Light up box dotted around the gardens? It could have different pieces of information written on them. Would make the gardens more accessible at night, and add a bit of colour to the park. Could each box be a similar colour to the roses it tells information about?
Architect Rick Reeder has designed a suite of Wayfinding products, all minimal and architectural in form, refined and sophisticated in presence, with integrated LED lighting. More information: http://www.landscapeforms.com/en-us/products/pages/Reeder.aspx