The reasons are manifold:

1.  Weeding should be done exclusively by hand and the scarce availability of resources and staff prevents weeding all the flower beds; alternatively the mowing, although occasionally practiced where possible, still ends up favoring a few really invasive species.

2.  Not even chemical or mechanical weeding, repeated several times, would guarantee the eradication of the most invasive species. In some cases, errors of this type have  been made, the negative results of which are still visible to the east, in the lower part of the GBH.

3.  Many spontaneous plants are not invasive plants and contribute to enrich biodiversity.

4.  The flowering of some invasive species, such as Oxalis, contribute to reviving GBH in certain periods.

5.  Sometimes the presence of a spontaneous and composite herbaceous layer helps to maintain a moist layer of soil, also favorable to cultivated plants.