Exhibitors came from Hampshire, Sussex, Kent and Surrey and included Exbury Gardens, Nymans and Borde Hill.
This is the second year running that High Beeches has won best in show.
High Beeches Gardens representative Sarah Bray said: "Winning best in show is a great honour for everyone at High Beeches Gardens. We had a hunch that Rhododendron macabeanum was going to do well. It looks stunning and shouts out 'look at me!'."
She added: "This is the first year for a very long time that the Magnolias have not been hit by frost so they are covered in exquisite flowers."
Best in Show was R. macabeanum, which was introduced to the UK by Frank Kingdon-Ward in 1928.
Other prize winning Rhododendron from High Beeches included R. neriiflorum. It was introduced to this country by George Forrest in 1910 and is found in China, Tibet and Burma. Another winner was R. lutescens, which was introduced from China by Ernest Wilson in 1904.
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