Tree care: When is a rootball not a rootball?

IN the last issue of essentialARB I wrote about how a lack of understanding of tree nursery production systems influenced transplanting success and longevity in the landscape post-planting.

There is no more prominent example of this than the question of what a rootball is and how this differs from something which only shares its appearance. Visually, any tree with soil around the roots wrapped in hessian and held together with a wire mesh can be called a rootball, but visual assessment alone can often be misleading.

A true rootball will have gone through a nursery production process over a number of years which has continued as the root system of the young tree develops. The production system is designed to ensure the root system lifted from the nursery fills and holds together the soil ball lifted, wrapped in hessian and consolidated with wire mesh.

The process involves the severing of the developing root system periodically as the young tree develops in the nursery field. This can either be achieved by undercutting the root system or lifting, pruning and replanting. More commonly, the process is completed by undercutting the root system. It is an expensive and time-consuming process and should be reflected in the final selling price.

What happens is that at each cutting the roots are severed. At the point of severance a proliferation of new roots is stimulated. As this process is continued layers of new root proliferations are stimulated resulting in continuous development of a full fibrous root system through the eventually lifted rootball.

This process is illustrated in the diagram below.

The numbers indicate the number of undercuts made and where they are made.

This process is clearly defined in the British Standard 8545 Trees: From Nursery to Independence in the Landscape. These standards are outlined in the table below.

It can be seen from the above that the criteria outlined for producing rootballs is clearly defined, but as has already been stated, not all trees wrapped in hessian and bound by wire mesh are rootballs. Certainly they look like rootballs but have not undergone the rigorous production process necessary to produce proper rootballs.

So what are they? Usually they are trees which have been lifted from the nursery field without any of the undercutting having taken place. The outcome of this is the severance of roots which have extended well beyond the crown of the nursery tree. These roots are damaged with very large pruning or cutting wounds apparent in the root system which is supplied. Research has indicated that as much as 95 per cent of the root system can be left in the nursery field if this procedure is used. It is then left to the tree to recover after planting into the landscape, often in conditions which are harsh and where maintenance is less than optimal.

It is fair to say that wrapping the root system of a bare root tree in soil and then employing hessian and wire mesh is a legitimate method of a nursery protecting the vulnerable root system during transport from the nursery to the planting site and consequently of value, but the trees are not rootballed.

It is fairly easy to differentiate between a correctly prepared rootball and an incorrectly prepared rootball on delivery. If the trunk of a correctly prepared rootball is moved from side to side, then the whole of the rootball should move with it. If the ball remains still when the lateral movement is occurring, then it is likely the tree has not been prepared correctly and it is what is known as a ‘made-up ball’.

The root framework apparent when the soil is removed from a correctly prepared rootball. (Image: essentialARB)

There is also much discussion of how the hessian and wire mesh should be treated when a rootballed tree is planted. In an ideal world, the hessian and wire mesh should be removed completely prior to planting, allowing room for the lateral roots in the ball to develop and extend laterally in an uninhibited fashion. 

However, there is often a discrepancy between the ideal and the practical, particularly with semi-mature trees where the size of the rootball and the difficulty in moving it with ease makes this impossible. In these cases, the British Standard referred to above recommends the hessian and mesh is cut and peeled back to between 30 and 50 per cent of the depths of the ball and rolled into the pit area. This facilitates the uninhibited development of the lateral root system.

It is also worth noting that often during the production process there is a mounding around the base of the tree which buries the root flare of the tree to be moved. This mounding is often incorporated into the actual rootball and therefore can contribute to the tree being planted too deep in the soil. It is advisable to remove this mounding at planting and expose the naturally occurring rootflare of the tree which will vary from species to species.

A hole left in the nursery field following the lifting of a rootball. (Image: essentialARB)

As with all nursery production methods, rootballing is perfectly legitimate and where best practice is followed it works perfectly well, but like all other methods of production it has advantages and disadvantages. 

ADVANTAGES

• The lifting and transplanting season is extended when compared to bare root trees. 
• Trees that have poor survival percentages when handled bare root can be transplanted successfully. 
• Trees may be lifted from the nursery field ahead of time and stored above ground, if handled correctly, thus extending the period for transplanting beyond the dormant season. 
• Trees can be lifted during the dormant season and stored in the nursery for summer planting. 
• Care between lifting and planting is less critical than for bare root trees, as the roots are kept moist and frost-free within the rootball.

DISADVANTAGES

• If nursery practice is poor then as much as 95 per cent of the root system can be lost on lifting from the nursery field.
• Actual lifting from the nursery field is limited to the dormant season for all but a very small number of tolerant species.
• Handling of large rootballs is labour-intensive, with rootballs being heavy and awkward to transport. 
• If the rootball is broken or allowed to shift during handling and dispatch, the chances of tree survival are reduced. 
• Field soil conditions can limit times of lifting, with frozen, very wet and very dry soils being unsatisfactory. 
• Rootballs are generally more expensive than bare root trees. 

So, it is not rootballs themselves which are problematic. It is an understanding of nursery production systems which can be a limiting factor and lead to disappointment and potentially tree failure post-planting.