You could look for gardeners with diplomas, associate degrees, bachelor degrees or certificates in horticulture, landscape design or garden design. The Royal Horticultural Society (RHS) is the UK’s leading gardening charity and runs specialist gardening courses for basic and advanced level gardeners.
Before you choose a gardener, you could read testimonials and read details of their services on their website.
You could also use Bark to look at their profile page and gardening reviews from past customers too. This might also help you get some ideas for the gardening service you want to book.