Mr Millar said Mr Miehe was not involved with the Salvation Army but had visited him four months ago.
"He said, 'I'm sick and I'm going to leave you the lot.' We were thinking six-seven years, we didn't think it would be so soon. We are very, very overwhelmed by the generosity and appreciate that he could identify with the work of the Salvation Army, helping us to do what we do," Mr Millar said.
"[These gardens] have been part of Rotorua's educative programme, we've heard stories of school children visiting and now they're grown up. Parents have brought their children in."
Mr Miehe was a big part of the fibre of Rotorua, he said.
"He would love to talk and just enthuse about the plants, telling stories. People have been blown away by the wide variety of plants here. He had a deep love for them and an incredible knowledge."
Mr Miehe's son, Horst, said his father started collecting cacti in the 1990s.
"If he had them in the greenhouse there was always a flower coming out, no matter what time of year. He liked the different sizes and the variety of cacti," he said.
"I think he really respected what [the Salvation Army] did for the community and felt he could add value to helping the wider community."
The sale of Mr Miehe's cacti will be held on Saturday with a collection of succulents also up for grabs.
"Some of the (cacti) would be worth hundreds of dollars but the maximum price at the sale is $30. They all must go. There are huge ones in the garden as well; with a digger someone could take them away," Mr Millar said.
The cash-only sale will be held at Mr Miehe's gardens, 70 Sala St, from 10am to 4pm.
