Drive to your favourite hardware store and look for parts that you need to repair roof borders, a famous internet translation tool says me that the english word for this is 'verge flashing', the bented aluminum sheet part I used is called 'Ortblech' in german. And in profile view it looks like this:

• measure the top of wall and cut aluminum profile with special scissors
• cut four small angle profile (aluminum)
• mark position of boreholes to join profiles via angle profiles and screw everything together
• fill interior with EPDM foil
• don't forget to cur hole in foil so water can flow out, dammed-up water kills your plants
• fill up interior with foamed clay (3cm)
• put flowe soil inside
• spread your plants so it looks pretty, buy only plants that need a few water
• don't forget to water your plants