There is also N1 the Garden Centre on Englefield Road, small but very high quality plants.If you are feeling adventurous there are lots of nurseries/garden centres in Crews Hill, NW of Enfield and a very good nursery called Clockhouse at Forty Hill.
How about Colombia Road on a Sunday morning, it gets packed this time of year though however there is huge variety and lots of bargains to be had, particularly with bare root herbacious plants, much cheaper than garden centres and good for bagels too