coffee grounds are excellent for holding moisture in dry weather,but you should add quality compost as coffee grounds hold no nutrients of note,but before you add this, put a circle of black plastic sheet in the bottom of your tyres with plenty of holes in ,this will suppress weeds and let any excess water drain away,.mix a little grit in the mix of compost and coffee grounds to help with drainage too,about 2 weeks before sowing or planting add a hand full of general purpose fertiliser such as growmore and mix in to the top third of your compost ,this should grow most things.....hope this helps ?