Containers Forest Seedling Propagation

Seedling tray design impacts on seedling propagation in several ways. Unfortunately, the tray which best suits the nursery manager (from the point of view of seedling propagation) is not necessarily the best one for the plantation manager or investor. Problems in the plantation can stem from poor root form and low vigour of the seedlings obtained. Some of these problems are now known to be attributable to tray design and steps can be taken to reduce these risks.

Overall tray factors

Construction design is often dictated by the material and method of manufacture. Currently, the most common method of manufacture is by injection moulding using either high density polyethylene or polypropylene plastic. This method and material allow a great degree of flexibility and control over intricate shapes, as evidenced by the most common trays in New Zealand nurseries.

Injection-moulding using other materials is not common in New Zealand, but expanded polystyrene trays are still used in Canada and South Africa. This material limits the degree of complexity in the cell architecture.
Copper root-pruning has been very commonly used in these trays, some designs are now also incorporating air root-pruning technology

Vacuum-forming is a process whereby a sheet of extruded plastic, commonly polystyrene, is sucked into the mould. This creates severe limitations on the intricacy of the cell design, especially with regard to any holes or slots required as these have to be made as a separate process.
For Eucalypt seedlings, only small plug trays with cells typically in the 3-7ml volume size, are made using this method.

To prevent root circling, copper root-pruning paint may be used, particularly limiting the copper to the base of the cell as the small cross-section of these cells can result in an excess of copper in the seedlings.

Air vents between cells have become common in a wide range of tray types. These vents promote air movement around the leaves. This air movement is credited with the commonly observed effect of fewer incidences of foliar diseases. In addition, where air root-pruning is used, this air movement between the cells assists the pruning action.

A wide range of tray sizes is available.
A number of "standards" have been developed, frequently based on a useful size at the time of design of the tray, and subsequently standardised simply through common use.
In some cases, the origins of the tray size are derived from the maximum surface area possible in the manufacturing machine!
Weight of the resultant filled tray is a common limiting factor, both for handling within the nursery and where the trays are shipped out with the plants. Bigger trays with more cells are more economical to move though, resulting in a compromise.
Cell configuration is largely limited to a regular square pattern, most suited to the widest range of seed sowing and seedling de-plugging machines.
Designs with alternate rows off-set are becoming less favoured, partially because of the limits on machine handling, but also because staggered rows reduce the plant density on the row edges.

Other practical considerations in tray design reflect the desire to have trays nesting for storage and shipping, special features on the outside edges of the tray to allow for automated de-stacking of trays onto the filling and sowing line, and an architecture that facilitates cleaning of the trays both physical removal of debris and rapid infiltration of chemical or heat sterilisation procedures.

Cell design

This is an area of major points of difference between competing trays.
Our understanding of the effects of cell size, spacing and architecture on plant growth in the nursery and after planting out has increased dramatically over the past 20 or so years. A number of practical constraints affect the suitability of certain cell designs.

Ease of filling the cell evenly is facilitated by a relatively shallow, open design. The deeper and narrower the cells the more difficult they are to fill. Angled cell walls are essential if the trays are to nest, and are also required to make for easy root plug extraction. Possibly more important than the nursery management impact is the effect these cell design features have on the root plug handling characteristics. For example, long, thin root plugs are easy to damage and difficult to plant correctly.

Reducing exposed surfaces of the plug reduce the chances of water loss. Unfortunately, this can also reduce root vigour as root growth requires a healthy balance between oxygen and water availability. Reducing the cell surface area either means plants are spaced more closely together, or there is a greater loss in water between the cells if the cell spacing remains the same. These are all compromises available between nursery management, economics and plant growth responses. A feature now almost universally advised from extensive research is that the cell architecture must not shape the roots, although it must necessarily shape the root ball. To achieve this, some means of root-pruning, or breaking of root apical dominance, is required. The object is to form a fibrous root system in the nursery stage that will promote many root tips, hold the root ball together for handling, and prevent a few dominant roots forming that will circle in the cell (in other words become pot bound). This root plug will then handle easily without damage and grow from active root tips on all sides, and eventually, through the re-establishment of apical dominance, lose the appearance of a planted root plug and take on a natural root form.

The advent of side-slot air root-pruning technology has allowed this method of root-pruning to now gain dominance.

A side effect of side-slot cell design is the increased exposure of the root system to air. This has a beneficial effect since plant roots require more oxygen than leaves. In a very real sense, the nursery should be aiming to grow a good root system, the stem and leaves are essentially secondary.

The size of the cell needs to be considered as small cells are cheaper for the nursery, but plants might not be suited to the plantation growing conditions. No definitive research appears to have been done to determine an optimum cell size and spacing, but cells broadly within the range of 400-850 plants per square metre and between 50 and 100 mm deep appear to work well in most plantation sites.

Eucalypts are small-seeded and produce a fine root system very early on.
This characteristic means that plug systems can be used, especially where germination is erratic or expensive growing conditions are required for this early germination stage.
Plug cells of 3-5 ml volume are commonly used.

Risk reduction

Tree plantations are an inherently risky proposition. The long time scale of the investment, and the exposure of this investment to many risk factors indicates that all practical efforts to reduce risk should be taken.
In general terms, three types of decision should be preferred to have a maximum effect for minimum input.
Firstly, if possible, push the decision up the management chain so that the person ending up making the decision is the one with more knowledge and more able to make an informed choice. For example, the inherent predictability of the root form of containerised seedlings compared to bare-root plants indicates that after due investigation, senior management can choose a containerised system and a container with characteristics that will normally prove advantageous.

Secondly, prefer a single decision that has multiple effects. For example, choose a container with side slot air-pruning rather than a container system that requires regular treatment and monitoring of the treatment.

Thirdly, prefer the decision to be made under conditions of closest scrutiny and/or best staff training available. For example, trained nursery staff grading and packing seedlings can be expected to not pack cull plants, while field planting staff are more interested in planting than selecting plant quality, especially if they have carried them a long distance

Container-related risk factors

Instability of trees.

Instability of trees is almost always related to a problem with the root system. Trees that snap at ground level or show marked butt sweep commonly display a ball and socket growth effect. This results when major roots are allowed to be shaped by the cell walls of the container, they are grown in (that is, are not pruned on the sides).
These root plugs typically have easily visible roots running down the outside of the plug and the root tips are all oriented downwards or are even angled in under the root plug.
Because new root growth is mostly only from the ends of these dominant roots, this poor root form is retained and the roots simply grow radially, pressing in upon each other and resulting in a very small cross section of attachment between the root system and the trunk. If allowed to grow, the tree might eventually completely enclose this mass and regain stability.

Butt sweep.

This is a milder form of instability and commonly associated with non-prune containerised seedlings. The root typically grow straight down from the root plug, with little lateral root growth. Economic impacts are considerable as it is the butt log, the biggest volume in the trunk, that is affected.
At harvest, a large volume of timber might be left standing if the cut is made above the sweep. If the sweep is harvested, then difficulty in loading and stacking logs occurs, plus the differences in timber properties between compression and tension wood that exist in the log can make down-stream processing is less efficient.

Poor vigour.

This is more likely to be associated with nutrient, moisture and temperature effects.
Container design can increase the risk of roots desiccating. It is commonly believed that roots on the outside of the root plug are to be encouraged as these will be in direct contact with the soil after planting.
This is a false assumption as these roots are more exposed to the risk of desiccation, in addition, water uptake and new root growth occurs from cut surfaces or active root tips.
Root-pruning, with its tendency to have many root tips, and these tending to be more within the protection of the growing medium in the root plug should be preferred strategies.

FOLR effect.

Research is indicating that plants with a greater number of first order lateral roots are inherently more vigorous. This appears to be largely under genetic control, but some cultural factors can affect it too. Plants without root-pruning have fewer FOLRs and therefore can be expected to exhibit reduced vigour. This effect appears to have not been tested yet.

Conclusion

Cell tray technology has shown some major advances towards reducing the risk to plantation owners through poor establishment or subsequent failure after planting.
Nursery risk is barely affected by container design, except within very broad parameters such as cost of manufacture, longevity of the tray, ease of handling etc.

Plantation owners need to have a clear understanding of the impact various container designs may have on their investment.
At present, the optimum cell design is likely to be close to a cube in shape, incorporate air root-pruning, be 60-100mm deep and plant density 820 plants per square metre or less. No particular benefit appears to arise from transporting the plants in the trays to the plantation, while sorting and packing in the nursery reduces freight and handling costs.