The legislation would be the bit that requires the OC to maintain its common property. I am in the ACT so I can’t point to the relevant chapter and verse elsewhere but it is a fundamental provision in all strata legislation.
Could you perhaps find 2-3 allies who would work on the courtyard garden with you? Perhaps you could offer to the committee that you would provide the labour if they agree to reimburse you for just the small cost of a few plants, some mulch, a bag of compost … Work out a budget of what you would need and present that to them.
Then, at the AGM, you don’t put a motion but you do give a short speech about how the OC really needs to accept its obligation to maintain the common property and put more into the grounds maintenance line of the budget, even if that means putting levies up a bit. Point to the improvements you have made to the courtyard and the small cost that was involved.
If they had not agreed to even let a few volunteers fix up the garden at very little cost, then you point that out at the AGM and shame them into letting you go ahead.