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He died on August 25, a death notice in the New Zealand Herald says.
Born in Portadown, Northern Ireland, Sam was only 2 when his father died leaving him heir to the family rose nursery, established by his great-grandfather, the first Samuel McGredy, in 1880.
The nursery was requisitioned during World War II for the growing of vegetables and on his return from the United States at the end of the war, the schoolboy found "half-a-dozen scungy glasshouses filled with tomatoes and no one who knew anything about roses".
At its peak under his stewardship, the nursery grew one million plants on 120ha and had 160 staff. From about 60,000 seedlings a year, two or three were chosen for release to the market.
After several friends and business associates, both Catholic and Protestant, were murdered during the troubles in Northern Ireland, Sam decided to move to another country, preferably one where he wouldn't be so reliant on greenhouses.
He and his family arrived in New Zealand in 1972.
The New Zealand Rose Society paid tribute to McGredy, calling him an icon of roses in New Zealand and around the world.
The Rose Society says he helped establish Plant Variety Rights in New Zealand, which gave plant breeders of a wide variety of species the ability to protect new cultivars and make a return on their investment in developing them.
It says he had also encouraged and supported other rose breeders in New Zealand, who are now producing many fine roses.
"May Sam rest in peace and let us treasure all those beautiful roses he created."
He is survived by his wife, Jillian, his children and grandchildren.
McGredy was a big rugby fan and his funeral will be held at Eden Park on Monday.