After most plants listed in the botanical lists there is the letter G, and the letter P, each followed by numbers, ranging from 1 to 10. The letter G stands for garden hardiness, and the letter P for suitability as a pot plant.
High numbers for G (ie. garden) indicate a plant that we consider to be hardy and reliable when planted in a suitable position, low numbers indicate a plant that may be less reliable, may need extra care in positioning, or in other words, might be a bit of a challenge. A rating with a wide middle spread, eg. G3-7, indicates either a plant that performs well in some gardens but not in others, or one that we don’t yet have an extensive knowledge of as a garden plant.
The rating with regard to pot culture, P, relates more to how easily one can present the plant in the pot over a period of time. It takes into account how much effort and care is required to have the plant looking good. A high maintenance plant, requiring regular trimming, fertilizing or repotting, will not score as highly as one which presents nicely without much attention. For example, a plant that will survive well in a pot, but quickly look straggly, will have a lower rating.
Example - The Pandani, Richea pandanifolia is not so easy to grow in the garden, requiring a well-composted, well-drained, moist and cool position. I would suggest a garden hardiness rating of G 2-6. However we have found it to be very reliable in a pot, even withstanding considerable drying out, and not requiring much attention, so its pot suitability rating could be P 6-9.