Part of a hope-based network
restoring and enjoying the Mahurangi

Editor Cimino Cole

Cimino 8 December 2009

View from the Tractor Seat: Seedlings growing in open-ground beds at the Forest Research Institute in the 1980s. Most aspects of the operation were performed from the seat of a tractor. Photographer Jonathan Barran

The scale is enormous.

Radiata pine seedlings are produced, and planted by the million.

But while scale is a factor, the main reason indigenous plants cost on average 10 times more than exotics is the labour-intensive production method. Radiata pine seedlings, in contrast, are literally grown from the tractor seat.

Preparing open-ground beds
Prior to full field cultivation, with a roller tiller, the subsoil may have to be loosened to ensure free drainage.

A bed-former is then deployed, similar to those used to prepare market garden beds.

(Confirmation that preparation of open ground beds is best done from the tractor seat was brought home at Ömaha in the first year of the trials. Lacking a locally-available bed former of suitable width, the final bed formation was done manually. Not to put a too fine a point on it, the company and conversation were more memorable than the productivity.)

Sowing
Radiata pine is readily grown from seed directly sown in open-ground beds. This will also be possible for a number of indigenous species that, like radiata pine, are natural colonisers. Tötara, being a coloniser, may be in this category. It is the predominant coloniser of pasture in Northland—including because, being comparatively unpalatable to sheep and cattle, it survives moderate grazing.

Harakeke is another coloniser that potentially can be grown readily from seed. It has the enormous advantage of suffering little transplantation shock and having an exceptional ‘shelf life’ between lifting and planting. Because it grows to a limited height, it is potentially invaluable, as a low cost filler species, in establishing target timber, amenity or restoration trees.

Other species, such as kauri, are not natural colonisers and establish better in other than full sunlight. Many indigenous species are also too frail as young seedlings to be suitable for growing open-ground directly from seed. This also applies to some exotic forestry species including eucalypts. Accordingly, such species are initially geminated and grown in plug trays indoors before being transplanted to open-ground beds.

Seed is drilled in straight rows at regular intervals. This aids subsequent root conditioning operations.

Currently in Aotearoa, plug-raised seedlings are manually transferred to open-ground beds. Transplanters designed to mechanically position up to 7000 vegetable seedlings per hour are being evaluated for forestry nursery use.

In the Mahurangi trials, both container and open-ground seedlings were raised in plugs.

Victorian Solution: Transplant Systems field transplanter can place 7000 seedlings per hour. Those being planted here, however, are lettuce! Photograph Transplant Systems

Root pruning
Aside from weed control, which can include both chemical and mechanical methods, the next intervention involves mechanical root pruning.

Undercutting is perhaps the most important mechanical operation in open-ground nursery practice as it largely determines the quality of the seedlings.

Undercutting causes a carbohydrate transfer from foliage to roots and therefore height growth is slowed and root growth is increased. The unconditioned root system of a seedling is impossible to extract from the nursery and does not transplant well.

When a seedling’s roots are conditioned by undercutting, wrenching and lateral root pruning, they respond by producing new roots from points further back along the severed roots.

Because of the impact on the plant, only one intervention should be performed at a time, with time allowed between for the seedlings to fully recover from the stress.

The other main intervention is lateral root pruning, whereby the lateral roots between the rows are pruned with self-aligning discs.

Pruning of lateral roots across the rows (cross-pruning) is desirable for some species, but does adds to production costs.

Lifting
Seedlings are lifted by hand—to date, mechanical lifting has proved damaging to delicate root systems.

Prior to lifting, the beds are worked by a reciprocating blade wider and thicker than the undercutting blade, inclined at 20° to gently lift and loosen soil and seedlings.

After lifting in handfuls, individual trees are teased apart and elongated roots trimmed.
Storage and transport
The seedlings, immediately after lifting, are packed wet into forestry boxes. These are fabricated from white fluted plastic sheet, which provides a measure of protection from the sun.

Ideally plants are stored and transported in cool storage at high humidity—desiccation, particularly of the root system is principal enemy of open-ground seedlings after lifting.

A subset of the trial involved seedlings held in cool storage for one week prior to transportation, but otherwise seedlings were transported in un-insulated white painted trucks and delivered within 24 hours of lifting.

Top Gear: An M-Planter in action in Finland. The machine, excluding reloading, is recorded planting at up to 351 seedlings-per-hour. Photograph Finnish Forest Research Institute

Planting
Forestry trees in Aotearoa are invariably planted manually.

In the United States, and subsequently in other countries including Australia, tractor-drawn furrow-forming tree planting implements have been used. The original machines were based on potato planters. Limitations include unsuitability for use on uneven terrain and undesirable consequences of furrow formation, including the drying out of ground.

In Finland, two different designs of furrow-less tree planting machine are in use. Being track, rather than wheel, -based, they are designed to cope with the range of terrain typical of Aotearoa.

In short
While much can be achieved in the context of urban and park restoration planting using the container methods that have prevailed since the 1970s, indigenous forestry means, as Jaap van Dorsser puts it, growing trees from the tractor seat.




Seed High-viability seed, resulting form intensive seed cleaning practices, is particularly important in direct sowing. If seed has low viability, appreciable gaps will result in the open-ground beds rendering the operation uneconomic.

High viability seed is particularly important in direct sowing. If seed has only low viability, so many gaps may result in the open-ground beds so as to render the operation uneconomic.

When raising monocots such as harakeke, seed viability is not nearly as critical. This is because several seeds can be planted in each interval to ensure a high probability of at least one successfully propagating. And if more than one seedling establishes, a useful clump results—whether subsequently planted intact or separated.

It is probable that various seed coating techniques available will render the seed of some indigenous species sufficiently manageable for direct sowing into open-ground beds.

Seedlings or plants Open-ground plants, or open-ground seedlings?

The correct term, if grown directly from seed, is seedling. However if the open-ground plant was first propagated in a plug tray, it is can correctly be called plant, or more technically, a transplant. When an open-ground nursery uses both methods, which generally will be the case, it will have open-ground plants and open-ground seedlings, which presents a problem of semantics when referring to them collectively.

In the indigenous forestry nursery context, seedling nicely conveys the phase of the plant.

That, and because plant is also used collectively for flora and to differentiate plants from trees and shrubs grasses—e.g. flax plant—the Mahurangi Magazine has simplified its open-ground terminology and standardised on seedling whenever it refers to open-ground seedlings, regardless of whether they are transplants.

Open-ground or bare-root While the term bare-rooted natives makes for racey headlines and titles, the Mahurangi Magazine judged that standardising on open-ground better conveys large-scale forestry-style seedling production.


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Part of a hope-based network restoring and enjoying the Mahurangi
 Editor Cimino Cole