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Award of Garden Merit (AGM)

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Choosing plants at RHS Garden Wisley

The Royal Horticultural Society's Award of Garden Merit (AGM) helps gardeners make informed choices about plants. This award indicates that the plant is recommended by the RHS.

AGM Plants - complete lists

What is the AGM?

With more than 100,000 plants available in the UK alone, how can you tell which plants are the best for all-round garden value? The RHS Award of Garden Merit (AGM) exists to help gardeners make that choice.

Criteria

The AGM is intended to be of practical value to the home gardener. It is awarded therefore only to a plant that meets the following criteria:

  • It must be of outstanding excellence for ordinary garden decoration or use
  • It must be available
  • It must be of good constitution
  • It must not require highly specialist growing conditions or care
  • It must not be particularly susceptible to any pest or disease
  • It must not be subject to an unreasonable degree of reversion in its vegetative or floral characteristics

Plants of all kinds can be considered for the AGM, including fruit and vegetables. An AGM plant may be cultivated for use or decoration. It can be hardy throughout the British Isles, or suitable only for cultivation under heated glass. It can range in size from Sequoiadendron giganteum to Cornus canadensis. 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.

Hardiness rating  

Every AGM plant has a hardiness rating. The hardiness rating is an integral part of the AGM, and should be included in any citation of the award. This is intended to serve as a general guide to growing conditions, and should be interpreted as follows:

H1: Requires heated glass
H2: Requires unheated glass
H3: Hardy outside in some regions or particular situations or which, while usually grown outside in summer, needs frost-free protection in winter (e.g. dahlias)
H4: Hardy throughout the British Isles
H1-2: Intermediate between H1 and H2
H2-3: Intermediate between H2 and H3
H3-4: Intermediate between H3 and H4
H1+3: Requiring heated glass; may be grown outside in summer

For plants requiring heated glass, further guidance is given as follows:

Minimum temperature (oC)

15: Hot glasshouse
10: Warm glasshouse
2: Cool glasshouse

Committee assessment

The AGM is only awarded after a period of assessment by the Society’s Standing and Joint Committees. Committees draw upon the knowledge and experience of a wide range of members, including nurserymen, specialist growers, and well-known horticulturists. Assessment for AGM takes place in one or more of the following ways:

  • during trials at one of the Society's gardens or at some other venue
  • during examination of specialist collections
  • during round-table discussions by committee members, with contributions by specialists when necessary

Each Committee is responsible for recommending plants for the AGM from within its own particular area of interest. Round-table discussion necessarily played a large part in the first ten-yearly AGM review, the results of which appeared in the 2003 edition, but trials continue to be the principal means of judging garden merit. This is especially the case in plant groups where large numbers of new cultivars are introduced each year.

There is no grading system within the AGM, and no attempt is made to distinguish the good from the very good. Committees are expected to set a particular standard against which each plant is to be judged: if a plant equals or exceeds the standard, it may be recommended for the award. No limit has been placed on the number of plants that may hold the award at any one time, but in groups that include many cultivars, standards have to be set especially high if the AGM is to offer helpful guidance to the gardener.

Who uses the AGM?

While the AGM focuses attention on a relatively limited range of plants there is no cause for concern that in doing so it will lead to a diminution of the general range of plants on offer in the trade. There will always be those who promote specialists' interests, who delight in growing something different or enjoy the challenge of a difficult plant. These are the people who are already plant enthusiasts.

Perhaps more than anything else the AGM will help those who wish to take advantage of the knowledge of professional gardeners when choosing plants, and their appetite for more (and perhaps the obscure!) may be whetted.

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Orchids

Orchids

The RHS Orchid Committee is re-introducing Awards of Garden Merit for Orchids; a new list of AGM orchids will be available in the New Year.

New plants

New plants

Well-respected plantsman, Graham Rice, uncovers all the latest new plants in his blog.

RHS plant bulletins

RHS plant bulletins

The results of many of our major trials are now being produced in full colour plant bulletins.