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202012 Fresh Quarterly Issue 11 01 A Strong Start
Issue ElevenDecember 2020

A strong start

The physical and physiological characteristics of nursery trees. By Grethe Bestbier.

How you start something often affects how you finish it, and the importance of a strong start cannot be overstated. In the same way, the physical and physiological characteristics of nursery trees play a significant role in how well the trees establish themselves. Knowing what to look for when buying young trees can mean the difference between the success and the failure of an orchard.

Why do nursery tree characteristics matter?

“The planting of an orchard is one of the largest investments a producer will make,” says Prof. Wiehann Steyn, assistant general manager at Hortgro Science. “Nursery trees are costly, especially when you plant a trademarked cultivar and rootstock at a high density. It is also a long-term investment. Depending on the cultivar, an orchard is planted to last for 20 years, more or less.”

If your orchard doesn’t perform as it should in the first two years, there is a risk that it will never be profitable. Nursery tree quality is therefore of the utmost importance.

“If you plant a tree and manage it correctly, you expect a certain height increase, and the development of a number of shoots to fill the space in the rows, if you plant at high density,” says Prof. Karen Theron, Hortgro Chair in Applied Preharvest Deciduous Fruit Research in the Department of Horticultural Science at Stellenbosch University.

“However, you will often find that a tree that grows poorly from the start, remains weak indefinitely. Even if you add extra nutrition, spray the leaves and approach consultants for advice, it could be too late. You might have to remove the trees entirely and start over.”

To ensure nursery trees of good quality, it is important to understand their physical and physiological characteristics. Theron explains that physical characteristics refer to the appearance of the trees, including size, root system and graft unions, while physiological characteristics refer to the tree’s internal functions and processes.

What you see: physical quality

The physical quality of nursery trees plays a huge role in how successfully trees establish. One important physical characteristic is the ratio between shoots and roots.

“There is a correlation between what happens above ground and below ground,” explains Steyn. “Hormones cross talk, so the shoots and roots are in communication about how their growth is faring. For every circumstance, the tree tries to establish a balance between shoot and root growth.”

Nursery trees are typically planted densely and pushed to grow under ideal conditions, leading to more shoot than root growth. After being transplanted to an orchard, the tree will first aim to re-establish a balance in its new environment, generally favouring root growth at the expense of shoot growth.

This disproportionality between shoot volume and root volume cannot always be avoided, but nurserymen and producers can improve matters by preserving as much as possible of the root system, and ensuring correct soil preparation and irrigation. Producing trees in bags is another option for reducing transplant shock due to root disturbance.

Besides the quality of the root system, tree size also matters. Larger trees tend to display more total new shoot growth, and to fill their allotted space earlier. Unfortunately, large trees often suffer more transplant shock due to a higher shoot to root ratio and relatively greater root loss during lifting.

“Studies show that smaller trees often don’t catch up with larger trees for the entirety of their lifespan,” says Steyn. “When planting, you want your trees to start carrying fruit as soon as possible. If you plant a tree that is almost fully grown, it can expand sideways and put on fruit. On the other hand, the first year after a too-small tree is planted is ultimately lost, since it first has to catch up size-wise.”

Length is not the only important factor. Stone fruit grower and technical adviser, Petru du Plessis, says that he would rather plant a thick tree that’s been properly hardened off than a long, thin tree with inadequate reserves.

It is also important that the shank of the rootstock is long enough so that the grower can plant the tree at sufficient depth without running the risk of scion rooting. Grafting too low can be a problem with trees derived from tissue culture.

Hidden characteristics: tree physiology

As active shoot growth comes to an end in autumn, nursery trees enter the paradormancy phase of development. During this initial resting phase, the tree not only develops the terminal and lateral buds, but also hardens off in preparation for winter and the entrance into endodormancy, or true dormancy.

“What this effectively means is that the tree needs a long enough period where it does not actively grow in terms of elongation visible to the eye, and where the leaves are still attached, for proper bud development and reserve build-up,” explains Theron. “So, what the nurseryman does is remove some, but not too much, nitrogen and water from the tree, to strain it a little.”

According to Theron, correct timing matters. For a tree to build up enough reserves and ensure good bud quality, it needs to spend between six and seven weeks in paradormancy. Trees also harden off during paradormancy to protect them from stress factors such as moisture loss during endodormancy. From paradormancy, the tree sheds its leaves and moves into endodormancy.

A too-short paradormancy phase caused by the tree’s continued growth prevents trees from properly hardening off before they are lifted. Dormancy can also be disrupted by premature defoliation, leading to the loss of a large portion of important reserves and poorer bud break and growth after planting irrespective of the amount of chilling received.

Nurserymen are often under pressure to produce the largest possible tree and, as we’ve seen, size does matter. But nursery tree quality is about more than just size. It’s also about producing a tree that is physically and physiologically prepared for life in an orchard

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