Sideritis cystosiphon is endemic to the island of Tenerife, Canary Islands, Spain (Gonzlez Gonzlez et al. 2004, Euro+Med Plantbase 2006-2010). Its area of occupancy is 2 km. This plant is only known from one location in the west of the island: Laderas de Tamaino, which comprises four subpopulations.
Sideritis cystosiphon is a xerophile and heliophile species. It occurs within communities Euphorbietum atropurpurae and Periploco laevigatae-Euphorbietum canariensis, on entisol and lithosol soil types. Frequent accompanying species include Lavandula buchii, Retama rhodorhizoides, Kleinia neriifolia, Euphorbia obtusifolia, Aeonium urbicum var. meridionale, Opuntia ficus-indica, Aeonium sedifolium, Hyparrhenia sinaica, Euphorbia canariensis, Argyranthemum foeniculaceum, Atalanthus capillaris, Salvia canariensis and Allagopappus dichotomus. Its also grows in the edges of the road that crosses the location. Flowering and fructification occur in July.
Gonzlez Gonzlez, R., Prez de Paz, P.L., Len Arencibia, M.C. & Reyes Betancort, J.A.
Bilz, M. & Peraza Zurita, M.D.
Sideritis cystosiphon is listed as Critically Endangered because its area of occupancy is 2 km, all individuals occur in a single location, and continuing decline has been observed in the extent and quality of its habitat and in the number of subpopulations, due to a nearby road and predation by rabbits.
A road runs through the population from which debris has been dumped (Gonzlez Gonzlez et al. 2004). Uncontrolled cleaning of the debris could reduce population size as 47 specimens grow on the side of that road. The predation of juveniles by rabbits poses a further threat.
Sideritis cystosiphon is listed as priority species on Annex II of the EU Habitats Directive and under Appendix I of the Convention on the Conservation of European Wildlife and Natural Habitats (Bern Convention). It is included as species "En peligro de extincin" in the Spanish catalogue of threatened species and as species "de inters para los ecosistemas canarios" in the regional catalogue of protected species of the Canary Islands. It is listed as Critically Endangered B2ab(iii,iv) on the Spanish Red List (Moreno 2008).
It occurs within the protected area SCI Riscos de Lara. Seeds are stored in the germplasm bank of the E.T.S. de Ingenieros Agrnomos de Madrid. Research about the genetics of the taxon is needed to clarify the degree of similarity and possible hybridisation with related species, mainly Sideritis soluta.