Have you chosen high school well? For a quiet, unassuming kid, it's important she doesn't get lost in the hustle and bustle of a big school.
Does she play an instrument? A school orchestra is a great activity for socially anxious children. They don't have to make much conversation but have a shared goal.
Choir is a good shout. Is there a cathedral choir where you live? If she is good, they'll bite your hand off to have her. Singing lessons are also good, but not that sociable, good for confidence.
The problem is, if she is only interested in gardening and nothing much else, it might be that she can't easily relate to groups of girls and seems uninteresting to her peers at this stage. It does help for her to be open-minded and have a range of interests, as well as taking a genuine interest in others.
Would you consider sending her to a summer residential? Encourage your DD to learn a sport, which is fantastic for secondary school, even if she just starts to run and gets into an athletics club. Sign her up for netball classes so that she gets picked for the team in September.
Drama is also great. Is there Stagecoach or something similar near you? If she isn't naturally sociable (yet), work on her talents. Also make sure she has a nice rucksack for school and generally good clothes when she starts high school. These things can smooth social difficulties out. Does she read? It is time to me her on to books that older girls read so she can take part in chats about that.
Sometimes it just takes that one friend, in or out of school, to give them the boost to strike up more friendships and be seen as someone who isn't friendless. Make sure your DD has a phone and is able to communicate with new friends in secondary school.
Having said that, girl friendships aged 9 - 16 can be nightmarish. Hormones, competitiveness, frenemies and a general lack of social nous at that age. Be prepared for a roller-coaster ride. Lots of supportive chats with your dd and try not to catastrophise.