When it comes to selecting plants for installation in a landscape, big is often better. Advanced trees and shrubs provide instant impact, but this benefit can come at considerable cost. Advanced plants are often container grown for the landscaping industry.
In general, specialist growers of advanced stock can provide a variety of landscaping trees and shrubs up to 6m in height, ready to make an instant impact. Although advanced plants are more expensive, they are used widely to provide that instant effect and are very popular in commercial landscaping projects, particularly in commercial developments where the more established landscape can be cost efficient from a sales point of view.
Setting the standard
The growing of advanced container plants is a long-term proposition for production nurseries and as such requires a commitment to quality. The Nursery Industry Accreditation Scheme Australia (NIASA) Landscape Tree Stock Specification is designed for production nurseries supplying into the landscape, wholesale and retail supply chains to ensure nursery stock meets stringent quality guidelines thereby reducing the risk of plant failure and improving useful life expectancy of plants in the landscape. The best management guideline specifications provide instructions and procedures for production nurseries to implement and adopt when producing tree stock for the landscape sector. These include areas such as:
• General quality: true to type and tree stock health.
• Above ground aspects: height, trunk diameter, formative pruning, stem and branch structure, crown distribution, compatibility of graft unions and trunk position.
• Below ground aspects: root ball diameter and depth, height of root crown, rootstock suckering, root ball occupancy, root direction, root division, self-supporting.
The aim of the specifications is to ensure that advance stock is grown in accordance with Australian Standard AS 2303:2018. Recent research undertaken by Horticulture Innovation Australia found that nursery stock measurements taken across Australia show that landscape trees have a much greater variation in size index than the current standard indicates. For landscapers selecting trees, this might mean that they are now rejecting trees based on a standard that is too limited for tree production. However, the introduction of specifications and guidelines does provide some assurance that the investment in advance container grown plants can be made with some degree of certainty as to the success of that stock post planting.
Container types
The range of containers that production nurseries utilise for advanced plant production is as diverse as the plants they grow. Advanced stock is usually categorised by container sizes ranging from 25 litres to 2000 litres. Standard round black plastic containers are often utilised to grow advanced plants due to their relatively cheap cost, the savings made however, do have limitations in terms of plant quality, particularly in the root zone. Standard round containers can impact plant development and root architecture as the smooth interior surface of round containers can lead to circling roots and plants becoming ‘root bound’, which may ultimately result in failure of the plant post planting through an inability to develop a well-formed structural root plate.
Several container manufacturers now produce what are referred to as root trainers. Root trainers encourage roots at the interface with the internal side of the pot to grow in a downwards direction. A root trainer usually has grooves on the inside of the container, encouraging the roots to grow downward, and therefore prevent the plant from becoming root bound in the container. Whenever the roots encounter resistance on the edge of the container, they change direction and are ‘funnelled’ towards the bottom of the container resulting in a straight, well-developed root system. This avoids the problem of circling roots that grow in a spiral pattern, which over time can cause plants to become root bound in a container. A critical plant characteristic to avoid when purchasing advanced container grown stock.
It’s in the bag
Grow bags or planter bags are also a popular method for growing advanced plants. Available in a range of materials, the high strength woven polypropylene planter bags are one of the most popular, designed to withstand our harsh climate with high levels of UV inhibitor. The porous nature of many grow bags due to their woven construction and drainage holes in the bottom and lower sides provide adequate drainage for good root development. The root bound effect that can occur in traditional plastic pots doesn’t tend to occur with planter bags. Instead, when the roots hit the edge of the bag, they come in contact with the air. Air contact causes the root to stop growing, this alternatively encourages new roots to begin to form on the plant. This ‘air pruning’ promotes many smaller roots and makes it easier to transplant or move to a bigger grow bag later on. The benefit to the end user is a root system that has developed a fibrous mass of feeder roots that will assist with plant establishment once planted. One dr wback with planter bag stock is that the porous nature of the bags can lead to plants drying out if watering is not effectively managed. As with all stock brought onsite, plants should be installed as soon as possible to eliminate the risk of roots being compromised through water stress, particularly during hot weather. The popularity of bags is their relative ease to move around and transport with stitched handles making it easy for onsite relocation of plants. There are also planter bags that are biodegradable, allowing for direct planting into site soil, although there are limitations on such practices, and it is usually best to plant with an exposed root system to allow for better root development.
3D root air pruning
An innovative technology that has been in use for a number of years now in the production of advanced tree stock is the Rocket Pot Tree Growing System. The RPTG system helps to produce a high-quality root system that supports developing trees. The Rocket Pot containers are comprised of cusps in the wall and floor of the container
that stops roots from circling and guides them to open space for air root-pruning. There is a 5 per cent open area in the walls and 10 per cent open area in the cuspate plastic base beneath the container that allows the tree to ‘breathe’ even with low porosity growing media. The deep cusps on the walls of the container guides root tips to the outer edge of the cusps exposing them to the air where they will desiccate through air pruning. This in turn stimulates the production of several new roots to emerge well back along the root and grow out to become leaders if the Rocket Pot is removed or become desiccated if it is not removed. The process is repeated over and over until the pore space is fully utilised. Once the tree reaches a state of equilibrium, little growth occurs until the Rocket Pot is removed and the tree is planted. Given a well-drained and aerated planting site, the tree roots can rapidly explore and establish the site soil.
Maintenance of advance tree plantings
Advance container stock is no different in its requirements for establishment in landscape installations to smaller stock plants. The regular regime of watering, fertilising, pruning and pest and disease control are paramount to successful establishment and ongoing plant health. With advance stock there may be additional requirements and it is always wise to consult the nursery supplier for any special requirements for preparation for planting and establishment of advance stock in the first year of installation, however establishment periods can vary from three months to three years. Several factors need to be considered for successful establishment including climate, microclimate, edaphic factors, plant size and type of plants.
While there are many types of advance container systems and plants to choose from, at the end of the day it comes down to selecting plants from a reputable supplier with a track record for quality. Investing in advance container plants for landscape installations is buying time and as with any investments, it may be advisable to consult an expert in the field of advance plant care and maintenance to help in making informed decisions and determining the best approach for achieving desired outcomes.