Hello! I'm in horticulture. I don't have a job title as such.
A couple of years ago I hit a real mental health low patch, and I was struggling with my previous career. My brother is a gardener and suggested I go ask the local nursery for a temporary job to give me something I could focus on. So I stumbled in (with my four-year-old daughter holding my hand). I had shopped there for years and knew the owner by sight. He offered me a job on the spot (and only remembered to ask salient details like my name, and why I wanted to work there, afterwards!).
At first I did three short days but now I am full time and quite often clock up a 40+ hour week. There is a large-ish team (three men and 8 women regulars, plus occasional staff, though most of us don't work full time). We do everything from weeding polytunnels, planting seeds or pricking out seedlings, to potting up rootball stock, planting hedges or pruning trees. It can be very heavy work and I am usually covered in mud and shattered, but I absolutely love it and find it really rewarding. It's amazing seeing stock through from the early stages to sale, and seeing the same people coming in to talk about their gardens, and so on.
I thought about qualifications, but I think I learn so much on the job, it's not really worth it for me (I'd only want the qualification in order to say I had one, if you see what I mean).
I'm sure it would depend on what nurseries are near you, but I'd really advocate for going and asking if you can do some work (NB: probably rocking up with a child, cheerfully admitting you're in the throws of a MH breakdown and want a temp job, not the best approach ...). It's so useful getting contacts and you will find you could easily pick up gardening jobs just from talking to people. I don't, but most of my colleagues do - having the nursery to use as a base is invaluable.