Linda M. Wilson, Cynthia Jette, John Connett, Joseph P. McCaffrey. 2003. Biology and Biological Control of Yellow Starthistle. USDA Forest Service FHTET-1998-17 2nd Ed.

Chapter 1: Getting To Know Yellow Starthistle

Yellow starthistle is an annual rangeland weed originating from the Mediterranean region. It became established in North America in the mid-1800s in contaminated alfalfa or other crop seeds. Yellow starthistle seeds were found in adobe brick in California beginning in the early 1800’s. First reports of yellow starthistle in the Pacific Northwest include an alfalfa field near Walla Walla, Washington at the beginning of the 1900s.

Yellow starthistle primarily infests annual and perennial grasslands, pastures, shrub steppe, open woodlands, and disturbed habitats such as hayfields, orchards, vineyards, roadsides and abandoned areas. Starthistle is present in 23 states, having the largest, contiguous infestations in California (about 15 million acres), Idaho (about 3 million acres), Oregon and Washington (about 150,000 acres each). It is estimated to spread at the rate of about 6 percent per year.

The thorny spines that surround the flower heads of starthistle interfere with grazing by livestock, recreation, and wildlife management. It is toxic to horses, causing a chronic and potentially fatal neurological disorder known as "chewing disease". It reduces biodiversity by displacing native vegetation in grasslands and woodlands.

Long-term strategies for management of yellow starthistle includes a combination of cultivation, hand pulling and mowing, herbicides, burning, managed grazing, biological control and other practices that suppress starthistle and enhance competition by desirable vegetation. Well-adapted, perennial grasses can limit yellow starthistle invasion and curtail its expansion.

Yellow starthistle is a winter annual. Seeds germinate in the fall and grow into overwintering rosettes. Under favorable conditions of temperature and moisture, germination can continue through the winter and early spring. Once established, fall germinated seedlings monopolize soil moisture and are highly competitive for soil nutrients and space. In the spring, plants bolt, producing a few to several branched, erect stems, each with a terminal flower head.

Yellow starthistle is renowned for its variable growth habits. This wide response, or plasticity, enables the plant to respond rapidly to wet or dry conditions, producing large plants with abundant seed during moist years, and small plants with few heads and seeds in dry years. For example, in a dry years, starthistle plants can be 6 inches tall with 1 to 2 flower heads, in contrast to moist years, when starthistle plants at the same site can be 2 to 4 feet tall with one to several hundred flower heads.

Like all members of the sunflower family, the starthistle seedhead, or capitulum, is an aggregation of 20 to 50 small, individual flowers. The individual flowers, or florets, are tightly clustered and anchored to a concave base, called the receptacle. The receptacle and florets are surrounded by an envelope of modified leaves, or bracts. Bracts of starthistle have a long, stiff spine at the tip, often 2 to 3 times the width of the head.

Yellow starthistle, Centaurea solstitialis L. Other common names: Barnaby thistle, Golden thistle, Cotton-tip thistle. Family: Sunflower family (Asteraceae or Compositae) Description: A winter-hardy annual that reproduces by seeds (Fig. 2). Height: Widely variable year to year depending on site conditions. The average rangeland size is 1 to 3 feet (0.3 to 1m), 4 to 6 feet (1.2 to 1.8 m) tall in shady, wet areas, or as short as 6 inches (15 cm) in dry, warm areas. Leaves: Basal leaves form a rosette beginning in the fall and continuing through the winter. Each leaf is divided into lobes with the end lobe larger and rounder than the side lobes, the stalk shorter than the leaf blade. Stem leaves attach directly to the stem by

Figure 2a. Yellow starthistle plant (UGA1350003).

Figure 2b. Yellow starthistle plant (UGA1350004).

Heads: Each flower bud appears as a small, egg-shaped swelling up to 0.75 inch (1.9 cm) long and enclosed by shingle-like layers of bud scales called bracts. A sharp, yellow-green spine appears at the tip of each bract can be 0.25 to 2 inches ( 0.6 to 5 cm) long after the flowers fully open. Buds are solitary at the ends of the branches. The base of the head is pubescent.

Stems: The stems are upright, stiff, winged and branched. Small plants can have an unbranched stem and one flower head; large plants have a stem with multiple branches and can have over one to several hundred heads.

Flowers: Bright yellow flowers, about 5/8 inch (1.6 cm) in diameter.

Seeds: Two types of seeds are produced: plumed and plumeless. Seeds are tan to black and about 1/8 inch (0.3 cm) long. Plumed seeds, located at the center of the seedhead, have a ring of fine, white bristles (called a pappus), 1/8 inch long and are easily dislodged from the seedhead by wind or other disturbances. Though not readily airborne, they disperse when they become lodged in hair, fur and feathers or are carried by water, vehicles or rodents.

Figure 3. Yellow starthistle seeds (UGA1350005).

Distribution: Figure 4 depicts the distribution of yellow starthistle in North America. Timeline of Attack: The relationship between biocontrol insect activity and starthistle growth stage is very important in starthistle biological control. Each of the insects’ lifecycle in closely tied to, or synchronized with, specific plant growth stages. Nine primary growth stages of yellow starthistle (see Fig. 5, page 9) are commonly used.

Figure 4. Map of yellow starthistle distribution in the United States. Yellow = Abundant. Gray = Present. White = Absent.

1. Seedling stage (Fig. 5a). Germination begins in the fall and continues through spring and is followed by the emergence of two oblong cotyledons or seed leaves. The plants then produce 5 or more basal leaves and 2 to 4 deeply lobed leaves.
   
   
2. Rosette stage (Fig. 5b). In the spring, 7 or 8 lobed leaves emerge to form a rosette as the plant grows in height and diameter ending with 20 or more leaves in the rosette. During this spring growth period, dense infestations of yellow starthistle can be identified from a distance by their characteristic blue-green color.
   
   
3. Bolting stage (Fig. 5c). The plant begins to bolt in late spring, sending up a rigid, winged flower stalk with a blue-green, cottony pubescence and tipped with a firm flower bud. The flower stalk can be simple in small plants, or branched in larger plants.
   
   
4. Floral bud stage. Learning to recognize the four floral bud stages is important for biological control planning.
   
   
• BU-1 (Fig. 5d) Small buds with yellow-green spines begin to be visible at the top.
  
  
• BU-2 (Fig. 5e) Spines protrude more than half of the bud length.
  
  
• BU-3 (Fig. 5f) Spines are equal to or greater than 45° angle from stem
  
  
• BU-4 (Fig. 5g) Spines are straw-colored and equal to or greater than 90° angle from stem.
  
  
5. Flowering stage (Fig. 5h). Bright yellow flowers appear in the summer.
   
   
6. Seed formation stage (Fig. 5i). There is a progressive loss of color in mid-summer, but the bud still retains some green.
   
   
7. Mature stage (Fig. 5j). By late summer, the leaves wither and dry, the bright yellow flowers fade, the plants take on a straw-colored appearance.
   
   
8. Seed dissemination stage (Fig. 5k). From late summer to early fall, the flower head dries to a tan color, the bracts dry and release the plumed seeds which are dispersed either by wind, water or by clinging to clothing, fur or feathers.
   
   
9. Senescence stage (Fig. 5l). The final stage begins in the fall and continues through the following spring when the plants continue to dry and lose their leaves, becoming silver-gray skeletons with heads that look like white, cottony tufts by December or January. The flower heads have lost most of their spines and the plumeless seeds by this time. Eventually the head disintegrates and plumed seeds are shed.
   
   

Figure 5a. Seedling (UGA1350006).		Figure 5b. Rosette (UGA1350007).
Figure 5c. Bolting (UGA1350008).		Figure 5f. BU-3 (UGA1350011).
Figure 5d. BU-1 (UGA1350009).		Figure 5e. BU-2 (UGA1350010).
Figure 5g. BU-4 (UGA1350012).		Figure 5h. Flowering (UGA1350013).
Figure 5i. Seed Formation (UGA1350014).		Figure 5j. Mature (UGA1350015).
Figure 5k. Dissemination (UGA1350016).		Figure 5l. Senescence (UGA1350017).
Figure 5. Yellow starthistle growth stages (UGA1350006 - UGA1350017).

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