The Open-Ground and Container-Raised Indigenous Plants Comparison is a Sustainable Farming Fund project.

Part of the Mahurangi Initiative: A hope-based
network restoring and enjoying the Mahurangi.

Cimino 9 November 2009

Stems Streets Ahead: After six months in the nursery, the stems of the open-ground plants, measured at the root collar, are respectively twice and three times the diameter of the potted and root trainer plants. Bar chart data Mark Kimberley

First surprise was the pronounced difference in size.

After six months in the nursery, the average diameter of the stems of open-ground plants was twice that of the potted plants and three times that of the root trainers.

The canopies were similarly larger.

Canopy, of course, is the key to the size difference. Plant growth is primarily dependant on sunlight. Hence the more foliage, the more photosynthesis can occur, and hence the greater growth. The open-ground plants, having virtually unlimited elbow room, were able to grow to their full potential.

In contrast the container-grown plants, spaced according to the dictates of their containers, have only a limited canopy space—25 and 225 square centimetres respectively for root trainer (Hillson-sized) and potted (PB3-sized) as compared with 400, at the open-ground spacing used.

The height of the plants also emphasised the imperative for photosynthesis. The densely spaced container-grown plants were not proportionately shorter, reflecting the competition for sunlight. The result was spindly plants—in the case of the root trainers, extremely spindly.

Many Measurements: Plant height, crown cover and stem diameter of at least 50 individuals of each six species by the three raising methods were recorded—3848 measurements. Pictured assisting is Tare Kaere, a forestry student from Vanuatu. Photographer Michael Bergin

Measuring more than a few
In order to produce an accurate picture of the comparative growth rates, at least 50 specimens of each of the six species, of each of three raising methods were measured—nearly 1000 plants.

This measurement programme would later be dwarfed by the number of measurements, 6242, taken in the field after the plants were established, which is where the size difference, it could be said, really matters.

Root circling
As mentioned earlier, significant concern with plants raised in containers is the phenomena of root circling.

Root circling results from the universal response of plant roots to detour when encountering an impervious surface. Unfortunately, when the impervious surface is wall of cylindrical container, roots are unable to respond other than to grow in gradually, but never-the-less ever decreasing, circles—as the space for root growth diminishes.

Even more unfortunately when established, while the no-longer-constrained roots can finally resume their growth away from the stem, the spirals lack any mechanism for straightening. Instead, with the root growing ever thicker, self and mutual strangulation occurs.

For monocots, the consequence of root circling is not nearly as injurious. But in species that rely on a well-developed buttress root system for support, the result can be catastrophic failure of the entire tree.

Roots Straight, Roots Circled: The open-ground plant on left displaying roots mechanically pruned rather than constrained to circle endlessly inside its plastic container. Photographer Jonathan Barran

Root system distortion and quality of quantity of feeding roots are largely ignored where container-grown stock is used in revegetation programmes (Bergin and Gea 2007).

After six months in PB3-sized planter bags and pots, all species were displaying root circling, some to a pronounced degree.



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