The importance of understanding nurseries in treecare

THE knowledge of nursery production systems by those who procure, plant and manage young trees is, in my experience, limited at best. Yet every tree planted has come through a production system of some sort and every production system has to make interventions in the way a young tree grows and develops. These interventions, while necessary if the range of species and variation in size, shape and form of young trees available is to be maintained, all impact on the evaluation of tree quality, planting methodology and eventually the likelihood of transplanting success and longevity in the landscape being achieved.

What follows is a summary of the principal nursery productions systems used to produce standard trees in the UK. It can only be a summary as space dictates, but hopefully it might encourage the reader to look more closely at nursery production and develop a greater understanding of the trees they are using from the nursery.

Poor or non- existent formative pruning. (Image: EA)

All trees, at some point, start from seed. The young seedlings may be allowed to develop into maturing plants, be used as understock for the budding of clonal selections or provide the parent plants for vegetative propagation.

READ MORE: Tree planting: Is the performance gap growing and not the trees?

So, to begin with the seedling. Growing naturally, the young seedling will produce a dominant tap root with a lateral root system produced just below the still developing and indiscernible root flare. This will vary from species to species and often between plants of the same species. It is here where the nursery will make its first intervention. Seedlings will largely be produced in open ground or protected seed beds or some form of cellular system. On lifting from the seed bed, the developing tap root is cut as the young plants are lifted. This results in the lateral root system, which would naturally occur just below the root flare, not developing with its natural vigour. A usually more vigorous, adventitious root system develops at the cut point, which is often significantly deeper than would occur naturally. The gap between the adventitious root flare and the natural, now weakened lateral root system is known as the root shank and is more or less visible on all nursery produced trees. This root structure is maintained throughout the life of the tree and has an impact on planting depth as the dominant root development is deeper than would occur naturally. This depth is transferred to planting and can exacerbate deep planting when transplanting trees.

Trees grown in cellular systems can also suffer root disturbance. But the principal potential problem is with root circling. Root circling is associated with a young tree being left in a container for too long and the young, developing roots following the side wall of the container and circling. Movement to a larger container can encourage lateral growth, but the circling problem will not go away and is there for the life of the tree with occasionally disastrous consequences which do not reveal themselves until the tree has been planted out into the landscape, with stability being the prime cause for concern.

Trees produced from cuttings tend to generate root initials at the cut end of the cutting being produced. Cuttings are generally inserted into compost for rooting and again there is a real possibility the young root system of the tree will be deeper than it would be if growing naturally, with consequences similar to those caused by the lifting of young seedlings from the nursery bed. There are many variations on this theme, with cuttings of varying sizes used for different species and cultivars, so it is not always apparent or even significant.

Birch lifted showing poor root system and evidence of deep planting. (Image: EA)

The important point is that those using trees produced on the nursery do not generally understand the process, usually cannot evaluate whether it has been carried out according to defined nursery best practice or assess the implications of the practices when it comes to transplanting into the landscape. 

This lack of understanding endures as the production process continues towards the size of tree that will be planted into the landscape. 

All cultivars which are clonal selections are produced by vegetative methods, that is to say by cuttings or more commonly by budding or grafting onto an understock produced as a seedling. For example, Sorbus aucuparia is invariably used as the understock for all cultivars of Sorbus. These are all genetically identical. Budding and/or grafting are both once again nursery interventions and these have implications with regard to the evaluation of quality and eventual planting success and longevity. Bud/understock incompatibility can result in distortion at the point of the union with stock and scion growing at uneven rates and even failure.

A girdled root system on Ginkgo biloba. (Image: eA)

So, the discussion continues. There are three principal methods of producing standard trees and above used in the UK. These are bare-root trees, root-balled trees and containerised trees. There are other methods to be found, but these are the most common. Each works and is of equal value, but each has had a different series of interventions made during the production process. Each system has advantages and disadvantages, and each has constraints. Understanding those advantages and disadvantages is critical if planting methodology is to be appropriately decided on. A full and comprehensive discussion on the advantages and disadvantages of each system can be found in BS 8545, Trees: From Nursery to Independence in the Landscape, published in 2014.

Then there is formative and structural pruning. The view seems to be that the tree from the nursery is the finished article and that the crown produced is the final version. This is obviously not true, yet the concept of further formative pruning and then pruning to develop the final canopy is not often considered. The nursery intervention is largely to produce a good strong and straight central leader which will be able to support the final canopy of the tree as it develops. Most nursery formative pruning is focused on this, with lateral branches subordinated to the central leader. This series of pruning interventions carried out on the nursery has to be understood and developed in the landscape after planting. It is therefore essential that a knowledge of nursery pruning and the reasons for it are understood, so that this can be continued as the young tree grows in the landscape and most young standard trees are still in ‘nappies’ when planted out. Failure to develop the canopy can lead to expensive and costly interventions beyond the nursery having to be made as the tree matures in the landscape.

BS 8545, mentioned above, has a full and comprehensive section on nursery practice and the implications of nursery practice. As was suggested at the beginning of this article, this has only been a summary and the interventions made by the nursery are many and descriptions of them and their implications far greater than can be expressed here. However, it remains true that having an understanding of nursery production systems can improve transplanting success and that nurseries will have to continue to make those interventions if the range of species, sizes, shapes and forms is to be maintained and even enhanced.