Keith Sacre considers a few of the most notable advantages and disadvantages of containerised tree production of which arborists should be aware.

THERE has been much written in these columns and other publications about the production of trees in containers. There are many advantages and disadvantages to containerised tree production.

READ MORE: Tree care: When is a rootball not a rootball?

These were comprehensively outlined in BS 8545 Trees from Nursery to Independence in the Landscape, published in 2014, and are set out below:

• The root system is entire and undamaged.
• Containerised trees can be planted at any time of the year, although soil conditions in the summer can be a limiting factor.
• Trees are generally easier to handle than root-balled trees.
• The trees are generally easier to store than trees from other production systems.
• Post-transplanting stress and shock is reduced to a minimum, consequently achieving earlier benefits from planting.
• They generally weigh less than root-balled trees, as the growing medium is usually peat-based rather than soil-based.

• The organic soil-less compost used in containerised mixes can shrink if allowed to dry out post-transplanting. This can lead to shrinkage of the compost, which can cause difficulties with lateral root formation into the indigenous soil.
• There is always the potential for root circling in any container. Irrespective of the container type, if a tree is left in any container for too long, its roots fill the pot, becoming distorted.
• It has been argued that the container compost media contains none of the beneficial micro-organisms found in soil.
• They are generally more expensive than bare root or root-balled trees.

Perhaps most notable among the disadvantages is the potential for root circling or girdling to occur. There are many products on the market which hold out the promise of preventing root circling, but experience suggests there is not a product which will prevent root circling from occurring. It is all a question of timing and, again, experience suggests any tree left in any container for too long will suffer from root circling and girdling. It is why Barcham Trees introduced the concept of ‘shelf life’. This concept ensures that trees are only in the container for a known period. To my knowledge, Barcham Trees is the only containerised tree nursery to make this provision as part of the process.

Forestry Journal: A serious case of more advanced root circling.A serious case of more advanced root circling. (Image: eA)

It must be remembered that root circling/root girdling, where the roots hit the side wall of the container and are directed around that container wall as they thicken, may only produce stability problems in the landscape many years after planting. 

It also must be remembered that the containerisation of trees, where the trees are lifted from the nursery field and containerised for a relatively short period of time, is different from trees which spend their life on the nursery in containers being moved from container to container as they grow and develop. This method of production also has advantages and disadvantages. These were also outlined in the above British Standard and are set out below:

• Trees never have to be lifted from the nursery field and are less likely to suffer root damage if handled correctly.
• Trees are grown in a controlled environment throughout the production process.
• Irrigation and nutrition can be regularly monitored and easily adjusted throughout the production process.
• Trees can be planted all year round, although soil conditions in the summer can be a limiting factor.

• Trees must be progressively moved from smaller to larger containers until the final dispatch container is reached.
• Root circling is potentially more likely to occur than with containerised trees. As the tree is pot-grown throughout its life, there is an opportunity for circling to occur at each stage when the root reaches the sides of the pot.
• Movement from smaller to larger containers must be carried out at the optimum time in terms of root development. Poor timing can result in root circling and root deformation. This will result in eventual failure in the landscape.
• Re-potting into progressively larger containers can result in the root flare becoming incrementally deeper in the container.

Again, root circling/girdling is considered to be a potential problem and it is a question of timing if this is to be avoided.

Increasingly, trees are being produced from seed sown in plugs, spending their whole lives in containers – and the plug must be considered a container – before being transplanted into the landscape. It is perhaps worrying that a system designed primarily to produce bedding plants is increasingly being adapted and used for the production of trees.

Close examination of tree seedlings in plugs, often just weeks after germination, will show the first signs of root deformation, with roots guided downwards along the wall of the plug itself and then turning back on themselves to move upwards through the compost. 

Forestry Journal: Root deformation is a common problem with plugs and cells.Root deformation is a common problem with plugs and cells. (Image: eA)

How detrimental this potentially is remains unclear as the movement to plug production is relatively (well, in tree time) new. Undeniably there are signs of root deformation at a very early stage, with lateral root development apparently inhibited.

A conversation with an established producer of forestry seedlings using traditional field seed bed production confirmed concerns over root deformation and his view was that the jury is out and it remains to be seen whether there is any long-term impact. 

As with the production of containerised and container-grown trees, it is all a question of timing and, while there is some scope and margin for error when moving trees from the nursery field into containers when they are 8–10 cm girth or larger, the margin for error is greatly reduced when considering plug production as the container is considerably smaller The time spent in one size of container is also shorter.

This is certainly an area of nursery production which needs further research and the implications tested and examined, if the search for efficiency and mechanisation does not lead to unanticipated consequences.