Rockrose in the "Fire Safe" Landscape
by Randy Baldwin, San Marcos Growers
Rockrose were introduced into the horticultural trade in California in the late 1800's and steadily became more popular as garden plants - for more details on cultivating this plant and its history of introduction, please see our Cistus Page. The steady increase in popularity was certainly due to their beauty and durability in the garden but there were other utilitarian reasons to plant them in the garden as well. Water conservation has increasingly become a valid reason to plant this group of plants and, in addition, many of the Rockroses became associated, rightfully or wrongfully, with fire resistance. In the 1960's even non-profits such as the Girl Scouts of America became involved - I distinctly remember my sister's troop providing Rockrose plants to residents on hillside properties in our Pasadena neighborhood to help with slope stabilization and fire retardation. In the 1967 the newly rewritten Western Garden Book was published with a general recommendation to plant Cistus in "fire hazard areas" and planting them in fire prone areas has been promoted by many municipalities. While these plants have long been recommended for landscaping in fire prone areas, it should be noted that much of the information regarding this is anecdotal and without enough research to back up all of the claims. More importantly, not all species or varieties can necessarily be treated the same in this regard and ALL plants will burn when they get hot and dry enough to do so.
In a publication by the Forest Service's Pacific Southwest Forest and Range Experiment Station published in 1977 by Eamor Nord and Lisle Green and titled "Low-volume and Slow-burning Vegetation for Plant on Clearing in California Chaparral" most highly recommended are planting of succulents and low growing groundcovers but it also advocated the planting of Cistus crispus and C. salviifolius (though mistakenly included them in as low fuel volume "native" plants). That the lower groundcover Rockrose would be better candidates for fire prone areas makes sense but often the larger ones are listed on "firesafe" lists as well though there is conflicting studies on these.
Most of these lists include Cistus x purpureus such as Dr. Claude Curran's 1978 paper "Wildfire Hazard Management in the Urban/Wildland Interface in Southern Oregon" where citing Lisle Green's research in "The search for a "fire resistant" plant in southern California" (California Division of Forestry. Fire Control Exp. 10. 2-12. 1965) notes that:
"Rockrose is an apparently slow-burning plant that should be
considered as a possibility for inclusion in some landscaping plans ... It seems to establish well, especially under poor site conditions and it remains relatively green well into the drought season. At least some species of Cistus (rockrose) are slower to ignite than some native plants and they tend to glow and blacken rather than burst into flame. There is less fuel volume than often found to occur with native vegetation. Species of Cistus that seem to be most favorable are orchidspot rockrose (Cistus purpureus), rockrose (C. villosus), gum rockrose (C. ladanferus), sagleaf rockrose (C. salvifolius), and hybrid rockrose (C. albidus x C. crispus)"
This is contradicted however in a study conducted in 2004 by Matthew G. Etlinger and Frank C. Beall of the University of California Forest Products Laboratory (International Journal of Wildland Fire 13(4) 479-488 ) that noted that the flammability under drought conditions was studied and led the researchers to reclassified Cistus purpureus from being acceptable in high fire hazard areas to being unacceptable. This means that where an irrigated plant may be acceptable the same plant not irrigated during times of drought would not be.
In addition it was noted by M. C. Juhren and Kenneth R. Montgomery in "Long-Term Responses of Cistus and Certain Other Introduced Shrubs on Disturbed Wildland Sites in Southern California" that "Cistus bushes became woody and senescent in about 12 years and they recommend "management to reduce the fuel load in case of fire, and to stimulate growth of new bushes."
Where one study may show Cistus ladanifer is more resistant to burning such as in "Moisture and Salt Effects on Fire Retardance in Plants" Kenneth R. Montgomery and P. C. Cheo American Journal of Botany;Vol. 56, No. 9 (Oct., 1969), pp. 1028-1032 anohter may suggest it a highly flammable species due to its external resins (Trabaud L (1981) Man and fire: impacts on Mediterranean vegetation. In: Di Castri F, Goodall DW, Specht RL (eds) Ecosystems of the world, vol 11. Mediterranean-type shrublands. Elsevier, Amsterdam).
Given the contradictory studies regarding these plants we only recommend the groundcover varieties of Cistus as suitable for non-irrigated areas near structures in fire prone areas. In addition, there is general agreement that ALL plants will burn and that the safest landscapes are devoid of any flammable material close to structures. If plants are placed near structures, they should be low in stature and fuel volume and high in residual water content (ie succulent groundcovers). When planting even away from structures plants need to be fire resistant, low in full volume, properly spaced at the time of planting, and properly maintained to remove any accumulation of dead leaves and stems.