This is the Royal Horticultural Society symbol for the Award of Garden Merit ‘AGM’. This award indicates that the plant is recommended by the RHS.
Awards are usually given after a period of trial at an RHS garden.
The AGM is intended to be of practical value to the home gardener. It is awarded therefore only to a plant that meets the tried and tested plant varieties using trial methods and criteria.
Though growing conditions and plant types may vary, the purpose of the award is always the same to highlight the best plants available to the home gardener.
No limit has been placed on the number of plants that may hold the award at any one time. In groups that include many cultivars, standards have to be set especially high if the AGM is to offer helpful guidance to the gardener. The award can be removed.
You can search for AGM plants at RHS sites. Detailed trials reports are usually such as this for carnations.
The Kiro LonKirok ‘was recommended for an AGM by the majority of the members present 6:1. The recommendation was proposed due to the outstanding quality of the flowers, the vibrant colour, form and its ability to provide flowers throughout the year together with a clove scent.’
‘Florence Franklin on the left was not recommended for any awards although it produced flowers for most of the year the flowers were small and the plant appeared to be reverting to Ann Franklin from which it sported.’
Read more on gardenerstips.co.uk