The peak of the market's thatched roof is planted in the traditional manner with the aptly named roof iris (Iris tectorum) and sempervivum.  These species are planted there for a reason.  Not only are they uniquely adapted to and able to thrive in this curious, nutrient-poor environment, but they serve the vital function of drawing the moisture out of the thatch at this crucial and vulnerable juncture point, thus preventing (or at least forestalling) leaks.  Talk about ecological building practice!