Authors: Tim Tunison, Global Invasive Species Team, The Nature Conservancy
Latin Name: Rubus argutus
Common Name: Prickly Florida Blackberry
Until the mid-1980s in hawaii, Rubus argutus was misidentified as Rubus penetrans, also from the southeastern US. However, Rubus penetrans is probably a segregate of Rubus argutus when R. argutus is treated in the broad sense (Wagner et al. 1990).
Rubus argutus is an erect, arching, or trailing shrub in the Rose family (Rosaceae). Stems are usually erect to arching in open areas and arching to trailing or decumbent in shaded areas. Primocanes (first year stems) are angled, 1-3 m long. Prickles are hooked or straight, up to 8 mm long. Leaves are palmately compound, typically with 3, sometimes 5 leaflets. Leaflets are elliptic, oblong-oblanceolate, or ovate. Terminal leaflet is 8-13 cm long and 3-8 cm wide. Leaflets are hairless on upper surface with soft, long hairs on lower surface. Leaflet margin is coarsely toothed. Prickles and leaves on floricanes (second year stems) are similar to primocanes but smaller. Flowers are arranged in short racemes on pedicels 1.5-5.0 cm long. Petals are white, 13-20 mm long. Fruit is black when mature, adhering to receptacle.
Blackberry is one of the more disruptive alien plant species in Hawaii. It is a very widespread plant often found at low densities which grows best in mid-elevation, mesic to wet habitats in exposed or partly exposed sites. Disturbance of the soil stimulates seedling recruitment; disturbance of the canopy stimulates vegetative spread. It should be controlled at Kamakou because it is localized and manageable. Its distribution and rate of spread should be monitored at Waikamoi to determine if control actions are needed. Insect biocontrol agents released to date are impacting blackberry to some degree, but prospects for the release of additional insect agents are poor. An apparently host specific, virulent rust may be the most promising biocontrol candidate for blackberry, but host testing in the state has not begun. The management of blackberry is in its infancy, and no control methods or strategies have proven overwhelmingly effective. Roundup® and Escort® are the most effective herbicides, but additional research is needed for basal bark, cut stem, and foliar applications of more selective herbicides such as Garlon 3A®.
Prickly Florida blackberry is native to the central and eastern United States. It was introduced to Hawaii in 1894 (Neal 1965), probably for horticultural reasons (Smith 1985). It is now naturalized on all major islands, but may be more widespread and disruptive on Hawaii, Kauai, Maui, and Oahu. It occurs in wet to mesic habitats mostly between 300 m and 2,500 m elevation (Gerrish pers. comm.). It is best developed from 1,000 m to 2,000 m elevation in mesic to moderately wet, relatively open, and disturbed habitats.
Blackberry reproduces both vegetatively and by seed. Flowers and fruits are produced more abundantly in open habitats, with seed production dropping markedly in subalpine locations or in deep shade. Blackberry seeds are dispersed by alien frugivorous birds (Smith 1985), as suggested by its attractive fruits and ubiquitious nature of its seed bank.
Blackberry can form clonal thickets by root sprouting, although aerial shoots may occasionally root when in contact with soil (Smith 1985). This species is habitat-disruptive when dense clonal thickets form. Most other native plants are excluded from these thickets when fully developed, although some, such as pilo (Coprosma spp.) and palapalai (Microlepia strigosa) tolerate or even penetrate a blackberry canopy. In marginal habitats, it does not spread much vegetatively.
Blackberry is stimulated by disturbance. Seedling establishment appears to be enhanced by soil disturbance, and vegetative spread is released by canopy removal, particularly windthrow. Blackberry is adapted to fire, although this is not an important factor in its spread in the state. The aerial portions of the plant are readily killed. However, it resprouts vigorously from the roots (Smith 1985).
Blackberry is moderately shade tolerant, and grows best in forest gaps or under a sparse canopy. It can be shaded out or severely suppressed by deep shade. Blackberry is basically deciduous in Hawaii, especially in more exposed sites, at higher elevations, and during colder winters.
Smith (1985) categorizes blackberry as one of the 86 most disruptive alien plant species in Hawaii. Blackberry is different from certain other disruptive plants such as clidemia (Clidemia hirta) or strawberry guava (Psidium cattleianum), low densities of which almost always predict eventual high densities. Blackberry is very widely distributed at low densities. It forms dense, monospecific thickets only in some habitats under the appropriate conditions.
Blackberry is a low to moderate threat to Kamakou Preserve. It grows in plantations at lower elevations, especially in disturbed areas such as roadsides. However, the clonal thickets are not especially dense or large. It is found in Kamakou mostly along open roadsides. It may pose a greater threat to Waikamoi Preserve, especially in upper elevation forests. Large thickets are found in this environment in nearby areas, and at least one large clonal thicket occurs in the preserve. Its threat to the subalpine shrublands is uncertain where blackberry is widespread. However, vegetative reproduction is limited, clonal thickets are not well-developed, and production of viable seed is limited (Art Meideros, pers. comm.). This results in scattered upright canes typical of marginal habitats.
At Waikamoi Preserve, recovery of native vegetation following removal of blackberry thickets is likely due to the predominantly native character of the community, although alien grasses may also invade these sites. Recovery of native vegetation depends partly on the damage to natives sustained by herbicides intended for blackberry or the stimulation of alien plants due to soil disturbance from manual and mechanical control measures. There was little evidence of native plant recruitment in colonies treated to identify an effective herbicide at Hawaii Volcanoes. These patches were located mostly in areas under or near native trees and within an alien understory (Santos pers. comm.).
Monitoring is needed at Waikamoi Preserve to determine if blackberry poses a threat. No monitoring is underway at the preserves. Hawaii Volcanoes is assessing recovery of herbicidally treated, single species stands of blackberry in mesic forest.
Kamakou should control blackberry. Even though it may not be especially distruptive in the preserve and at present is located in disturbed sites, it is currently localized and manageable. Waikamoi should monitor blackberry as described above before initating a control program. Controlling blackberry at Waikamoi would require a program similar in magnitude to feral pig control, and probably take many more years.
Manual removal, possibly aided by mechancial removal, may provide a control method in selected habitats in which the soil is deep and loose and large clonal thickets have not developed. Manual/mechancial control will be very time-consuming. Resprouting from roots not removed or from seeds in disturbed soil may result in recovery of blackberry. Current tests at Waikamoi may indicate if manual control is feasible in some subalpine grasslands.
Control of blackberry by cut stem application of undiluted Roundup ® or Rodeo ® may be less time consuming and more practical than manual control when canes are scattered and soil in not deep and loose. Canes should be cut as close to the soil line as possible. Lower concentrations than those used by Smith may be effective.
Foliar applications of 2% Roundup® or Escort® at 28 g/l are probably the most effective herbicidal means of control known. However, both of these are very damaging to non-target vegetation in foliar treatments.
Translocation of existing biocontrol agents is not recommended. Biocontrol is only partially effective, and currently released insect agents are already widespread in Hawaii (Nagata and Markin 1986), and adversely affect native Rubus (Markin pers. comm.).
Management of blackberry in native ecosystems has only recently been attempted, and the results or too provisional for recommendations. The staff at Mt. Kaala Natural Area Reserve on Oahu are controlling blackberry in boggy sites using undiluted Rodeo® in a cut stem application (Smith pers. comm.). However, Smith reports that this is a very labor-intensive practice (Smith pers. comm.), and progress has been slow. Hawaii Volcanoes is currently testing foliar applications of 2% Roundup® followed by cut stem treatments on resprouting stems. Waikamoi has established small test plots for manual control.
Management Research Needs
Distribution and rate of spread monitoring are needed to determine the magnitude of the threat posed by blackberry at Waikamoi Preserve. Distribution monitoring should emphasize the inventoring locations and sizes of large colonies in the upper forest. Rate of spread monitoring should use on point intercept transects to determine cover changes in subalpine plant communities with a significant blackberry component. Density plots to monitor chanages in density of canes could be used to augment cover transects.
An investigation of the ecology and life-history of blackberry would aid management state-wide. This is an appropriate study for a University researcher, and is not needed specifically for TNCH preserves. However, a study of blackberry demography at Waikamoi Preserve would augment rate of spread monitoring recommended above. Also helpful to management would be research to identify basal bark, cut stem herbicidal control methods with an herbicide more selective than Roundup ® and Escort ®. A candidate to test is triclopyr, to which blackberry is sensitive and grasses and many ferns are not. Research is needed to identify and test potential biological control agents specific to blackberry and harmless to native Rubus. The use of pathogens promises the most effective and host-specific agent (Markin pers. comm.)
Gerrish is mapping the distributions of all Rubus species in Hawaii (Gerrish, pers. comm.). No studies of the ecology of the species are known.
Motooka et al. (1986) found blackberry to be sensitive to triclopyr. The Research staff at Hawaii Volcanoes National Park found that a foliar spray of 2% Roundup ® or Escort ® at 28 g/l were relatively effective in controlling blackberry, although mortality was less than 50% (Cuddihy pers. comm.). Smith (pers. comm.) found that undiluted Rodeo was very effective in controlling blackberry growing at low densities when used as a cut stump treatment.
Biological control has been partly effective (Nagata and Markin 1986). Five insects were released 1963-1969 as biocontrol agents. Three of these became established, and were causing extensive damage by the mid-1960's (Nagata and Markin 1986). Nagata and Markin (1986) found substantial populations of these species and concluded that they were having some effect in controlling populations of blackberry, particularly the Schreckensteinia festaliella, a leaf skelentonizer, and Croesia zimmermani, a leaf roller. Markin (pers. comm.) is currently attempting to determine why released insects are not more effective, and feels that parasites on the biocontrol insects may limit their effectiveness as biocontrol agents. There is a large complex of insects that attack blackberry. However, new introductions are on hold because historically released species also attack native Rubus species.
Biocontrol is currently focusing on pathogenic rust fungi because of their virulence and host-specificity (Gardner pers. comm.). Gymonconia nitens is a powerful pathogen of Rubus argutus in its native range where it produces systemic infection. Preliminary tests indicate severe inoculation of Rubus argutus, that did not affect native Rubus spp., although these tests were inconclusive due to difficulties in maintaining the native species. Gardner, National Park Service Plant Pathologist (pers. comm.) has applied for permission to bring Gymonconia nitens to Hawaii for further research, although questions about the availability of quarantine research facilities can affect the timing of the research.
Cuddihy, L.W. 1991. Biological Technician, Hawaii Volcanoes National Park. Person communication with with Tim Tunison, P.O. Box 754, Volcano, Hawaii 96785, June 15, 1991.
Gardner, D.E. 1991. National Park Service Research Biologist, Coop. Natl. Park Resour. Studies Unit, Univ. of Hawaii. Telephone conversation with Tim Tunison, P.O. Box 754, Volcano, Hawaii 96785, April 1, 1991.
Gerrish, G. 1991. Lecturer, University of Hawaii. Telephone conversation with Tim Tunison, P.O. Box 754, Volcano, Hawaii 96785, June 15, 1991.
Markin, G.P., 1991. Research Entomologist, US Forest Service. Telephone conversation with Tim Tunison, P.O. Box 754, Volcano, Hawaii, 96785, April 1, 1991.
Meideros, A. 1991. Biological Technician, Haleakala National Park. Telephone conversation with Tim Tunison, P.O. Box 754, Volcano, Hawaii, 96785, March 15, 1991.
Motooka, P. 1986. Summaries of herbicide trial for pasture, range, forage crops, and non-cropland weed control, 1985. Hitahr Brief 52. Hawaii Institute of Tropical Agriculture and Human Resources.
Nagata, R.F. and G.P. Markin. 1986. Status of insects introduced in Hawaii for the biological control of the wild blackberry Rubus argutus. In Proceedings, Sixth conference in natural sciences, Hawaii Volcanoes National Park. Univ. Hawaii Coop. Natl. Resour. Studies Unit.
Neal, M.C. 1965. In Gardens of Hawaii. Bishop Museum Press. 924 pp. Santos, G. 1991. Biological Technician, US Department of Agriculture Germ Plasm Laboratory, Hilo. Personal communication with Tim Tunison, P.O. Box 754, Volcano, Hawaii 96785. March 1990.
Smith, C.W. 1985. Impact of alien plants on Hawaii's native biota. pp. 180-250 In C.P. Stone and J.M. Scott (eds.). Hawaii's Terrestrial Ecosystems: Preservation and Management. Univ. Hawaii Coop. Natl. Resour. Studies Unit.
Smith, D. 1991. Natural Area Reserve Biologist, Hawaii State Division of Forestry and Wildlife. Telephone conversation with Tim Tunison, P.O. Box 754, Volcano, Hawaii, 96785, March 15, 1991.
Wagner, W.L., D.R. Herbst, and S.H. Sohmer. 1990. Manual of the Flowering Plants of Hawaii. University of Hawaii Press and Bishop Museum Press. 1853 pp.