Varieties, Rootstocks, Seedling, Hybrid and Clonal Plants

Varieties, rootstocks, seedlings, hybrid and clonal rootstocks, pollinizers... As a new grower, you will come across these terms regularly, and for very good reason. For me, all of these terms were confusing and hard to grasp at first. On my journey, first as a grower and then as a plant supplier and consultant, I learned why each of them matters. So I thought it would be a good idea to shed some light on these concepts to help beginners. As in all my earlier articles, the language will not be academic or scientific. Instead, I will try to explain these concepts, and why they matter so much, from a practical point of view.

Rootstock, Variety and Grafting

Just as humans are biologically classified under Genus Homo and Species Homo sapiens, the plants that produce walnuts as we all know them are biologically classified under Genus Juglans and Species Juglans regia. However, Genus Juglans has 21 known species, 20 of which are "cousins" of Juglans regia, the species grown for walnut production.

Although these other species do not produce edible fruit, some of them, such as Juglans hindsii, commonly known as the Northern California Black (NCB) walnut, are known for their tolerance of difficult soil conditions (excess water or salt) as well as for their more vigorous growth. Because of these useful traits, a hybrid plant called PARADOX was developed through artificial crossing between NCB and Juglans regia (the common walnut-producing tree, also known as the Persian walnut).

What Is Grafting?

Grafting is the joining of the tissues of two different plants so that they continue to grow as a single plant. It can be done using various techniques, each with different success rates under different conditions and in different seasons.

There Are Two Common Reasons for Grafting Walnut Plants:

1. Changing the variety: If you plant a walnut and give it some care, the seed you planted will produce a walnut tree. Even if the first walnut you plant comes from a known variety (e.g. Chandler), you can never be sure which variety the resulting tree will be. Why? Simple genetics: the seed you plant may come from a Chandler tree (the mother). But the pollen that pollinated the Chandler tree may have come from a different tree. As a result, the plant grown from this seed will not necessarily show the characteristics of its mother (Chandler). To resolve this uncertainty, the plant grown from seed is grafted with tissue, known as scion wood, taken from a tree of a known variety.

If the graft is successful, the upper part of the plant grows as the true variety (it will be the fruiting part, meaning it determines crop quality). The lower part is called the rootstock, and it interacts with the soil and supplies the resources the tree will need.

2. Using a specific rootstock: A hybrid or clonal rootstock known for better soil interaction is grafted with the desired variety, producing a plant that combines the best traits of both the rootstock and the variety. Paradox hybrid plants have been used as rootstocks in commercial orchards for decades.

Variety and Trueness-to-Type

In commercial fruit and nut production, it is essential to identify and use the variety (also known as the cultivar) best suited to your soil conditions, your climate and the commercial preferences of your market. Once the decision has been made, every effort must be taken to ensure that all the trees in your orchard — apart from the pollinizers that complement the main variety — belong to the chosen variety.

The presence of (unintended) different varieties makes orchard management more difficult and reduces quality and productivity. The situation can be even worse when there are inconsistencies or poor choices among the pollinizers.

Pollination, Pollinizers and How to Choose Them

Walnut trees are monoecious. This means that walnut trees bear both male flowers (catkins) and female pistillate flowers. The pollen produced by the male flowers pollinates the female flowers, and this is how walnuts are produced. However, in most walnut trees the female and male flowers mature at different times — this minimises the chance of self-pollination in most walnut varieties. Flowers that are not pollinated during the period when they are receptive drop from the tree.

To maximise tree pollination and, with it, the productivity of a commercial orchard, one or several walnut varieties known to produce male flowers that overlap with the female flowering time of the main (production) variety are planted. These trees are called pollinizers.

Matching Pollinizers: The Chandler Example

The male and female flowers of the popular Chandler variety overlap only partially (about 35%). However, the male flowering time of Franquette overlaps completely with the female flowering time of Chandler — making it an ideal pollinizer for Chandler.

Although overlap of flowering periods is the most important factor in determining the right match between production and pollinizer varieties, there are other factors to consider as well:

• Some varieties, such as Franquette, produce more catkins at maturity; this makes them poor pollinizers at a young age.
• Some varieties are known to be more susceptible to bacterial diseases.
• For this reason, using 2 or even 3 pollinator varieties that complement one another creates the most effective pollination.

How Many Pollinators Are Needed?

Young orchards located near mature orchards that produce plenty of pollen can benefit from it and may not need many pollinator trees. Others have no external pollen source at all. As a general rule, it is recommended to have 5-6% pollinator trees in your orchard. These should be split between different varieties. For example, in a Chandler orchard:

• 2% Franquette
• 2% Fernette
• 1% Ivanhoe (Ivarto)

Pollinators should be distributed evenly throughout the orchard, taking into account the prevailing wind conditions at flowering time.

Placement and Marking

Knowing and managing the location of your pollinator trees is a vital practice. They may require different treatments at different times, such as different sprays or different irrigation regimes. To do this easily and reliably, it is good practice to plant your pollinators in rows.

Why Should We Mark Pollinizers?

Pollinizers leaf out at a different time than the production cultivars.

Walnut trees are primarily wind-pollinated — meaning they are not as dependent on insect pollinators such as bees as some other crops are. Wind plays an important role in carrying walnut pollen over long distances to reach the female flowers.

In a few walnut varieties, the male and female bloom periods overlap. These varieties generally do not need a pollinizer. Tulare is a good example.

Hybrids and Clonals

The walnut we know, Juglans Regia, is a member of the Genus Juglans L. There are 20 other species of Juglans L., each with different characteristics.

Hybrids

Hybrids are produced by cross-pollinating two different species of the same genus in order to combine the superior traits of both species in a "hybrid". Paradox, one of the first popular walnut hybrids, was produced by cross-pollinating Juglans Regia and Juglans Hindsii (Northern California Black - NCB). Paradox has been the preferred rootstock in California for decades because of its vigour and its resistance to adverse soil conditions. Seeds produced from Paradox trees are used to grow other Paradox trees for use as rootstock. It is important to understand that Paradox is a hybrid seedling (grown from seed).

Limitations of Hybrids

Despite its superior traits, like any hybrid, Paradox has certain limitations stemming from the genetic variability in Paradox seeds. Each seed (even when taken from the same tree) 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 every seed is different from the next. One seed may be more vigorous, one may be more susceptible to phytophthora, one may encourage greater fruit production, while another encourages more leaf and branch growth."

MJ209 is a hybrid produced by cross-pollinating Juglans Regia and J. Major (also known as Arizona Walnut), and it is widely used in timber production because of its vigour. Just like Paradox, MJ209 is a hybrid seedling, not a clonal rootstock like Vlach, RX1 or VX211.

Clones: Genetic Uniformity

Scientists have solved this problem of variability by propagating a single plant with superior characteristics and creating tens of millions of new plants with the same traits — this is called cloning.

A good way to compare hybrids and clones is this analogy: while every hybrid is a sibling or cousin of the others, every clone is a propagation of the same living organism. Plants are cloned in tissue culture laboratories, using specialised growing media under very precisely controlled climate and lighting conditions.

Hybrid and Clonal Production

Producing plants on hybrid rootstock is very easy and inexpensive compared with clonal rootstocks, which require an expensive tissue culture laboratory, technology that must be purchased or licensed, and sophisticated greenhouses. The sterile laboratory environment and soilless propagation of clonal rootstocks (in tissue culture media) also prevent root diseases, as is the case with seedlings grown from seed (including hybrids).

Vlach, RX1 and VX211

The most successful clonal walnut rootstock still in very widespread use is Vlach — it was produced by cloning a single Paradox plant with superior characteristics, located in Modesto, CA. During various trials over the last 15 years in California, two very promising new clonal rootstocks, RX1 and VX211, were developed — both show superior characteristics compared with Vlach. At present, the commercial sale of RX1 and VX211 outside the USA is not permitted.

Clonal or Seedling?

In most cases, a walnut tree grafted on a clonal rootstock will be more expensive than a similar tree grafted on a seedling rootstock.

On certain soil types — deep, well-drained and free of any significant physical or chemical imbalance — a seedling plant with a good root system will perform "almost" as well as a clonal plant. The difference lies in maximising the uniformity of tree performance: clonal trees, which supposedly share the same genetic traits, will all perform in the same way, whereas differences arising from genetic traits will be observed in seedlings.

When "money is no object" — which is usually not the case — clonal trees should be preferred. In the real world, however, when a grower has to make decisions that involve trade-offs over where to allocate limited financial resources, given the soil characteristics I mentioned above, it may not be wrong to use good seedling plants and put that money into an area that creates more value.

Root System Differences

Another factor to consider when choosing your rootstocks is the difference in root system characteristics between seedling and clonal rootstocks:

• Seedling trees tend to produce large pivot roots (also known as taproots, which grow vertically downward).
• Clonal trees (such as Vlach) tend to produce a denser mass of fibrous roots that explore the soil laterally and stay close to the soil surface.

In addition to the many functions they perform for the plant, root systems also act as a temperature sensor, triggering many of the tree's critical phenological functions and serving as the tree's body clock. In regions where summer temperatures are extremely high, walnut trees slow down their metabolism to prevent tissue damage. They do this by assessing the ambient temperature through their roots. Because root systems that go deeper into the soil are more insulated from heat, this protective mechanism is triggered at much higher temperatures. A root system closer to the soil surface, on the other hand, tends to be more affected by heat.

Source: Selecting the 'Right' Walnut Rootstock, Pacific Nut Producer Magazine, 10 September 2020.