Planting on a field scale is one thing, and would be agriculture, I guess. Turning agricultural land into a vegetable garden would surely require change of use at the very least.
The 'suck it and see' approach is risky - you could invest time and money and then have to rip it all up and make good.
Personally I would make a telephone enquiry of my local planning office. Or, if you don't want to contact them, your local planning agent.