Advanced plants stock selection

Jonathan Garner underlines important considerations when choosing garden plants and trees.

We’ve discussed what to look for when selecting garden plants in smaller containers. Before we talk about selecting advanced stock, think about how you get your plants from the nursery to the site. If it’s on a ute or truck and they’re not covered, you really need to think about lifting your game. Your uncovered plants have gone from a protected environment to experiencing galeforce winds where the foliage is thrashed, and the root balls are often horizontal with potting mix spilling out.

Cover them with shadecloth.

The material will greatly soften the thrashing and will see them arriving to site in a much better condition. Enough said.

If you’re transporting plants on a ute or truck and they’re not covered, you need to lift your game. Image: Shutterstock

Advanced stock

Now it’s time to talk about selecting advanced stock, feature or specimen plants.

These treasures will be intended to be in the landscape for a very long time. Most of the smaller growing species being planted have lifespans that can range from three to five years for plants such as lavender or rosemary, five to 15 years for plants such as gardenia, 15 to 25 years for plants such as murraya, then 25 to over 100 years for treasure shrubs such as camellias, magnolia or feature trees intended for shade, privacy, focal points or avenues. Most of these highvalue and long-lived specimens are supplied in container sizes that range from 25 litres to over 1000 litres.

When it comes to planting for the long game, and definitely when planting trees, we need to ensure:
1. There is sufficient soil volume
2. Soil preparation is appropriate, and
3. Planting practices are correct.

Beyond these, we also need to be sure the specimen has been grown properly, has the potential to grow into the intended feature or specimen plant, and that either we or the grower has performed the appropriate formative pruning (a topic for sometime next year).

Criteria for above-ground assessment of tree stock include: true to type; height and calliper; health; crown symmetry; injury; stem taper, selfsupporting, stem and branch structure, formative pruning, trunk position; compatibility of graft unions; and freedom from pests and disease. Image: Shutterstock


Back in the 1990s, a highly respected industry player, Ross Clarke, developed specifications for landscape trees called ‘Natspec’. This specification grew from the original British Standard 3936 and has now evolved into Australian Standard AS 2303:2018 (Tree stock for landscape use). Just like the soils standards discussed last year, AS 2303 provides clear and strict specifications when it comes to growing trees. It specifies the criteria for the assessment of above-ground and below-ground characteristics of tree stock supplied for landscape use. It covers container-grown, bare-rooted and ex-ground tree stock. The standards are applied to all stages of growth and are broken into three sections.
* Criteria for above-ground assessment of tree stock describes ‘the tree you can see’, and includes: true to type; height and calliper; health; crown symmetry; injury; stem taper, self-supporting, stem and branch structure, formative pruning, trunk position; compatibility of graft unions; and freedom from pests and disease.
* Criteria for below-ground assessment of tree stock describes important aspects of the tree below-ground, some of which you can see and others you need to dig for. They include: root-ball diameter, root-ball depth, height of root crown, non-suckering rootstock, pests, diseases and weeds, root direction, and root division.

Tree stock balance assessment

Balance is the relationship between the visible parts of the tree (above ground) and the invisible parts (below ground). Apart from bamboo, a plant that’s 1800mm tall shouldn’t be growing in a 25 litre container.

The benefits of buying from a grower that adheres to industry standards is being able to rely on the grower that when he or she says the plants are good, they will be good. If you can’t be guaranteed the plants are grown to standard here’s a checklist roughly based on the standards which should help with what to look for inspecting stock in the nursery:
1. Is the plant true to type and labelled correctly?
2. Is it healthy and free from any active pests or diseases? Is the crown symmetrical and the tree hasn’t suffered any significant injury?
3. Is the stem tapered and can the plant support itself without stakes or other artificial supports?
4. If it’s a tree, then Is the clean stem height less than 40 per cent of total tree height and the branches smaller than the stems (Ideally less than 50 per cent of stem diameter immediately above the branch junction)?
5. If a defined central leader is appropriate for the size and species – such as conifers – is the terminal bud intact and is the main stem upright?
6. If it’s a multi-branched species such as Manchurian Pear, are the terminal buds on each branch intact? Are the stem and branch unions sound, and does the clear stem height equal or exceed any specific requirement?
7. Is the trunk roughly in the middle of the root ball? If there are any graft unions present do they look sound? Are the girths consistent above (the scion) and below the union (the understock) and are they free of splits?

If you’re happy with the tree you can see and the balance looks okay then it’s time to check for the important aspects of the tree below ground before accepting the stock.
1. Is the root flare at the uppermost surface of the root ball? If not, how deep is it? (to be discussed later during our planting chat)
2. Is the root ball wider than the height and doesn’t exceed 660mm high?
3. Is there suckering growth at the base?
4. Is the plant free from pests, diseases and weeds within the root ball?
5. Have the roots fully occupied the pot or container?
6. Are the roots heading in an outwards or downwards direction, and is the root ball free from: J roots (circling roots within the root ball); kinked roots; girdling roots; or woody circling roots at the extremity of the root ball?

Determining and addressing these in pots greater than 450mm can be very tricky. Regardless of how tricky, it’s very important to address problem roots.

In multi-branched species like Manchurian Pear check the terminal buds on each branch are intact, stem and branch unions sound, and the clear stem height equals or exceeds any specific requirement. Image: Shutterstock


When trees are grown in containers, any interaction between roots and the container always leads to deformities in the roots. The most common of these deformities are girdling roots. These are created when the emerging roots make contact with the walls and base of the container and then track around the container edge. If these are left untreated during production or at planting they can cause a number of serious longterm problems. Strangulation, lack of support and reduced resource uptake are the three major issues which we’ll cover next issue.

Think now for the future

Beyond the aesthetic of seeing attractive, long-living plants, everyone now understands the importance of growing trees within the built environment. The urban heat-island effect is serious. Heat waves are occurring every summer and top the strike-rate for deaths from natural disasters. A healthy tree canopy can reduce pavement temperatures by between 11 degrees and 25 degrees. If flora isn’t grown and planted properly it will never achieve the designed intent.

Landscape professionals now play a vital role in the development and health of the built environment. Our time is now. It’s time landscapers step up and take this planting gig seriously.

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