Currently we measure pots in a variety of ways but in short, small pots, tubes and punnets are normally measured by size (diameter) of the container and larger containers, grow-bags and rocket pots are commonly referred to by their volume. It is very confusing for landscapers sourcing plants for their clients to simply compare sizes – and therefore apples with apples - if supplier ‘A’ has 40cm stock and supplier ‘B’ has 50 litre stock.
So which pot offers better size value?
Like quality, container size is an important factor to understand. Assuming you have plants of equal quality, being able to compare pots for size value alone, is a significant problem for the industry. Add to this dilemma the numerous pot styles available to the consumer that claim to be reflective of a certain common size, and the confusion instantly triples. From the Garden City Plastics catalogue, Australia’s biggest manufacturer of plastic pots, containers and bags for the horticultural and nursery industry, a 20cm pot can be supplied in five different styles with a varying volume of between 3.1 and 5 litres.
Could this be any more confusing? Well yes it can. Should you be sourcing material both locally and interstate, only the most pot weary members among us will be aware that a 50 litre container is sold in some areas of Australia as a 75 litre container. Similarly a 40cm diameter pot in some areas of Australia is known as a 40 or 45 litre container, but it actually holds only 27 litres. These are only two examples that readily come to mind, but it highlights the point that being able to make a value call by pot size alone is a very complicated task.
A long time ago when Terracotta pots were the only pots readily available, identifying pot size was as simple as finding a number stamped into the bottom of the pot. Apparently this digit referred to the number of pots that could be ‘thrown’ from a standard block of clay. In Australia before plastic pots but after terracotta, we sold plants in cans and tins; jam cans, fruit cans and kerosene tins. Of course these were measured by volume such as one quart or one gallon. This made perfect sense at the time and was uniform across the market.
With the development of plastic containers for growing plants, initially with bags and then to hard wall pots, there has been no move to regulate product sizes. If you go and buy a bottle of water, in fact any liquid, there is a volume printed on the container you are looking to purchase. If you buy a powder or any other solid material from store or supermarket, the volume or weight is clearly printed on that container. Look at soap, beer, sugar, fuel - all have a printed indication of what you are buying in terms of weight or quantity.
In most other industries you are obliged to put the volume of what you sell on the container, and that in turn makes the manufacturer or supplier obliged to provide accurate, auditable information. In the nursery industry, volume is seldom listed and within the industry there are calls to now have this addressed and remedied.
With this standardisation of weight and volume comes the need to settle on a common language if you like - so we can compare apples with apples, which takes either diameter of pot or capacity of pot as its descriptor in all cases. An obvious place to start would be to talk in terms of litres, as this is a standard measure that we all understand readily, and a metric measurement that can be tested or checked easily. Once guidelines have been set and a language of litres embraced across the industry, it will become a requirement for all pots, bags and containers to advertise their size.
At this time of year (July), being able to show the difference in growth between pot sizes is a lot easier with evergreen trees than deciduous, although once in foliage, size difference will also be evident for deciduous too. Here we compare a 50 litre Waterhousea with a 40cm specimen. While we interchange wording - '50 litres' with '50 cm', one refers to volume and the other diameter. It is the ONLY size that comes close, in all other pot sizes volume and diameter do not correlate.
Like Waterhousea floribunda 'Whisper', here is another example where you get a much bigger specimen in 50L than in a 40cm (27 litre) container. Size is quite significant. Our Arbutus is kept quite upright and tight with branching all the way to the ground. This is standard in many of our lines as it gives the purchaser flexibility to grow the tree as a single feature specimen or in multiples tightly for a hedging affect.
Natives are often easy to spot the size difference as in larger pots they shoot up pretty quickly and don't put much on in caliper until much older. Remember the difference between the 50 litre and 40cm container is some 23 litres of potting media and as much as eight to twelve months in extra growing time depending on the species.
An evergreen favourite, Magnolia grandiflora 'Teddy Bear' naturally grows quite short and round but what is evident here is the extra height and extra volume of tree you get when buying the 50 litre specimen over the 40cm one. Quality is apparent in both sizes, but the extra time in the nursery provides greater height and plant volume.
If you are buying in a large container or bag, look for labels or notations like ours. These give you either volume size (over 50 litre) or a measurement of the diamension of the pot (measured in centimeters) for material under 50 litres. Here is our 50 litre Waterhousea floribunda 'Whisper' clearly marked as 50L on the label. If you are in any doubt, we'd be happy to take you through pot sizing in more detail.