Varieties, Rootstocks, Seedlings, Hybrids and Clonals, Pollinators... As a new grower, you will be exposed to these terms regularly. And there is a good reason for this. For me, all of these terms were so complicated and difficult to understand initially. Over the course of my journey first, as a walnut grower, than as a plant supplier and advisor, I grew to learn the importance of each. That is why, I thought that it would be a good idea to shed some light of them so I could be of help to beginners. While doing that, as all other postings I have done previously, my wording will not be academic or even scientific, but I will try to explain these concepts and why they are so important from a practical point of view.
Rootstock, Variety, Grafting
Just like the human beings are biologically classified to be under the Genus, Homo andSpecies, Homo Sapiens, plants that produce walnut that we all know have been biologically classified under, the Genus Juglans and Species of Juglans Regia. That being said, there are 21 known species of the Genus Juglans, the 20 of them being cousins of Juglans Regia, which is grown for the production of the walnut. Although they do not produce edible nuts, some of these other species, for example, Juglans hindsii, also commonly called the Northern California Black (NCB) walnut, are known to tolerate complicated soil conditions, such as excess of water or salt in addition to be more vigorous. Due to these interesting characteristics, a hybrid plant named PARADOX has been developed by the artificial hybridization between NCB and Juglans Regia (common walnut producing plant, also called the Persian walnut).
Grafting is joining the tissues of two different plants so that they continue growing as asingle plant. This can be done in a variety of techniques, each having different success rates in different conditions and seasons.
There are two common reasons for grafting walnut plants:
1- Variety change: If you plant a walnut and show some care, the seed that you plant will produce a walnut tree. Even if the initial walnut that you plant is coming from a known variety, e.g. Chandler, you can never be sure about the variety of the tree that it will produce. Why ? Simple genetics: The seed that you plant may come from a Chandler tree (the mother). However the polen that pollinated the Chandler tree may come from a different tree. Therefore, the plant that will be produced by this seed, does not need to present the mother´s (Chandler) characteristics. To resolve this uncertainty, the plant produced from the seed, is grafted by a scion - a tissue taken from a known variety tree (a Chandler in this example). If the graft is successful, the upper part of the plant, grows to be the true Variety of the plant (that will become the fruit producing part of the plants that will determine the food quality), whereas the bottom part is called the Rootstock, that will interact with the soil providing with the resources that the tree will need. Using grafted plant is one of the most reliable and practical ways of ensuring variety reliability.
2- Using a specific rootstock: A hybrid or clonal rootstock that is known to have better soil interaction capabilities is used as a rootstock and is grafted by the desired variety to make a plant that consists of the best characteristics of a rootstock and a variety. Paradox hybrid plants have been used as rootstocks in commercial orchards for several decades.
Variety and its reliability:
In commercial fruit and nut production, it is important to determine and use the most suitable variety (also called cultivar) with regards to your soil conditions, your climate, as well as commercial preferences in your market. Once the decision is made, every effort should be made to ensure that all the plants in your orchard belong to this selected variety, except for pollinators that complement your main variety. Having (unintentionally) different varieties makes orchard management difficult, lowers quality and productivity. The situation can even be worse if the inconsistencies or poor choices are on the pollinators. Supplying its clients plants with very high variety reliability is one of the most important qualities of a good commercial nursery, as realizing variety inconsistencies in an orchard several years after plantation is a major economical loss for growers. A good commercial plant nursery should have internal controls in managing variety reliability as delivering customer orders correctly cannot be left to “best intentions”. Not relying external grafting materials, good record keeping are among these controls methods. As of the writing of this blog, I still see significant inconsistencies in plants provided by nurseries in Europe, in a level that cannot simply be labeled as mistakes.
Pollination, Pollinators and how to chose them:
Walnut plants are monoecious. Which means that walnut plants carry both male (catkins) andfemale pistillate flowers. Pollens produced by the catkins pollinate the female flowers and this is how nuts are produced. However, most walnut plants have different maturity times for their female and male flowers, that minimizes the chance of most walnut varieties pollinating themselves. Flowers that are not pollinated during the period that they are receptive are aborted from the tree. In order to maximize pollination of plants and therefore the productivity of a commercial orchard, one or more variety of walnut plants that are know to produce male flowers that overlap the female flowering time of the main (production) variety are planted. These plants are called the pollinators. The correct pollinator varieties can be determined by reviewing the flowering or pollination chart, similar to the simplified one below:
As you can see, Male and Female flowers of the popular Chandler variety only partially overlap, apprx 35 %. However, Franquette´s Male flowering time completely overlaps Chadnler Female flowering time, which makes it an ideal Pollinator for it. While the overlapping of flowering period is the most important factor that determines the correct match between the production and the pollinator varieties, there are other factors that need to be taken into account. Some varieties, such as Franquette, produces higher quantities of catkins at mature ages, making them weak pollinators in younger ages. Some varieties are known to be more susceptible to bacterial diseases. Therefore, looking at these factors, it is advisable to use 2 or even 3 pollinator varieties that will complement each other in creating the most effective pollination.
How much: Some young orchards that are located to other mature orchards that already produce abundant quantities of pollen can benefit from this and may not need many pollinator plants. Some on the other side do not have any external source for pollen. It is usually advisable to have 5-6% of pollinator plants in your orchard. This should be divided into different varieties. Such as having 2 % Franquette, 2 % Fernette and 1 % Ivarto plants in a Chandler orchard. Pollinators must be distributed within the orchard evenly, taking into account the prevailing wind conditions during the flowering time.
Placement and Marking:
It is a vital practice to manage and know the location of your pollinator plants. They may require different applications, such as spays on different times or different irrigation regimes. In order to do these easily and reliably, it is a good practice to plant your pollinators in rows.
Your main variety is Chandler:
Your pollinators are: 2 % Franquette, 2 % Fernette and 2 % Ivart = 6 % Pollinators. Below you will see that there is a pollinator row in every 9 tree rows. And 1/2 of the plants in a pollinator row is a pollinator tree planted in alternate order.
Why must we mark pollinators:
The leafing time of the pollinators are different that the production variety plants.
Walnut trees are primarily wind-pollinated, meaning they don't rely on insect pollinators such as bees as heavily as some other plants. The wind plays a significant role in carrying walnut pollen over longer distances to reach female flowers.
A few walnut varieties have male and female flowering times that overlap. These varieties usually do not require pollinators. Tulare, is a good example for this.
Hybrids and Clonals:
Walnut we know, Juglans Regia, is a member of the genus Juglans L. There are 20 other species that are members of the Juglans L genus, each with different characteristics.
Hybrids are produced by cross pollinating two different species of the same genus to incorporate superior characteristics of each of these species into a “hybrid” specie. One of the first popular walnut hybrid is Paradox, which has been produced by cross pollination of Juglans Regia and Juglans Hindsii (also called Norther California Black Walnut - NCB). For decades, due to its vigor and resistance to adverse conditions in the soil, Paradox has been the preferred rootstock in California. Seeds produced from a Paradox plant is used to produce other paradox plants that were used as rootstocks. It is important to understand that Paradox is a hybrid seedling (produced from seeds).
Despite its superior characteristics, like every hybrid, Paradox has certain limitations due in part to the genetic variability, or genetic differences, in Paradox seeds. Every seed (even if taken from the same plant) is a different genetic organism.
“University of California Walnut researchers, specialists, and farm advisors studied the genetic background of Paradox seedlings and found high variability from one seed to the next. This means that each seed is different from the next one. One seed might be more vigorous, one seed might be more susceptible to phytophthora , one seed might encourage more seed production, while another encourages more leaf and branch growth.” 
MJ209 is a hybrid produced by cross pollination of Juglans Regia and J. Major (also called as Arizona Walnut) which is popularly used in timber production due to its vigor. Just like Paradox, MJ209 is a hybrid seedling, not a Clonal rootstock like Vlach, RX1 or VX211. New plants are produced by planting MJ209 seeds into the soil. The plant is then grafted with the desired variety. MJ209 rootstocks share the same limitations of all the seedlings as each plant shows high variability due to genetic differences of one seed to the next.
Scientists have solved this variability issue by developing the cloning, technique of multiplication of a single plant showing superior characteristics and creating tens of millions of new plants with the same exact characteristics. A good way of contrasting hybrids and clonals is the use of the following analogy: While each hybrid is a sibling or cousin of each other, each clone is the multiplication of the same living organism. Plants are cloned in tissue culture laboratories using special plant mediums under very precise climate and lighting conditions.
Producing plants with hybrid rootstocks are very easy and inexpensive compared to clonal rootstocks that require expensive Tissue Culture Laboratory settings, technology that needs to be purchased or licensed as well as sophisticated greenhouses. Just like planting Juglans Regia seedlings require use of any variety walnuts, producing Paradox or MJ209 requires hybrid seeds that can easily be purchased in large quantities. Sterile laboratory settings and soilless propagation (in tissue culture mediums), unlike in seedlings including hybrids, also prevents root diseases in clonal rootstocks.
The most successful clonal walnut rootstock that is still being used very widely is Vlach, which is produced by cloning of a single Paradox plant located in Modesto, CA, that showed superior characteristics. During several trials made in California in the last 15 years, two new very promising clonal rootstocks, RX1 and VX211, have been developed, each showing even superior characteristics when compared with Vlach. Currently commercial sale of RX1 and VX211 outside of US is not allowed.
Hybrid rootstocks have taken walnut plantation to a step forward and have paved the way to the creation of Clonal rootstocks. However, due to the consistency and superior characteristics of Clonal rootstocks, hybrid seedlings have almost entirely been abandoned in commercial walnut production where clonal rootstocks are available.
In most cases, a walnut tree grafted on a Clonal rootstock will be more expensive than a seedling plant with similar graft on. In certain types of soils, deep, well-drained and with no significan physical or chemical imbalance, a seedling plant with a good root system will perform "almost" as good as a clonal plan.The difference will be in maximizing the homogeneity of these trees´ performance as clonal trees with "presumably" the same genetic characteristics will all perform in the same way, while differences due to genetic characteristics will be observed with seedlings. While "money is not an issue", which is usually not the case, clonal trees should be preferred. But in a real world where a grower needs to make comprimizing decisions on where to allocate his/her limited financial resources, given the soil characteristics I have mentioned above, perhaps it will not be a mistake to use good seedling plants and to allocate this money in a more value-creating area.
An other consideration that should be taken into account while chosing your rootstocs is the difference in characteristics of the root systems of seedling and clonal rootstocsk. While the seedling plants tend to produce large pivot-roots, also known as the axonomorphic root, or taproot, is the root that grows downward vertically, clonal trees such as Vlach, tend to produce a denser body of hairy roots that explores the soil laterally, remaining closer to the soil surface. In addition to their many functions the root systems perform for the plants, they also function as a temperature sensor, triggering many critical functions of the tree phenology, acting as a body-clock for the tree. In areas where the summer temperatures are extremely high, walnut plants slow down their metabolism to protect tissues being harmed. They do this by assessing the ambient temperature through their roots. Root systems that grow deeper into the soil, are more insulated from the heat so that this protection mechanism is trigged in much higher temperatures. Whereas, a root system located closer to the soil surcface has a tendency to be affected by the heat more.
General Rule: Learn more about your soil. Drainage issues, high clay percentage, pH issues, salt: Do not think. Buy Clonal rootstock. Financial issues... Go for Hybrids.
Sandy soils, good drainage, decent soil parameters, (and you are not extremely rich), pick a good seedling plant with excellent root structure. They will get the job done.
 Taken from Selecting the ‘Right’ Walnut Rootstock on Pacific Nut Producer Magazine, Sept 10, 2020