The Award of Garden Merit (AGM) is a long-established annual award for plants by the British Royal Horticultural Society (RHS). It is based on assessment of the plants' performance under UK growing conditions.
Awards are made annually after plant trials intended to judge the plants' performance under UK growing conditions. Trials may last for one or more years, depending on the type of plant being tried out, and may be performed at Royal Horticulture Society Garden in Wisley and other gardens or after observation of plants in specialist collections. Trial reports are made available as booklets and on the society's website. Awards are reviewed annually in case plants have become unavailable horticulturally, or have been superseded by better cultivars.
If a plant has been awarded this mark, you can be confident that it is suitable for the majority of British gardens and it should do well in yours. The absence of the mark does not imply that a plant is difficult, or will not thrive. The RHS award is applied to a limited number of plants each year.