A new UK-wide study by Hedges Direct ranked the Vale fifth in Wales and 77th in Britain, scoring 56.5 out of 100.
The rankings are based on seven factors, including climate, garden size, soil quality, and access to public gardens.
Martin Chaplin, head of horticulture at Hedges Direct, said: “The conditions gardeners are working with vary much more than people tend to realise.
“There are more than five months between the shortest and the longest growing season in this study, and a typical garden runs from under 50 square metres in some areas to more than 700 in others, so two gardeners doing exactly the same things in different parts of the country will get very different results.”
The Vale of Glamorgan has a 320-day growing season, 77 days of ground frost, and a topsoil pH of 5.93.
The area also has 18 gardens on the historic register and 20 that open to the public.
Anglesey took the top spot in Wales with a score of 61.5, placing 13th in the UK and finishing ahead of Pembrokeshire and Monmouthshire.
Anglesey was praised for its long growing season, mild climate, and large, affordable gardens.
Pembrokeshire came second in Wales and 23rd in the UK, while Monmouthshire ranked third in Wales and 30th in the UK.
Powys was fourth, earning the highest score in the UK for gardens to visit – 51 historic gardens and 80 open to the public.
Blaenau Gwent placed lowest in Wales and 337th nationally, with just one historic garden and no public gardens.
Ceredigion, ranked sixth in Wales and 82nd in the UK, was noted for having the largest gardens in Wales, averaging 393 square metres.
Six of Wales’ 22 areas made it into the UK’s top 100 gardening locations.
Mr Chaplin also highlighted the importance of garden shelter.
He said: “Wind does more damage to plants in an exposed garden than cold does, and a hedge filters it rather than forcing it over the top the way a solid fence does, which makes a real difference to what will grow in an exposed spot.”
