Malta Starthistle

Centaurea melitensis

Malta starthistle blooming at Academy Village

This is an INVASIVE PLANT!

Malta starthistle is an invasive annual from the Mediterranean region. It was introduced to California in the 1700s and has since spread to Arizona, New Mexico, Oregon, and Texas. It has the ability to form dense stands and outcompete natives for resources. Activities in areas with Malta starthistle are limited because the flower heads are covered with sharp spines.

Malta starthistle was added to the Arizona State Noxious Weed List in January, 2020.

Malta starthistle seeds germinate in winter and grow to form a rosette of leaves. As the weather warms, stems begin to grow, potentially reaching to 3 feet. Flower heads form on the ends of these stems in April. Flowering continues for several weeks.

Very small Malta starthistle with flower head starting to form at ground level

Malta starthistle can be hard to control because the first flower head can form when the plant is still quite small, only an inch tall and two inches wide; because each plant produces many seeds; and because the seeds are viable for several years, remaining in the soil until conditions are right for germination. Effective control requires persistent monitoring so any infestations are noticed before seeds have formed. To control, remove plant, including the roots, as the first stems begin to elongate. Make sure to discard plant in the trash - don't leave it on the ground.

Notes: Malta starthistle can be confused with yellow starthistle (Centaurea solstitialis). The leaves and stems of the two plants are similar. The flower heads are different: the spines on Malta starthistle are less than one-half inch long while the spines on the flower head of yellow starthistle are an inch in length.

More Information

Weekly Plant on Malta starthistle

Managing Malta starthistle from USDA

Identification and Management from Sonoran Desert Cooperative Weed Management Area

Map of distribution in US (light blue indicates exotic, pink indicates exotic and noxious; not updated for new 2020 AZ Noxious Weed List)

Technical botanical description from SEINet

ID Characteristics 

This plant is in the Asteraceae - the aster family.
Malta starthistle can grow to 3 feet in height. Small plants only have 1 or 2 stems; large plants with ample moisture may have 10 or more (see second photo above).
The first leaves may be only an inch long, without lobes (note flower head in center of rosette).
As the basal rosette develops, the leaves become lobed and can grow to 6 inches in length. These leaves will often die back as flowering begins. Several leaves in the picture above are starting to die and turn brown.
As the plant begins to flower, several stems begin to grow. The leaves on the stem differ from the basal leaves. They become shorter as the stem elongates. They lack the deep lobes of the basal leaves but often have small lobes. Their base extends down the stem, giving the appearance of "wings".
The base of the flower head is covered with short spines that usually have several smaller spines on each side. The flower head is less than one-half inch long, tinged with brown or purple, . The flowers are yellow, constricted at their base. Overall the flower head is usually less than an inch tall.
As the fruit matures, the spiny base of the flower head curves backward. Many seeds develop in the center of the flower head. The seed is white and tan striped with many short bristles at one end.