3.2.2. The 30-cm Plots

These dry matter calculations were based on the 30-cm depth substrates, and the aboveground phytomass was harvested as described in Section 2. The 3 groups of 13 planter boxes were used in the following statistics. In the years between planting in 1999/2001 up to the first harvests in 2011 on building 2 and in 2013 on building 3, no harvesting of the plants influenced the typical growth of these plots. Working on the principle that extensive green roofs do not need maintenance, we allowed them to grow untouched for the first 10 years. The decline in the appearance was the initial reason for starting maintenance, which included mowing of the meadow. This means that the first years of harvesting, 2011 and 2013, respectively, have higher values than the annual productivity in the following years; see Table 9 with the standard deviation values.


**Table 9.** Examples of the harvested dry aboveground phytomass in g/m2 and SD from the 30-cm-depth planter boxes, calculated as dry matter/m2.

The 13 boxes with the same growing media should have similar growth rates, but as can be seen in Figure 7, the values for the phytomass vary widely. These variations are caused by several factors but are ultimately due to impacts by the users of the roof. The extensive roof growing media is poor media with no additional fertilizer. The annual aboveground productivity on this 30-cm media is between 100 and 400 g/m2. The boxes on the Op media on building 2 showed the best rates. The annual productivity of the dry mass ranges between 100 and 400 g/m2. The 8.5 million m2 of new green roofs in Germany [13] will result in fixation of between 1551 and 6205 t CO2/year.
