The garden, based near Probus, is one of six in Cornwall that has been involved with the NGS since its inception in 1927.
Initially founded to raise money for the Queen's Nursing Institute, the charity continues to open gardens of quality and interest to the public 85 years later supporting nursing, caring and gardening causes.
Trewithen's Head Gardener, Gary Long, said:
"We are proud to have supported the NGS from the start. It is an extremely worth while cause and we will continue to do so in the future."