For indoor & outdoor plants you should go to the Omani souq or the various garden shop near the wholesale market.
To get there, go from Ramada over the bridge to Qatar Decoration roundabout. Go back towards Midmac and take the first street onto the sliproad. After Bukanan Furniture turn right. Now you are on the street that has many garden centres on your left. The omani souq is the hangar looking structurre on your right when you go over the 1e speed bump.
Check out the places before you buy as they have differences in prices. Specially at the Omani souq where I find the prices are over the top, but they have more choice.
Aisha-Taweela