Numerous spontaneous herbaceous species grow on the natural travertine outcrops and spread throughout the garden like the ancient channels and the rocky ridge; these are xerophilous species, whose presence is linked to the arid conditions of the substrate. Among them we find some rare and highly local species, such as Ajuga chamaepitys (L.) Schreb., Allium chamaemoly L., Asphodeline lutea L. Rchb., some orchids such as Ophrys sphegodes Mill. subsp. passionis (Sennen) Sanz & Nuet, and the spontaneous hybrid Ophrys x grottaliensis P. Delforge & C. Delforge.

What enriches this natural flora is Santolina etrusca (Lacaita) Marchi & D\’Amato, a very rare and endangered species in Latium, introduced as part of an ex situ conservation project of species of the spontaneous flora of Viterbo. Within the project, both seeds and specimens were collected in nature, the former kept in the Germplasm Bank and the latter planted on a travertine outcrop to reconstruct the original environment.