigorous variety will require good management (pruning, irrigation, fertilization) to balance vegetative growth and future production.

• Compatibility: Fortunately, within Pistacia vera and its most common rootstocks (P. terebinthus, P. atlantica, UCB-1), graft compatibility is generally very good. Incompatibility problems are rare if quality materials are used.

The true art lies in the correct execution of the chosen technique, the selection of healthy and vigorous plant material, and the right timing. The choice of Kerman, Larnaka, Sirora, or any other, will influence nut characteristics, but the physical joining process through grafting follows the same biological principles.

Why Trust Professionals for Grafting?

Although grafting may seem simple in theory, practice requires skill, experience, and deep knowledge of tree physiology. A poorly performed graft can result in a low take percentage, weak unions, or the introduction of diseases. At Agro Vivero del Mediterráneo, our team of experts not only performs grafts with maximum precision, but we also offer comprehensive advisory services for your plantation.

Acquiring pistachio plants already grafted by professionals like us saves you time, reduces risks, and ensures starting material of the highest quality, fundamental for long-term plantation profitability.

Grafting is a fascinating and essential technique in the pistachio world. Mastering it is key for any producer seeking to maximize their plantation’s potential. We hope this detailed guide has been useful to you. If you are thinking of starting a pistachio plantation or need expert advice, do not hesitate to contact us or request a quote through our reservation-quote form. At Agro Vivero del Mediterráneo we are here to help you cultivate success! 🌱💚