Malta starthistle
Centaurea melitensis
Washington State Classification
Class B — Control Required (Designated)
This is a new noxious weed for 2018.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
Malta starthistle is an annual that begins as a basal rosette of leaves, then grows stems up to ~3 feet tall, though often shorter, with spiny yellow flowerheads that resemble yellow starthistle, Centaurea solstitialis, a Class B noxious weed. Plants have simple to branching wiry stems, covered in hairs, being loosely gray-tomentose.
Leaves
Like stems, leaves are often grayish-green, covered in stiff hairs and minute resin glands. Often, the older leaves have sparse, fine, white cottony hairs. Basal rosette leaves and lower stem leaves have petioles or tapering leaf bases, with leaf blades that are longer than wide, with typically the widest point toward the tip, and tapering to the base. Leaves 0.8 to 6 inches (2 to 15 cm) long. Leaf margins are smooth to toothed, or deeply lobed. The terminal lobe is usually the largest and rounded at the tip.
Flowers
Like yellow starthistle, flowerheads of Malta starthistle, are comprised of yellow disk flowers. Flowerheads are single or in clusters of two to three. Base of the flowerhead is egg-shaped, 8-15 mm, loosely cobwebby-tomentose or becoming glabrous. Main bracts have a slender, central spine at the tip, 5-12 mm long that is often purplish to brownish colored. At the base of the central spines are lateral spines, usually in 3 to 4 pairs, with the upper pair near the middle of the central spine. The yellow disk florets can be sterile or fertile.
Fruit & Seeds
Each fertilized flower can produce a dry, single-seeded fruit with attached bristles, called a cypsela. Cypselae are dull white, to gray to tan, often with slightly darker stripes, and are about 2 to 3 mm long. The pappus is made up of many white to pale tan, unequal, stiff bristles, 1 to 3 mm long. The base is deeply notched with a scar.
Impact
This is a new noxious weed for 2018. Malta starthistle causes detrimental impacts to ecosystems, being able to displace native plant species and impact wildlife. It has been found to negatively impact endangered plant species in California, reducing other plants' abilities to reproduce. While invading disturbed habitats, it can also establish in undisturbed habitats, competing with other species for resources such as nutrients, light, and water. It may poison and cause horses to develop "chewing disease" (Equine nigropallidal encephalomalacia), but as yet, its toxicity has not been confirmed experimentally.
Ecology & Spread
Habitat
Malta starthistle grows in open, disturbed habitats such as grasslands, rangelands, creek beds, fields, pastures, roadsides and waste places. In California, plants have also established in less disturbed to undisturbed communities such as pine-oak woodlands, chaparral, coastal dunes and bluffs, wetlands, and even serpentine habitats. Click here to see a county-level distribution of beach grasses in Washington.
Spread Mechanisms
Reproduction
Malta starthistle reproduces by seed. Seed production per plant is highly variable. Seeds fall near the plant and also may be dispersed a short distance by wind. Seeds can then be further dispersed by people, animals, water and soil movement.
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