Privets are any of a number of shrubs or trees in the genus Ligustrum, many of which are invasive. The genus contains about 50 speciesWebb, C. J.; Sykes, W. R.; Garnock-Jones, P. J. 1988: Flora of New Zealand. Vol. IV. Naturalised Pteridophytes, Gymnosperms, Dicotyledons. 4. Christchurch, New Zealand, Botany Division, D.S.I.R. native to the Old World and Australasia. Many members of the genus are grown as ornamental plants in parts of the world.
Several species of privet have become a nuisance in regions outside their ranges. In these conditions they are most commonly found wherever there is disturbed soil, soil that is physically perturbed from its natural state through fire or mechanical machinery, such as along fencerows, old fields, ditches, and forest margins.Maddox, V., J. Byrd, Serviss, B. (2010). Identification and Control of Invasive Privets (Ligustrum spp.) in the Middle Southern United States. Invasive Plant Science and Management. 3:482-488. Privet grows particularly well in riparian forests, which are found throughout the Southeastern United States.Hanula, J.L, Horn, S., Taylor, J.W. (2009). Chinese privet (Ligustrum sinense) Removal and its Effect on Native Plant Communities of Riparian Forests. Invasive Plant Science and Management 2:292-300. Although tolerant of varying soil and light conditions, including a tolerance for shade, privet survives best in mesic soil with abundant sunlight.Urbatch, L. Chinese Privet: Plant Guide. USDA and NRCS.<https://plants.usda.gov/DocumentLibrary/plantguide/pdf/pg_lisi.pdf>Retrieved 19 April 2022
Economic and cultural effects
Privet is considered a huge problem in New Zealand and the east coast of Australia (Ligustrum lucidum, Ligustrum sinense and Ligustrum vulgare). It is banned from sale or cultivation in New Zealand because its pollen is known to cause asthma and eczema in sufferers. Privet can be removed by contacting local government agencies to report its presence. but in Auckland it is only a surveillance pest plant in the 2012 Regional Pest Management Strategy. The NZ Weedbusters site provides guidance on how to get rid of the plant.
Where privet is an invasive species, the cost of controlling and removing privet is economically detrimental, something that is problematic for conservation efforts. The annual cost of removing Chinese privet in the United States is estimated to be $737 per acre when a mulching machine and two-person herbicide application crew are employed.Environmental Assessment: Zebra Project (Timber Scale). (2010) USDA. <http://a123.g.akamai.net/7/123/11558/abc123/forestservic.download.akamai.com/11558/www/nepa/63900_FSPLT1_026073.pdf > Retrieved 15 March 2013 The cost for foliar glyphosate applications on privet is approximately $130 per acre for chemical and surfactant treatments.
Invasiveness
Privet is a successful invasive species because of its ability to outcompete and therefore displace native vegetation. This competitive superiority to native vegetation is connected with the plant's ability to adapt to different light conditions.Lornalkfosko[ris, Jeffrey L. Walck, Hidayati, S.N. (2002). Growth and Reproduction of the Invasive Ligustrum sinense and Native Forestiera ligustrina (Oleaceae): Implications for the Invasion and Persistence of a Nonnative Shrub. International Journal of Plant Sciences. 163: 1001-1010. For example, in low light environments, privet is able to produce fewer and larger ramets than its competitors. These larger ramets make privet more tree-like, making privet better able to compete for light than its more shrub-like native counterparts. Privet is an ideal invasive species because it reproduces both sexually and asexually. Through sexual reproduction, privet produces seeds that are easily dispersed by wind and animals. These seeds can rapidly colonize disturbed soil such as that perturbed by fires, forest clearings, erosion, or abandoned agricultural land. Privet matures quickly, which allows for a short generation cycle and even greater dispersal. The roots of privet can reproduce asexually through root suckers. This vegetative reproduction makes privet difficult and costly to control because root fragments left in the soil can sprout and grow new plants.
One reason why privet is so invasive in the United States is because it has few native shrub competitors.Greene, B.T., Blossey, B. (2011). Lost in the weeds: Ligustrum sinense reduces native plant growth and survival. Biological Invasions. 14:139-150 In a sense, privet is invading and exploiting an open niche within the southern U.S. floodplain ecosystem. Prior to privet invasion much of the native land was clear and open; currently, privet forms a dense thicket which chokes out other, usually native, plant life. Thus, privet is believed to be phylogenetically distinct compared to its native cousins.
Ecological impact in the U.S.
Introduction
All nine species of privet currently in the southeast U.S. are invasive. The first species of privet was introduced into the United States in the 1700s as an ornamental plant used as a hedge or foliage for gardens. Glossy privet arrived in the U.S. in 1794, Chinese privet in 1825, Japanese privet in 1845, California privet in 1847, and Amur privet in 1860. Privets escaped cultivation in the early 1900s, but became widely naturalized during the 1950s-1970s or later. Currently privet is designated as a foreign invasive plant in Alabama and Georgia and considered a severe threat in North Carolina and Florida. It is estimated that Chinese privet alone occupies over one million hectares of land across 12 states ranging from Virginia to Florida and west to Texas.
The full ecological effect of privet is still being studied. However, data suggest that forests containing large amounts of privet tend to have fewer trees, less shrub diversity, and decreased density of herbaceous plants.Zhang, Y., Hanula, J., Horn, S., Braman, S.K., Sun, J. (2011). Biology of ‘'Leptoypha hospita'’ (Hemiptera: Tingidae), a Potential Biological Control Agent of Chinese Privet. Ann. Entomol. Soc. Am. 104: 1327-1333. When introduced to an ecosystem, privet grows quickly and, given time, will produce a thick layer under the forest canopy preventing sunlight from reaching the native plants below. In some cases, this can drive native populations to extinction. If left unchecked, privet may result in large-scale ecosystem modification and an overall loss of native species diversity and richness. The Sierra Chicas of Cordoba have experienced just such a widespread landscape change since 1970.
In a study by Greene and Blossey using field observations and a transplant experiment, a significant negative correlation was found between percent Chinese privet cover and herbaceous cover, species richness, and plant height. In a comparison of two experimental gardens, each with four native plant species, the plot with Chinese privet contained almost entirely nonnative plants after 64 weeks. Out of approximately twenty plants per species, only single individuals of Acer negundo, Chamaenerion latifolium and C. tribuloides survived the entire study when in the presence of Chinese privet. Surviving plants had lower leaf counts and stunted height relative to their counterparts in privet-absent plots. None of the Boehmeria cylindrica survived.
Insects are greatly affected by the proliferation of invasive species such as privet.Hanula, J. L., Horn, S. (2011). Removing an invasive shrub (Chinese privet) increases native bee diversity and abundance in riparian forests of the southeastern United States. Insect Conservation and Diversity 4: 275-283. For example, one study found the abundance and diversity of butterflies increased following privet removal to almost the same abundance as that of a similar forest community, with no history of privet invasion. In a study conducted in Georgia, privet was found to decrease the diversity of native honeybee colonies. Plots removed of privet resulted in four times as many bee species as control plots in which privet was not removed. Traps placed in undisturbed forest plots with no history of privet caught an average of 210 bees from 34 species, while traps placed in privet-infested plots caught an average of 35 bees from only 9 species.
Another example of how privet can have a negative ecological impact in ecological communities is its ability to threaten endangered species.Zhang, Y., Hanula, J., Horn, S., Braman, S.K. and J. Sun. 2011. Biology of Leptoypha hospita (Hemiptera: Tingidae), a Potential Biological Control Agent of Chinese Privet. Ann. Entomol. Soc. Am. 104: 1327-1333. For example, privet threatens the Miccosukee gooseberry, a native plant species that privet is similar to, by encroaching on the gooseberry's habitat and slowly displacing it.Langeland, K. A., Burks, K.C. (1998). Identification and biology of non-native plants in Florida's natural areas. University of Florida, Gainesville. 165 pp.
Biological species invasion is considered a main component of global ecosystem change due to changes in biogeochemical cycles and disturbance regimes.Gavier-Pizarro, G.I., T. Kuemmerle, L.E. Hoyos, S.I. Stewart, C.D. Huebner, N.S. Keuler, and Radeloff, V.C. (2012). Monitoring the invasion of an exotic tree (Ligustrum lucidum) from 1983 to 2006 with landsat TM/ETM + satellite data and support vector machines in Córdoba, Argentina. Remote Sensing of Environment 122: 134-145. Invasive species such as privet are known to alter the dominant vegetation type, soil properties, animal behavior, and the natural cycling of resources. For example, Chinese privet increases the decomposition rate of leaf litter by 2.6-fold in riparian forests.Mitchell, J. D., Lockaby, B.G., Brantley, B.F. (2011). Influence of Chinese Privet (Ligustrum sinense) on Decomposition and Nutrient Availability in Riparian Forests. Invasive Plant Science and Management 4:437-447.
However, because the usual method employed to study the impact of privet is to compare invaded and non-invaded areas, available data may be inaccurate. This is because these types of comparison studies have trouble controlling for abiotic and biotic habitat conditions that could be negatively affecting native plant growth.
Effect of climate change Evidence suggests that climate changes, brought about by increased CO2 concentrations, will increase the spread and proliferation of privet.Bradley, B.A., Wilcove, D.S., Oppenheimer, M. (2010). Climate change increases risk of plant invasion in the eastern United States. Biological Invasions 12:1855–187 Climate change is predicted to increase competition between native and invasive plants. Additionally, the increased temperatures that result from climate change are predicted to expand the range of plants typically restricted to the deep south where it is warm and moist. In a projection based on the current and predicted CO2 levels, temperature, and precipitation, it was predicted that by the year 2100 privet will have spread as far north as Maine, sweeping across the mid-western United States into Nebraska. Thus, states that are currently free from privet invasion (Michigan, Ohio, Indiana, Illinois, Kentucky, Virginia, Pennsylvania, etc.) are all predicted to become invaded with privet within the next 100 years unless either privet is contained or conservation efforts decrease the effect of pollution on climate change.
Control and removal methods
Methods of control