Friends of Miss Murrell and her family and campaigners will walk through the Maengwynedd Valley at Llanrhaeadr, near the Shropshire/Mid Wales border, on Saturday to place stones on the memorial.
The trek will be led by Rob Green, Miss Murrell's nephew, and the event will celebrate Miss Murrell's life and her love of the Welsh hills.
Maengwynedd Valley was her favourite place and mentioned in the last entry in her diary before her death.
The death of Miss Murrell, whose body was found in a field three days after her disappearance from her home in Shrewsbury in March 1984, sparked off conspiracy theories that she was killed because of a paper she was writing on the nuclear industry.
In 2005 Andrew George, who was 16 at the time of her death, was convicted of her kidnap and murder and sentenced to life imprisonment.
Her murder and the discovery of her body and abandoned car five miles south of her home threw the international spotlight on the 78-year-old's interest in nuclear power.
Marnie Sweet, from Llanrhaeadr, is one of those involved in the commemoration.
"I didnt know Miss Murrell but her death had a profound effect on myself and my husband. We moved from London to Llanrhaeadr and became involved in an annual walk to Maengwynedd valley were the cairn has been built up.
"Miss Murrell loved this part of the world. When she retired she bought a cabin near Llanymynech and loved visiting it. And Maengwynedd was her favourite place."
She became an expert botanist, and extracts from her nature diaries were published in 1987 illustrated with her coloured photographs and drawings.
The walk to the cairn will be followed by a simple ceremony at a tree planted in her memory at the home of Mrs Sweet.
Miss Murrell was head girl at Shrewsbury Girl's High School before graduating from Cambridge in the 1920s. She joined her father's rose nursery and took over as director in 1937. Just three weeks before her death rose grower, David Austin gained her approval to name a rose after her.