Malta starthistle is a close relative of yellow starthistle (Centaurea solstitialis). Other common names for this plant include Tocalote, Napa starthistle or Maltese centaury. The plant is an annual or biennial and reproduces from seed. The plant forms a rosette during the first growing season. The leaves of the rosette are deeply lobed and the surfaces are covered with stiff, thick hairs and resinous dots. The mature plants of Malta starthistle have stiff, upright stems are openly branched from near the base. The plants have vertical ribs along the stems that are extensions of the leaves. These extensions are sometimes referred to as “wings”. The wings on Malta starthistle are approximately 0.1 inch wide. Wings on yellow starthistle are somewhat larger. The flowers of Malta starthistle occur singly or are groups of 2 or 3 at the ends of the stems. Vigorous plants can develop flowers in the leaf axils. Malta starthistle has slightly smaller seed heads compared to yellow starthistle. The spines on the phyllaries are about ¼ inch long, brown to purple tinged and are branched near the midpoint. The spines are flattened when observed in cross section. Malta starthistle has a single taproot. Sources for new infestations may be contaminated seed or vehicles traveling from infested areas. Unlike its close relative yellow starthistle, Malta starthistle does not cause chewing disease in horses.
Northern Arizona Localities:
Currently, there are only small, isolated populations of Malta starthistle in Northern Arizona. There is a small population in Dead Horse State Park near Cottonwood, and infestations in Verde Valley. Most of the current infestations in Arizona are found below 4,000 feet in altitude, in Apache, Yavapai, Maricopa, Pinal, Graham, Pima, and Cochise counties. Malta starthistle grows in a variety of open areas including disturbed sites, grasslands, rangelands, and woodlands. Its ability to adapt to a wide variety of habitats gives it the ability to invade many habitats throughout the Coconino, Kaibab and Prescott National Forests.
Origin & Impacts:
Malta starthistle is a native of Southern Europe where it grows in dry places and disturbed ground. It grows in a variety of open areas including disturbed sites, grasslands, rangelands, and woodlands. Its ability to adapt to a wide variety of habitats gives it the ability to invade many habitats throughout the Coconino, Kaibab and Prescott National Forests. All species of the genus Centaurea are less palatable than native or exotic forage plants. Occupancy of sites by one or more species of the genus can cause a decrease in forage production. This can impact range carrying capacity of livestock allotments and decrease the quality and quantity of wildlife habitat. Replacement of native perennial grass species by Malta starthistle or other members of the genus contribute to soil erosion. The finely divided root structure of perennial bunch grasses secures the topsoil and prevents erosion from wind and water. Malta starthistle and other members of the genus Centaurea have a single taproot. This leaves many areas of the soil exposed to the elements of erosion. Any protection that Centaurea spp. may provide the soil is generally lost in the fall when plants senesce. However, perennial bunchgrasses provide protection for the soil even when they are dormant. Recreation and scenic values of natural areas can be impacted by the presence of Malta starthistle and other exotics. Malta starthistle plants have spines and can decrease the enjoyment of hikers and animals using the area. Yellow starthistle (Centaurea solstitialis) is a close relative of this species.
Integrated pest management systems are most effective, combining the suppression of Malta starthistle with the enhancement of perennial grass species. Grazing always gives starthistle an ecological advantage over grasses, so 12-18 months of rest from grazing may be required after treatment. Prevention and detection of new populations, as well as eradication and containment of existing populations all need to be addressed to achieve control over Malta starthistle. Mapping of distribution and extent as well as consistent monitoring are also necessary to determine which combinations of control methods will be most effective in each circumstance. Most local land management agencies have adopted policies to map and monitor noxious weed populations through the collaboration of South West Exotic Mapping Project (USGS).
Prevent Malta starthistle from becoming established. Do not drive through areas that are infested with Malta starthistle. Check and clean all footwear, livestock and vehicles for attached starthistle plants when leaving infested areas. Purchase hay that is certified as being weed-free.
Hand pulling or grubbing Malta starthistle is most effective for very small infestations, perimeter populations, or along riparian zones. Pull plants after they have bolted and before their flowers have bloomed. Repeat every 2-4 weeks during the growing season, removing as much of the root as possible. Cultivation, where feasible, will control (reduce) Malta starthistle within two years. Tilling must be done five or six times a year, two weeks apart, starting in June. Remember that seeds may stay dormant in the soil for at least ten years. Mowing is not recommended, as plants can produce side branches with more flowers. A study in California found that the yellow starthistle seed-bank can be reduced by burning the areas at least three years in a row just as the plants are beginning to flower. For burning to be a useful control agent in Northern Arizona, biannual burn/torch plans must coincide with Flagstaff’s biannual rainfall.
Chemical Control (Noted here are chemical control techniques that have been used in other areas. Always check with weed specialists or chemical suppliers before treatment to ensure correct dosage and application. Mention of these products does not imply endorsement by the USDA Forest Service, Northern Arizona Weed Council or The Nature Conservancy.):
Selective soil residual herbicides applied at the correct time may control weeds long enough to establish competitive grasses. Initial treatment should be in early spring, with follow-up before plants bolt. Picloram (Tordon 22K), dicamba (BanvelÔ), clopyralid (for high water table), and triclopyr (Garlon 4 ) have been used, but repeat applications are needed. Other favorable broadleaf species will also be affected. 2,4-D has also been used, but plants should not be treated more than once every 2-3 years to minimize selection for herbicide-resistant Malta starthistle plants.
Biological Control (No exotic species should be introduced into an ecosystem without extensive research into the long-term effects. Mention of the species below does not imply appropriateness for use in Northern Arizona.):
Lasioderma haemorrhoidale a beetle that feed on the seed heads of Malta starthistle was inadvertently introduced from the Mediterranean area. However, it does not eradicate infestations of Malta starthistle. There are no approved biological control agents available for use on Malta starthistle. Some of the same agents that are available for yellow starthistle may also work on Malta starthistle. Using a biological control agent on a species for which it is not intended could lead to serious ecological harm and loss of the financial investment. However, due to the large number of seeds and seed longevity of Malta starthistle, insects alone cannot completely control an infestation.
Note: No single control method, or any one-year treatment program, will ever achieve effective control of an area infested with Malta starthistle. The fast growth, high seed viability, and long seed dormancy of this plant require long-term cooperative integrated management programs and planning in order to contain and reduce Malta starthistle infestations.
Coconino NF/ SFPWMA Page 4/15/2016