Qualifications improve your chances; they do not gate the job. Level 3 is the most recognised.
Level 3 most recognisedOpen entry
Your route in
You can start without qualifications - direct entry/volunteering; a qualification improves your chances
1
Qualifications employers recognise (Level 3)
Any of these strengthens your application.
Direct entry possible with no qualifications; horticultural worker/operative (L2) progresses to technician/supervisor level (recognised Level 3 Diploma/Advanced Apprenticeship) then horticultural manager.