Centaurea melitensis: Characteristics, Ecology and Impact as an Invasive Plant

Introduction to Centaurea melitensis: An invasive plant with multiple names and uses

centaurea melitensis, commonly known as centaury, darnel, weed, or Maltese centaury, is a herbaceous plant that has captured the attention of botanists, farmers, and environmentalists due to its impressive invasive potential. Original From the Mediterranean region, its presence extends significantly across multiple regions of the world, especially in disturbed, agricultural and natural areasIts spread, ease of adaptation, and the ecological problems it creates make it one of the most illustrative examples of invasive exotic plants with global influence.

This species stands out not only for its biological characteristics, but also for its invasion dynamics, its potential historical or ethnobotanical uses, and the risks it poses to biodiversity and the agricultural economy.

Taxonomy and common names

La centaurea melitensis belongs to the genus knapweed, within the extensive family of Asteraceae. It is classified within the subgenus Solsitiaria, although the systematic review places it in the section Mesocentron.
Taxonomy:

• Kingdom: Plant
• Division: Magnoliophyta
• Class: Magnoliopside
• Subclass: Asteridae
• Order: Asterals
• Family: Asteraceae
• Tribe: Cynareae
• Gender: knapweed
• Species: centaurea melitensis L.

Among the most widespread common names for this species are fist opener, tares, weeds, and in areas of America such as California and the Southern Cone it is called tonkaIn the Canary Islands it is also known as Christ herb. Its specific epithet, melitensis, refers to the island of Malta.

Detailed morphological characteristics of Centaurea melitensis

This plant is a annual herbaceous —sometimes biennial— that can reach considerable heights, generally between 20 and 100 cm, although there are records of specimens that exceed 80-85 cm.

• Stems: Erect, robust, winged above, and often branched on their upper segment. They have a pubescent or slightly hairy texture.
• Sheets: The basal leaves are lyrate-pinnatifid, petiolate, with a sinuate or lobed margin, sometimes glaucous green, and with a scabrid surface. The upper leaves are sessile, decurrent, linear-lanceolate, and generally simple or sparsely lobed.
• Inflorescences: They are grouped in ovoid capitula approximately 1-2 cm in diameter, solitary or in groups of 2-5, growing at the end of stems or short branches.
• Involucre bracts: Very characteristic, imbricated, with a terminal spiny appendage up to 1 cm long, often branched into small lateral spines, which constitutes their main identifying feature along with their yellow coloration.
• Flowers: All are florescent, yellow, tubular, and about 10-12 mm long. Flowering usually occurs from late spring to midsummer.
• Fruits: the seeds are achenes Elliptical or oblong, pale, 2,5 mm long, sometimes compressed, with up to 12 ribs; they have a scaly pappus as long as the achene itself and a basal-lateral insertion scar. The fruit surface is delicate, reticulate-areolate, and slightly hairy.

The life cycle of centaurea melitensis It develops from germination in autumn-winter, growth and vegetative development in spring, and flowering at the beginning of summer.

Geographic distribution and occupied habitats

La centaurea melitensis It is native to the Mediterranean basin, including southern Europe, North Africa, and regions of Macaronesia such as the Canary Islands, Madeira, the Azores, Cape Verde, and other Atlantic islands. However, due to human activity and its surprising adaptability, it has spread globally as an introduced and invasive species, colonizing:

• North America (especially in the southern United States and California, where it is a pest that is difficult to eradicate)
• South America
• Pacific Islands
• Oceania

The plant is especially common in:

• Roadsides and ditches
• Open grasslands and prairies
• Agricultural areas, especially areas of extensive crops
• Road margins
• Vacant land, urban lots and disturbed areas
• Altered soils, with some preference by nitrified or clayey substrates

In the Canary Islands, it is widely represented on all the main islands, adapting to both coastal and mid-mountain areas.

Mechanisms of reproduction and dispersal

La centaurea melitensis bases its reproductive success on a combination of key factors:

• Sexual reproduction exclusively by seeds (achenes).
• Each plant can produce a high quantity of seeds per vegetative cycle.
• Seeds present mechanisms of efficient dispersion: The pappus facilitates its transport by the wind, although the main route is accidental adherence to animals, agricultural machinery and people.
• Agricultural activities and human traffic favor its introduction and spread to new areas.
• Seeds can remain viable in the soil for long periods, allowing the formation of a persistent bank of seeds.

Anemochorous (by wind) and zoochorous (by animals) dispersal have been verified in different studies in Mediterranean environments as well as in America and the Atlantic islands.

Invasive traits and their ecological and agricultural impact

The real ecological and agricultural problem of centaurea melitensis derived from:

• Aggressive growth and high population density: It can form authentic monocultures that displace native vegetation and alter the structure of the plant community.
• Reduction in forage value of the soil: Displaces species useful for grazing and reduces the diversity of plants consumed by livestock.
• Threat to native speciesBy rapidly occupying open spaces and colonizing altered habitats, it endangers the native flora and fauna of reserves and natural ecosystems.
• increased erosion: By reducing natural vegetation cover, it increases the vulnerability of soils to erosion and desertification.
• Contamination of agricultural seeds: Its ability to mix into agricultural seed lots makes it a difficult weed to eradicate even with crop rotation.
• Toxicity and problems in cropsAlthough not considered particularly toxic, it can interfere with crop growth and increase agricultural production costs due to the need for additional controls.
• Increase in management costsEradication and control require ongoing measures and represent a considerable economic burden.

colonization of centaurea melitensis It also negatively impacts natural forest regeneration, hinders ecological succession and impoverishes landscape quality.

Ecology and life cycle

centaurea melitensis It displays outstanding ecological plasticity, meaning it can adapt to a wide range of habitats and environmental conditions. Its life cycle can be summarized in the following stages:

1. Germination: It occurs after the first autumn rains.
2. Vegetative growth: During winter and spring it quickly reaches its maximum leaf expansion.
3. Flowering: Generally between late spring and summer, depending on latitude and weather conditions.
4. Fruiting and seed release: After flowering, the seeds mature and disperse, remaining in the soil or attached to carriers.
5. Senescence: The aerial part dies with the arrival of drier conditions, although the seed bank guarantees its future survival.

This cycle allows it to take advantage of temporary niches and quickly colonize disturbed areas.

Differentiation from similar species

It is important distinguish Centaurea melitensis from other similar species of the genus:

• Centaurea solstitialis: It is often confused because it shares habitats and general appearance, but its involucral bracts have longer and less branched appendages.
• Centaurea benedicta: It has a different appearance and morphology, especially in the shape of the chapters and the coloration of the flowers.
• Other centauries:
• Centaurea cyanus (cornflower), whose flowers are blue.
• Centaurea calcitrapa, with very prominent spines.

Factors that favor its invasiveness

The invasive success of centaurea melitensis It is due to the combination of factors such as:

• High fertility and seed production, very resistant and adapted to accidental transport.
• Drought tolerance, thanks to its rapid vegetative cycle and development in conditions of water scarcity.
• Ability to colonize degraded soils, vacant lots and areas with low agricultural maintenance.
• Absence of natural enemies specific in invaded areas, allowing it to grow without biological limitations.
• Herbicide resistance conventional agricultural practices, which makes it difficult to control once established.
• Mimicry and similarity with other native plants, making its early detection and eradication difficult in some ecosystems.

Distribution and presence in the Canary Islands and other island regions

The Canary Islands have extensively documented the presence of centaurea melitensis on all the major islands (El Hierro, La Palma, La Gomera, Tenerife, Gran Canaria, Fuerteventura, and Lanzarote). Its preference for roadsides, agricultural land, peri-urban areas, and disturbed spaces makes it one of the most widespread species in disturbed environments.

• Historical records confirm its presence in the flora of Lanzarote, La Graciosa, Famara, Timanfaya, La Palma and other protected areas.
• Studies show that its density can vary depending on agricultural pressure, road maintenance, and annual rainfall patterns.

It is included in catalogs and biogeographical databases, as well as in lists of exotic flora and naturalized species in the Canary Islands, where its management is a cause for concern for the administration and managers of natural areas.

Social, economic and environmental impacts

centaurea melitensis affects various human activities and ecological processes:

• In agriculture: It competes with crops, reduces forage yield and quality, and increases management and control costs.
• In conservation: It poses a serious threat to native and endemic species, contributing to the loss of local biodiversity.
• In soil erosion and degradation: By reducing vegetation cover and forming monocultures, it promotes water and wind erosion.
• In public healthAlthough it is not toxic, its presence in large quantities can make agricultural work difficult and encourage the proliferation of other pests.
• In landscaping and environmental restoration: It eliminates the ornamental and natural value of spaces, prevents effective restoration and complicates the reintroduction of native flora.

Identification methods and keys for their recognition

Identify centaurea melitensis is key to its management. The following aspects help differentiate it from other species:

• Intense yellow inflorescences and branched spiny appendages on the involucral bracts.
• Ovoid capitula with leathery bracts and triangular base.
• Lower leaves lirado-pinnatifid and linear and decurrent superiors.
• Winged stem on top.
• Double pappus in the fruits (cypselae), almost the same size as the achene.

Control and management measures

Handling centaurea melitensis As an invasive plant, it requires an integrated approach:

• Prevention: Prevent seed dispersal by cleaning agricultural machinery and vehicles, and rigorously controlling commercial seeds.
• mechanical control: This includes weeding before flowering, manual uprooting, and removal of entire plants. It is essential to perform these tasks before seed dispersal.
• Chemical control: Use of selective herbicides in early stages of development. However, their effectiveness may be reduced in dense or advanced populations.
• Ecological management: Promote restoration with native species and vegetation cover to reduce the spaces available for the invasive species.
• Monitoring and follow-up: Implement early detection, inventory, and population monitoring programs, especially in natural areas and sensitive crops.
• Outreach and education: Raise awareness among farmers and environmental managers about the importance of their identification and timely eradication.

In island contexts or protected areas, it is recommended to conduct periodic inventories and define priority areas for monitoring, given the particular vulnerability of these ecosystems.

Ethnobotanical considerations and historical uses

Although primarily considered a weed today, there are historical references to some Cultural uses of Centaurea melitensis:

• In some regions of the Canary Islands and agricultural areas of Lanzarote, it has been used as a fodder plant in times of scarcity, although its nutritional value is low.
• In ancient times, some centaureas have been valued for their medicinal properties, although centaurea melitensis It is not among the most used for this purpose.
• The robustness of its stems and its rapid growth allowed it to be included in grass mixtures, although its low productive value and tendency to become a pest currently discourage such use.

References to the Centaurea family in the popular treatment of certain conditions appear in traditional literature and herbalists, although scientific documentation on the specific properties of the species is scarce.

Conservation status and regulation

centaurea melitensis It is not considered to be in danger of extinction. On the contrary, it is recognized as invasive exotic plant in numerous catalogues, which implies control protocols and, in some jurisdictions, restrictions on its marketing and movement between regions.

• In international legislation, its invasive potential is recognized in regional lists of exotic species.
• In areas of the Americas, it has been the target of control and eradication plans within restoration programs.
• In the Canary Islands and other Atlantic islands, special monitoring is carried out within the biodiversity and naturalized exotic flora databases.

Recommendations from public entities and international organizations point to the importance of citizen collaboration in monitoring and early detection of new outbreaks.

Cytology and synonymy

From a cytogenetic point of view, centaurea melitensis It has a chromosome number of 2n = 24. Its taxonomic history has given rise to various synonyms and alternative names, historically used in different regions and scientific publications:

• American Centaurea Spreng., name illeg.
• Centaurea apula The m.
• Centaurea congesta Willd. ex DC.
• Centaurea glomerata Webb & Berthel.
• Centaurea patibilcensis DC.
• Centaurea sessiliflora The m.
• Cyanus melitensis Gaertn.
• Calcitrapa melitensis (L.) Dum.Cours., among others.

In the popular and regional sphere, it is common to find variations such as abrepuño, abrepuños, abremanos, ardolla, arzolla, cardo escarolado, cardo escrolado, centaura menor or risillas de la suegra, depending on the country or province.

Key research and studies

Over the years, numerous studies have documented the expansion, ecology and management of centaurea melitensis:

• Research on their distribution in the Canary Islands, island by island, has allowed us to understand their invasion patterns and priority areas for intervention.
• Floristic and vegetation studies in the Mediterranean, California, and Latin America detail its behavior as an invasive plant and its effects on native communities.
• Experimental research has been carried out on the effectiveness of different control methods (mechanical, chemical and ecological).
• Reports from biodiversity banks and monitoring programs have highlighted the need for continued monitoring and citizen participation in early detection and eradication.

Consulting technical literature and flora catalogs allows you to track the growth and decline of this species in different latitudes and ecosystems.

Frequently asked questions about Centaurea melitensis

1. Why is Centaurea melitensis considered an invasive plant?
   Due to its rapid expansion and its ability to displace native species, alter ecosystems, and reduce agricultural productivity.
2. Is it toxic to humans or animals?
   It is not considered toxic in the quantities typically found in pastures or crops, although its consumption is not recommended for livestock due to its low nutritional value.
3. What is the best way to control its spread?
   Integrated control (mechanical, chemical and ecological), accompanied by early detection and cleaning of agricultural machinery.
4. Which habitats are most vulnerable?
   Altered areas, roadsides, poorly managed agricultural soils and areas degraded by human activities.
5. Can it be confused with other centauries?
   Yes, especially with Centaurea solstitialis y Centauria BenedictaThe key is in the spiny appendage of the bracts and the color of the flowers.

Knowledge, awareness and timely management are essential to prevent centaurea melitensis continue to colonize and deteriorate natural and agricultural ecosystems, especially in regions where it has not yet achieved a dominant presence. Integrating local expertise, scientific monitoring, and citizen collaboration will be crucial in managing this and other challenges related to invasive alien species.