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Ligustrum

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Ligustrum

Privet
Ligustrum vulgare
Scientific classification
Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Asterids
Order: Lamiales
Family: Oleaceae
Tribe: Oleeae
Genus: Ligustrum
L.
Species

See text


A privet is a flowering plant in the genus Ligustrum. The genus contains about 50 species of erect, deciduous or evergreen shrubs, sometimes forming small or medium-sized trees,[1]native to Europe, north Africa, Asia and Australasia.[2] Privet was originally the name for the European semi-evergreen shrub Ligustrum vulgare, and later also for the more reliably evergreen Ligustrum ovalifolium used extensively for privacy hedging, though now the name is applied to all members of the genus. The generic name was applied by Pliny the Elder (23 CE – 79) to L. vulgare.[3] It is often suggested that the name privet is related to private, but the OED states that there is no evidence to support this.[4]

History and Underlying Biology

Description

Privet (Ligustrum sinense Lour.) is a perennial shrub or tree that is shade-tolerant, evergreen,[5] and a member of the Olive Family.[6] It can grow up to 9 m tall [7] with grey bark and branching reddish stems.[8] Privet’s leaves are opposite, oblong, leathery, and pointed.[8] Its fruit grows in spiral clusters and range from purple to black in color.[8] Although considered a weed, it is often used as an ornamental plant in gardens due to its flowers.[9] The flowers are small, white, unpleasantly pungent,[2] and borne in panicles. They have four curled-back petals and two high stamens with yellow or red anthers, between which is the low pistil; the petals and stamens fall off after the flower is fertilized, leaving the pistil in the calyx tube. Flowering starts after 330 growing degree days.

Privet is 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.[10] Privet grows particularly well in riparian forests, which are found throughout the southeastern United States.[5] Although tolerant of varying soil and light conditions, including a tolerance for shade, privet survives best in mesic soil with abundant sunlight.[9]

History in the U.S.

All nine species of privet, currently in the southeast U.S., are invasive.[10] 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.[10] 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.[10] Most privet escaped cultivation in the early 1900s, becoming naturalized during the 1950s-1970s.[9] Currently Privet is designated as a foreign invasive plant in Alabama and Georgia and considered a severe threat in North Carolina and Florida.[10] It is estimated that Chinese Privet alone occupies over one million hectares of land across 12 states ranging from Virginia to Florida and as far west as Texas.[5]

Uses and Cultivation

In addition to being cultivated to create ornamental hedges and foliage, privet is also widely used in horticulture and flower arrangements.[9] Privet is used to create living screens and privacy barriers, while its flexible twigs are sometimes used as cords for lashing.[9] In the U.S., nurseries continue to sell Chinese Privet, and it remains recommended in several planting guides.[11]

Additionally, Chinese privet is used in traditional herbal medicine.[12] The decoction of privet leaves or bark helps to treat diarrhea, stomach ulcers, chronic bowel problems, chapped lips, sore mouths and throats, and a wash for skin problems.[12] Privet leaves and bark have bitter properties that make a useful tea for improving appetite and digestion in chemotherapy patients.[12]

Economic and Cultural Impact

Privet produces a fruit, which is mildly toxic to humans.[9][13] Symptoms from eating privet fruit include nausea, headache, abdominal pain, vomiting, diarrhea, weakness, low blood pressure, and low body temperature.[9] In large amounts, the odor produced from privet’s flowers can cause respiratory irritation and its pollen can cause an allergic reaction.[9]

Privet is considered a huge problem in New Zealand and the east coast of Australia (Ligustrum lucidum, Ligustrum sinense and Ligustrum vulgare[14]). 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.[15]

When privet is invasive species, the cost of controlling and removing privet is economically detrimental, something that is problematic for conservation efforts.[9] 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.[16] The cost for foliar glyphosate applications on privet is approximately $130 per acre for chemical and surfactant treatments.[16]

Ecological Effect

A plant may produce thousands of fruits, most of which are eaten by birds. Privet is used as a food plant by the larvae of some Lepidoptera species including Common Emerald, Common Marbled Carpet, Copper Underling, the Engrailed, Mottled Beauty, Scalloped Hazel, Small Angle Shades, The V-pug and Willow Beauty.

Invasiveness

Privet is a successful invasive species because of its ability to outcompete and therefore displace native vegetation.[9] This competitive superiority to native vegetation is connected with the plant's ability to adapt to different light conditions.[17] For example, in low light environments, privet is able to produce fewer and larger ramets than its competitors.[17] These larger ramets make Privet more tree-like, making privet better able to compete for light than its more shrub-like native counterparts.[17] Privet is an ideal invasive species because it reproduces both sexually and asexually.[9] Through sexual reproduction, privet produces seeds that are easily dispersed by wind and animals.[9] These seeds can rapidly colonize disturbed soil such as that perturbed by fires, forest clearings, erosion, or abandoned agricultural land.[9] Privet matures quickly, which allows for a short generation cycle and even greater dispersal.[9] The roots of privet can reproduce asexually through root suckers.[9] This vegetative reproduction makes privet difficult and costly to control because root fragments left in the soil can sprout and grow new plants.[9]

One reason why privet is so invasive in the United States is because it has few native shrub competitors.[18] In a sense, privet is invading and exploiting an open niche within the southern U.S. floodplain ecosystem.[18] Prior to privet invasion much of the native land is clear and open, afterwards privet forms a dense thicket that chokes out other plant life.[18] Thus, privet is believed to be phylogenetically distinct compared to its native cousins.[18]

Ecological Impact in the U.S.

The full ecological effect of privet is still being studied. However, data suggests that forests containing large amounts of privet tend to have fewer trees, less shrub diversity, and decreased density of herbaceous plants.[11] When introduced to an ecosystem, privet grows quickly and, if given enough time, will produce a thick layer under the forest canopy preventing sunlight from reaching the native plants below.[11] In some cases, this can drive native populations to extinction.[9] If left unchecked privet may result in large-scale ecosystem modification and an overall loss of native species diversity and richness.[5] The Sierras Chicas of Cordoba have experienced just such a widespread landscape change since 1970.[5]

In a study by Greene and Blossey using field observations and a transplant experiment, a significant negative correlation between percent Chinese Privet cover and herbaceous cover, species richness, and plant height was found.[18] 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.[18] Out of approximately twenty plants per species, only a single individual of A. negundo, C. latifolium, and C. tribuloides survived the entire study when in the presence of Chinese Privet.[18] These surviving plants had lower leaf-counts and stunted height relative to their counterparts in Privet-absent plots.[18] None of the B. cylindric survived.[18]

Insects are greatly affected by the proliferation of invasive species such as privet.[19] 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.[19] In a study conducted in Georgia, privet was found to decreases the diversity of native honeybee colonies.[11] Plots removed of privet resulted in four times as many bee species as control plots in which privet was not removed.[11] 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.[11]

Another example of how privet can have a negative ecological impact in ecological communities is its ability to threaten two endangered species (SchweintzOz sunBower and Miccosukee gooseberry).[20] Privet threatens the Miccosukee gooseberry, a native plant species that privet is similar too, by encroaching on the gooseberry's habitat and slowly displacing it.[21]

Biological species invasion is considered a main component of global ecosystem change due to changes in biogeochemical cycles and disturbance regimes.[22] Invasive species such as privet are known to alter the dominant vegetation type, soil properties, animal behavior, and the natural cycling of resources.[22] For example, Chinese Privet increases the decomposition rate of leaf litter by 2.6-fold in riparian forests.[23]

However, because the usual method employed to study the impact of privet is to compare infected and uninfected areas, the current available data may be inaccurate.[5] 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.[18]

Effect of Climate Change

Evidence suggests that climate changes, brought about by increased CO2 concentrations, will increase the spread and proliferation of privet.[24] Climate change is predicted to increase competition between native and invasive plants. Additionally, the increased temperatures that result from climate change is predicted to expand the range of plants typically restricted to the deep south where it is warm and moist.[24] 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.[24] 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.[24]

Control and Removal Methods

Methods of Control

The same qualities that allow Privet to outcompete native species make it extremely difficult to control and eradicate.[9] Privet is manageable with mowing or cutting consistently, as closely to the ground as possible.[25] This will prevent the spread of the privet but will not eradicate it.[25] There are no known biological controls of privet.[25] Even methods such as controlled burning have proven ineffective and actually aid privet growth because privet recovers better than native plants from controlled burns.[25]

Mechanical removal of privet, especially for younger plants or smaller areas of growth, can be effective; however, all of the root must be removed to prevent root re-sprouting.[6] Mechanical removal can prove difficult for more large-scale invasions.[6] When such large invasions occur in the natural environment, herbicide use is usually avoided due to the negative impacts on native plants.[6] Thus, the use of many workers or heavy machinery such as bulldozers is the most effective option.[6] However, the soil disturbance and resulting erosion are important considerations when using such measures.[6]

When herbicide use is warranted, several methods have proven effective.[6] In all cases, soil disturbance following herbicide treatment is inadvisable.[25] This is due to privet's preference for disturbed soils and its increased ability to compete in such an environment.[6] The length of time that the soil must be left undisturbed varies with the treatment.[6] Foliar applications of glyphosate, cut-stump applications of glyphosate or triclopyr, and basal bark treatments for stems less than 0.5 inches is effective.[25]

The foliar treatments are most effective against dense thickets of privet.[6] The best time for such treatments are in late fall and early spring when many of the native species are dormant.[6] Care must be used with such an application to avoid spraying non-target plants.[6] This treatment is generally considered to be less effective than the cut-stump and basal bark methods.[6]

The cut-stump method is most useful when treating single plants.[6] This method consists of thoroughly covering the stump of a recently cut plant with herbicides, such as glyphosate and triclopyr.[25] The effectiveness of this treatment is increased by cutting a hole into the surface of the stump.[25] Use of this treatment when the plant breaks the dormant stage as well as when the ground is frozen is advised against as it greatly reduces effectiveness.[6]

The basal bark treatment consists of application of herbicide to all basal parts of the plant below 12-15 inches.[25] The bark must be thoroughly wet with herbicide for this treatment to be most effective.[25] The USDA suggests that 25% triclopyr and 75% horticultural oil is the most effective herbicide composition for basal bark treatments.[6]

Agencies Working to Control Privet

The Federal Interagency Committee for the Management of Noxious and Exotic Weeds (FICMNEW) engages with public and private organizations in the effort to combat noxious and exotic weeds, among which are Chinese and European Privet.[26] Privet is on the agenda of the US Forest Service via their Invasive Species Program while the National Park Service deploys Exotic Plant Management Teams.[27] Each team has jurisdiction over ten national parks and working with local volunteers, contractors, and service organizations.[27]

Agencies work to eliminate, contain, and/or remove privet in an attempt to keep forests sustainable.[18] This is because much of the land invaded by privet, such as the Piedmont floodplains, are important areas of biodiversity, carbon storage, and resource production.[18]

Selected species

The Integrated Taxonomic Information System lists eleven "accepted" species of Ligustrum.[28] Additional species are listed in other references.[29][30][31]

  • Ligustrum amamianum (China)
  • Ligustrum amurense – Amur privet (Northern China)
  • Ligustrum angustum (China)
  • Ligustrum australianum (1865) – Australian privet (Endemic, Wet Tropics, NE. & C. Qld, Australia)[32]
  • Ligustrum chenaultii (Yunnan Province, China)
  • Ligustrum compactum (Himalayas, southwest China)
  • Ligustrum confusum (Himalayas, Khasi Hills)
  • Ligustrum delavayanum (Southwest China, Myanmar)
  • Ligustrum expansum (China)
  • Ligustrum glomeratum (Thailand, Malesia)
  • Ligustrum gracile (China)
  • Ligustrum henryi (Central China)
  • Ligustrum ibota (Japan)
  • Ligustrum indicum (Himalayas, Indochina)
  • Ligustrum japonicum (Japan, Korea)
  • Ligustrum leucanthum (China)
  • Ligustrum lianum (China)
  • Ligustrum liukiuense (Taiwan)
  • Ligustrum longitubum (China)
  • Ligustrum lucidum - Chinese privet, Glossy privet (China, Japan, Korea)
  • Ligustrum massalongianum (Himalayas)
  • Ligustrum microcarpum (Taiwan); often included in L. sinense.
  • Ligustrum morrisonense (Taiwan)
  • Ligustrum obovatilimbum (China)
  • Ligustrum obtusifolium – Japanese deciduous privet (East Asia)
  • Ligustrum ovalifolium – Japanese privet, Oval-leaved privet (Japan)
  • Ligustrum pedunculare (China)
  • Ligustrum pricei (Taiwan)
  • Ligustrum punctifolium
  • Ligustrum quihoui (China)
  • Ligustrum retusum (China)
  • Ligustrum robustum (China)
  • Ligustrum sempervirens (Western China)
  • Ligustrum sinense – Chinese privet (China, Taiwan)
  • Ligustrum strongylophyllum (Central China)
  • Ligustrum tenuipes. (China)
  • Ligustrum tschonoskii (Japan)
  • Ligustrum vulgare – Common privet, European privet (Europe, northwest Africa, southwest Asia)
  • Ligustrum xingrenense (China)
  • Ligustrum yunguiense (China)

See also

References

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