Nursery Quality to Tree Planting Success – the “Airpot”


We can be proud of many landscape tree outcomes, but there are some alarming tree failures in our parks and streets that are costing us millions. Up to six parties are responsible for landscape tree outcomes: the designer, propagator, grower, planter, waterer and maintenance crew. Too often the grower alone is held to account for any problems.

We are fortunate to have so many excellent participants, but there are weak links in the chain of responsibility. These weak links are exacerbated by price competition in the absence of adequate quality standards.

I noticed the first dead eucalypt in a nearby park early in 2010, about seven years after it had been planted. I happened to walk past as the contractor was removing it, so asked to have it to take home. “No problem mate! There are plenty more like that”. So I threw it over my shoulder and took it home to dissect. The cross section view of the root ball shows a girdling root that has strangled the tap root.

Three more trees have died recently in the same park. The trees were in their eighth year. Failure occurred in the wind on a wet day – the root ball then exposed was encircled several times by thick roots. Trunk caliper was 150 mm and height 3.0 m.

The problem. Landscape tree seedlings are produced in tubes or cells developed for forestry. Delay in using the seedling can cause fatal root system flaws. Landscapers often need to cope with delays of weeks or months. Tubes and cells were designed for a shelf life of days. The seedlings shown at the end of the Y2010 trials were all sown on the same day 9 months earlier and are now all past their use by date.

The solution? First, get the right tools and procedures. Then skilled workers; trained tree planting tradesmen would make a big difference. “Advanced Tree Teamwork” is essential. Finally, measure outcomes and publish the results.

Tree costs. City of Hume cost data show that the cost to buy, plant, mulch and stake the typical tree is about $200 (AUD). The cost to water it to self sufficiency West of Melbourne is about $200. Total costs are about $400, if there are no failures. The Hume data suggest that the cost of a typical tree failure after one year is $288 (Tree Cost $110+Plant, stake $51+, first year mulch, water (24 visits) and spray weeds $127). $288 is more than 35 times the cost of an elite propagated seedling. Why skimp on the seedling and risk wasting another $288 needed to plant a second time? Using anything less than a perfect seedling is false economy.

About 2 million advanced trees are planted annually in Australia. The annual cost of planting and bringing these trees to self sufficiency may be in the order of $800 million. So what proportion of advanced landscape trees we plant fail to survive and prosper? We should measure and know the Australian figure but we don’t. Consensus opinion among five tree growers with vast experience was that:

More than 20% of advanced trees fail after planting and even more are seriously compromised.

The annual cost probably exceeds $100 million ($280 x 400,000). What other industry could tolerate 20% defective product? Cars? Appliances?

Managing container grown trees through their root system

After 8 years of trials, we have learned to manage trees by pruning their roots in all three dimensions 24/7 – starting a few days from germination. Our research is unfunded and basic. It seems that basic horticultural practices are being overlooked.

I believe that Industry R&D funds could be used to fine-tune a set of products and practices that will lead to consistent propagation, growth and establishment of superior ornamental trees.

Direct sowing is essential for good root architecture

In our small trial nursery, we hand fill pots, direct sow seed with a depth-controlled dibble (nut and bolt with flange), cover the seed with the same mix and water it in. We hand water until the radicle reaches the root-pruning base of the pot – then start flood and drain watering. We select the best seedling when it is about 50 mm tall.

Procedures in large nurseries should include mechanised pot folding, machine filling and machine seeding. It is already possible to stack palletized pots in blocks under existing overhead spray systems.


We are using relatively new technology in these trials. It’s generic name is three dimensional air root-pruning (3DARP) – This may be defined as:

“The use of containers with vertical walls and elevated flat bases with at least 20 mm air gap above recycling water flow, with both wall and bases fitted with at least 400 open ended root guidance cusps per square metre”.

I have spent about 20 years seeking to control a tree’s growth in a container, through its root system. This picture is the culmination of that work. For the first time, downward pointing roots can be made to grow on down and colonise the planting site below the root ball. We just have to have 24/7 automatic air root-pruning for direct seeding eucalypts in larger pots.

Caliper – Wise buyers purchase trees and tree seedlings for caliper and not height

We found that we can keep seedling height down and caliper up by:

– Limiting fertilizer incorporated in the media to less than 4 kg per cubic metre of 9-12 month release.
– Letting in light as the seedling matures.
– Rotating inside seedlings to outside.
– Sheltering the pot against evaporative cooling and root scorching.
– Keeping the root ball warmer than the trunk and of course
– Air rootpruning

24/7 with 3DARP. We attempt a caliper at least 1% of height but it is species specific


Our trials have led to a better understanding of root behaviour in air root-pruning systems. This picture shows the effect of pot size on seedling caliper in the first 28 days from seed. These Corymbia citriodora were direct sown with the same seed, sowing date, water and nutrients but different pots.
The diameter of radicles were:
– 0.5 mm for cell trays (RHS)
– 1.0 mm for forestry tubes
– 1.0 mm for 1.5 litre pots and
– 1.2 mm for the 1.5 litre and deeper 3DARP pots (LHS) So pot size does matter to seedlings.

Note that air root–pruning has started already on the 3DARP seedling over 200 seedlings were “key-hole dissected” with a compressed air jet during the trials. In every case an unbranched radicle had air root-pruned at the base. These two typical root systems at day 133 from seed show how the radicle is surrounded by an array of laterals uniformly arranged in 3D space.

Part of understanding root behaviour in 3DARP pots is to study root systems suspended in water.The specific gravity of the fresh seedling roots is fractionally higher than water (1.0). The washed out root ball hangs naturally in an aquarium tank.

Notable findings being used in the design of new 3DARP pots are:

  • The radicle runs quickly to the base of the pot and air rootprunes each and every time – if the humidity in the basal air gap is low enough.
  • Lateral root growth is stimulated and root-pruned at the wall – if the humidity in the air between pots is high enough.
  • Three or more roots emerge from each root that is air root-pruned, then nine, then 27 and so on.
  • The rate of root tip development becomes exponential.
  • In our Melbourne summer trials of fast growing eucalypts, colonization often occurred quite suddenly at about 110 days from seeding. (8 litre) and 90 days (1.5 litre)
  • The volume of wood in the trunk increases fourfold if the volume of mix in the 3DARP pot increases fourfold.
  • Germination in this windy site increased from 80% to near 100% when we increased dibble depth from 5 mm to 10 mm. The length of the seed radicle when first pruned is probably determining the future shape of the tree, but much more research is needed to verify and detail this finding.

The seedling’s natural shape fights with and suffers from the normal 7 degree inverted pyramid taper in smooth wall pots.
Consider two pots of the same diameter.
– A 200 mm 3DARP pot normally has an effective depth of 240 mm and 7.5 litres of useable volume.
– A 200 mm smooth wall pot normally has a depth of 190 mm and 4.5 litres of useable capacity.
– Parallel wall pots give about 50% more root space – just where it is needed.
– The shelf life of a seedling germinated in a 200 mm diameter 3DARP is at least 4 times the shelf life of one in a 200 mm smooth wall pot.

The trial equipment we used has led to the design of new procedures and racks. Notable findings from the trials include:

  • We still need to cull 20% of the crop – despite multiple seeds per pot.
  • Watering by weight works well. We weigh a sample at field capacity and monitor it daily. When the pot loses 30% of its original weight we flood, dwell for an hour and drain. In 8 litre pots, 1.2 litres of air is expelled and replaced with water.
  • Watering is every 3 days in windy hot weather but normally less frequent.
  • Filtration and sanitation with a 24 watt UV Bio-filter works well – running 50% of the time.
  • Salinity is measured and adjusted by dilution if it exceeds 1.3 ms (rare).
  • Culling 20% of a rack after 10 weeks allows re-spacing to let in more light to increase caliper and minimize height.
  • Lifting 8 litre pots repetitively is stressful. If the 8/10 litre pot slides (or better – glides) across the rack floor it is very easy to grade, cull and respace stock.
  • Correct working height and full access around the rack are essential for good OH&S.
  • Carrying pots individually from nursery to truck and truck to planting site is expensive and dangerous. It can be mechanized.
  • New pots and racks have emerged as modules of a shipping container.
  • Racks are fitted with a “mezzanine floor” that maintains an air gap of 50 mm beneath the pots for air root pruning and hygiene. This floor is a key component of the 3DARP system


Palletised Flood and Drain Tank for Propagation

Rocket® Tank for Mechanised Propagation

  • Grow, relocate and ship seedlings in the same Rocket Tank = labour savings to pay back capital in one year
  • Flood and drain watering = uniform water application to each pot at very low labour cost
  • New air root-pruning method = reliable radicle pruning and then all round root pruning
  • The complete nursery can be sold and moved as a going concern at any time
  • Tanks are at working height and can be accessed all round
  • Legs allow 50 mm adjustment on uneven ground
  • Levelling is easy – a group of 100 tanks can be levelled precisely
  • One piece moulding in recycled plastic with a five year guarantee = 20 year life expectancy
  • Contractors can load tanks with prefilled pots and deliver in bulk ready to sow

Rocket® Tank for Water Conservation


  • Closed circuit water system recycles all water with no ground contact = minimum water use
  • Sanitation by 12 Watt UV and bio filtration running 24/7 in season = clear water, no pathogens
  • 20,000 litre storage serves 40 tanks with occasional 10,000 litre top ups in mid-summer

Rocket® Tank for Precise Watering

  • Smart Tank option waters “just in time” = significant improvement in seedling growth rates
  • Smart Tank option = Automatic watering for unattended use in remote locations
  • Unique identification by NFC tag = tanks could be leased to conserve capital



“Best by” dates of 90 days in 1.5 litre and 110 days in 8 litre 3DARP pots

The benefits of using the Rocket Pot Tree Growing System

Using Rocket Tank with Rocket Pots brings all the benefits of the Rocket Pot Tree Growing System (RPTGS):


Pruning the radicle gives elegant root arrays when seeds are direct sown to 3DARP pots.

  • Grow quality seedlings with elegant root systems and better than 1% trunk taper
  • Quality seedlings yield quality trees that sell at a premium and grow so much faster in the landscape
  • Reduce failure rates by sowing direct in a single step to the pot size that will tolerate likely planting delays
  • – IPPS Sydney 2011 suggests that the loss rate in Landscape Tree Seedlings exceeds 20%.
    – BS 8545 Trees: from nursery to independence in the landscape – Recommendations – Introduction includes the comment: “… much as 25% of all planting undertaken in the public sector actually fails”.
    – Rocket Tank is an important first step in reducing the Australian failure rate where failure modes are dominated by self-strangulation of the radicle: particularly notable where seedlings have been kept too long in tubes/cells. Commonly caused by project delays.

  • Grow seedlings of twice the size in half the time – direct sow in a single step to bigger 3DARP pots
  • Reduce labour cost by eliminating drudgery and leave the propagator with fingertip control
  • – Cuts out pot handling between the nursery and its customer. No need to handle individual pots

  • Bulk handle pots with a pallet truck (concrete) or a fork truck (gravel) and halve labour costs
  • Conserve every drop of water. Use only what the tree needs – often saving more than 80%
  • Rocket Tank can be used to flood and drain with any pot


Pruning the radicle gives elegant root arrays when seeds are direct sown to 3DARP pots.

Papers containing tree trial results that justify these statements include

IPPS Melbourne 2000
Innovative Tree Production Systems 2003
IPPS Christchurch 2004
New pots and procedures for propagating landscape trees.pdf – IPPS Sydney 2011
New pots and procedures for propagating landscape trees.pdf – Published version



20 foot shipping container showing 5 available stacking options

  1. Single 23 litre + > 800mm trees
  2. Double 8 litre < 800mm trees
  3. Triple 1.5 litre < 500mm trees
  4. 6 stack of filled pots or germinated seedlings
  5. 10 stack of empty tanks


System Setup

Tank layout – 100 tanks can fit on an area the size of a tennis court in rows of 20. It leaves room for a small forklift to pick up any tank.

Level all the tanks by connecting them to the water supply and watching the water level in the tank as the hand operated levelling screw is adjusted. This can be done while standing.

Rocket Tanks are best installed with buried 32 mm LDPE pressure lines, 25 mm risers, 1000 watt submersible pumps (stainless steel) and 600 mm plastic headers that also act as sumps.

Costs of trenching, placement and back filling of a typical tanks installation is 70% of the tank cost. We can provide lists of fittings if required.

The recommended pumps work at low pressures. When the tank is full, surplus water overflows through the inbuilt outlet and returns to the sump.

The pumps are bi-directional and can be allowed to drain the tanks to sumps. This also allows the tanks to be used to collect rain water.

The sump design allows for surges of used water while being pumped back to storage.

Water Conservation

The Rocket Tank flood and drain irrigation system is a closed system. All excess water flows back into the sump including rainfall. No water reaches the ground. Water losses are restricted to evaporation and transpiration.

A 20,000 litre supply tank has proved adequate during trials of 2,000 8 litre eucalypts since 2002, supplemented by about 8 x 10,000 litre loads of water delivered in that time , usually mid-summer, to dilute for salinity management. Two 20,000 litre plastic tanks are adequate if supplementary water can be drawn from the mains.


Designed and made in Australia
Telephone 0419 323 050 or 03 97961422 – Fax 03 9796 1422
[email protected]

Smart Rocket Tank

Smart Tank puts the propagator in finger-tip control of a precision process

Flood and drain watering seedlings gives precise watering: all seedlings get the same amount of water. The smart tank system introduces a controller that automates the watering process based on field capacity. 100% field capacity is established by weight of the tank, pots, soil and tree immediately after watering.

Years of trials of the Rocket Tank Tree Growing System, growing eucalypts, have shown that the optimum growth occurs when the field capacity at rewetting, has dropped to just above the threshold of 70% field capacity. If the field capacity drops any further we start to see stress at 65% FC, considerable stress at 60% FC and death at 55% FC.

This rewetting threshold will vary for different species of trees and plants but the principle remains the same.


The controller avoids the field capacity dropping too low on a hot windy day.



  • The propagator elects the re-wetting field capacity level to suit the trees he is growing
  • The propagator sets the length of time they wish to soak the plants at each watering
  • The propagator can manually override the system at any time
  • System allows for remote monitoring and control
  • One controller can be used to control watering in a monoculture nursery
  • All components and electronics are sealed and weather proofed to guarantee long life
  • System may include or be linked to a weather station and log data for research and analysis purposes

Display may show info such as

  • FC = Current Field Capacity %
  • W = weight kg
  • FC Set point = Rewet Field capacity

Controls include:

  • Manual override
  • Field capacity set point adjustment
  • Soak time adjustment
  • Menu navigation


  • One Smart Tank can be used to measure transpiration losses for a group of similar trees
  • We recommend one Smart Tank per 20 tanks generally
  • One for each 50 is sufficient in a monoculture nursery
  • One in every 5 tanks may be needed in a nursery growing a diverse range

Leave a Reply

Your email address will not be published. Required fields are marked *

Related Articles

Back to top button