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Publication #ENH1271

Biology and Management of Thickhead (Crassocephalum crepidioides) in Ornamental Crop Production1

Allison Bechtloff, Shawn Steed, Chris Marble, and Nathan Boyd2

Species Description: Dicotyledonous plant

Family: Asteraceae

Other Common Names: Redflower ragleaf, ebolo, gbolo, fireweed, hawksbeard velvetplant

Life Span: Summer annual (flowering and fruiting August–December)

Habitat: Occurs in moist, newly disturbed areas from spring to fall. Usually found in full sun in wet areas.

Distribution: Thickhead is native to tropical Africa, but has been widely cultivated in Asia as a medicinal and nutritional herb. It was introduced to the United States and can be found in Florida, Hawaii, and Puerto Rico. In Florida, it has been reported in Escambia, Alachua, Marion, Hernando, Orange, Brevard, Hillsborough, DeSoto, Martin, Lee, Collier, Palm Beach, Broward, and Miami-Dade counties.

Growth Habit: Erect (upright), sparingly branched, herbaceous annual, growing up to 4 feet tall (Figure 1).

Figure 1. 

Upright growth habit of thickhead (Crassocephalum crepidioides).


Credit:

Chris Marble, UF/IFAS Mid-Florida Research and Education Center


[Click thumbnail to enlarge.]

Seedling: Cotyledons are light green with opposite, spatulate (spatula-shaped) leaves with smooth margins. The first true leaves will be light green, elliptical (oval-shaped) with toothed margins growing in a rosette. Veins are slightly red in color (Figures. 2 and 3).

Figure 2. 

Thickhead cotyledon and first true leaf.


Credit:

Annette Chandler, UF/IFAS Mid-Florida Research and Education Center


[Click thumbnail to enlarge.]

Figure 3. 

Mature thickhead leaf.


Credit:

Shawn Steed, UF/IFAS Extension Hillsborough County


[Click thumbnail to enlarge.]

Shoot: Stems are round and ribbed with short, thick hairs near the apical portion of the plant on the stem and branches. The lower plant is glabrous (smooth/without hairs). Newer leaves are alternate, green, elliptical (oval-shaped), 1.5–8 in long, and 0.4–4 in wide, with irregularly toothed margins. Leaves will be slightly lobed near the apex, with only a small pair of lobes near the base of the leaf, and deeply lobed in older growth (Figure 3).

Roots: Fibrous

Inflorescence: From summer to fall, clusters of reddish-orange to reddish-pink flower heads develop at the apex of the plant. Flowers are cylindrical, about 10 mm long and 4 mm wide. Flower heads droop downward as if from water stress during flowering (Figure 4).

Figure 4. 

Thickhead stem and typical drooping flowering stalk.


Credit:

Shawn Steed, UF/IFAS Extension Hillsborough County


[Click thumbnail to enlarge.]

Fruit and Seeds: Each flower head produces multiple achenes about 2 mm long that are dark-reddish brown and attached to silky hairs (pappus) that are about 9–10 mm long. The hairs allow the achenes to be wind dispersed.

Similar Species: American Burnweed (Erechtites hieracifolia) does not have leaves as thick or deeply veined as thickhead. Brazilian or Tropical Burnweed (Erechtites valerianaefolia) has more flower heads than thickhead, and the flower heads are mauve colored. Emilia (Emilia sonchifolia var. javanica) has similar features, but the flower heads do not droop and are bright pink; the leaves are more hairy than thickhead. Emilia has only been introduced to Hawaii in the United States at this time. Other Crassocephalum species also have similar features but need to be examined to be certain.

Plant Biology

Thickhead generally grows from spring to fall in north and central Florida and year-round in south Florida. It germinates over a wide range of pH, salt, and temperature conditions and preferentially when the seed is on or slightly below the media surface (Nakamura and Hossain 2009, Chen et al. 2009). Thickhead germination is higher in warm, saturated, or flooded conditions; and, as a result, it typically occurs in poorly drained fields, irrigated areas, roadsides, or during periods of heavy rainfall (Nakamura and Hossain 2009). Each flower can produce 91–150 seeds, producing up to 750–1,150 seeds per plant (Sakpere et al. 2013). Persistence in fields and container production is due to the thousands of seeds produced per plant over the course of a growing season (Nakamura and Hossain 2009). Seeds can germinate anywhere from 50°F–86°F (optimal = 59°–68°), at a pH of 2–12 (optimal 4–10). Seeds have a 90% germination rate from mature (wind dispersed) seeds at 2–12 days after making contact with media (highest germination rate is day 6) (Chen et al. 2009). Flowers can set seed in 10–12 days from initial flower opening (Sakpere et al. 2013). Thickhead grows aggressively in containers and can outcompete nursery crops for water, nutrients, and light. Thickhead is typically found in shadehouses and shaded areas of nursery production.

Management

Physical and Cultural

Hand weeding when plants are small is the best physical preventative measure. Increasing light levels and reducing application of irrigation water can help prevent germination. Using a bark mulch that prevents seed-to-soil contact might prevent seed germination. If thickhead plants escape control by various methods, the plant should be prevented from flowering in the nursery or liner bank. If flower stalks are evident in a localized area and seed dispersal is imminent, as a last ditch effort some growers have cut the stalks with machetes or hedge trimmers to prevent wind dispersal of the seeds. Seed dispersal by wind will cause a wide-spread problem throughout the nursery. Keeping field margins and border areas clean will help prevent seed dispersals. All liners and bought-in plants should be isolated and monitored for thickhead seedlings before being dispersed into the nursery.

Chemical Control

Preemergence

Very few preemergence herbicides labeled for use in nursery production or landscapes contain recommendations for control of thickhead. One herbicide that lists thickhead as a controlled species is flumioxazin, which is sold as Broadstar™ and SureGuard®. Broadstar™ is a granular formulation of flumioxazin and can be applied to container-grown ornamentals and to established plants in landscapes. SureGuard® is a dry flowable sprayable formulation and is only labeled as a directed application around most ornamentals. Other herbicides that provide suppression or control of thickhead include indaziflam (Marengo® or Specticle®) and products containing dimethenamid-p (FreeHand® and Tower®). Other preemergence herbicides are listed in Table 1. Additional herbicide recommendations are available at https://www.cals.ncsu.edu/plantbiology/ncsc/containerWeeds/Crassocephalum_crepidioides.htm.

Postemergence

There are many different postemergent herbicides that are effective for thickhead control but all should be applied as a directed application. Effective active ingredients would include glyphosate (RoundUp® and many other trade names), glufosinate (Finale®), diquat (Reward®), pelargonic acid (Scythe®), among several others. Both systemic and contact herbicides typically provide effective control, but thorough coverage is needed for contact herbicides such as diquat or pelargonic acid. Postemergent herbicides are most effective when the weeds are small and actively growing—this is especially true for contact action herbicides. Always consult the manufacturer’s label and follow all precautions when applying herbicides.

References

Chen, Guo Qi, Shio Liang Guo, and Qiu Sheng Huang. 2009. “Invasiveness evaluation of fireweed (Crassocephalum crepidioides) based on its seed germination features.” Weed Bio. And Mgmt. 9:123–128.

Mallory-Smith, C.A. and E.J. Retzinger Jr. 2003. “Revised Classification of Herbicides by Site of Action for Weed Resistance Management Strategies.” Weed Technology, 17(3):605–619.

Nakamura, Ichiro and Mohammad Amzad Hossain. 2009. “Factors Affecting the Seed Germination and Seedling Emergence of Redflower Ragleaf (Crassocephalum crepidioides).” Weed Bio. And Mgmt. 9:315–322.

Sakpere, A.M.A., O. Adedeji, and A.T. Folashade. 2013. “Flowering, Post-Pollination Development, and Propagation of Ebolo (Crassocephalum crepidioides (Benth.) S. Moore) in Ile-Ife, Nigeria.” Journal of Science and Tech. 33:37–49.

Tables

Table 1. 

Preemergence herbicides labeled for use in ornamental plant production and landscapes to control thickhead.

Common name (active ingredient)

Example trade name and formulation

WSSA Herbicide Group1

Efficacy2

Container production

Field production

Greenhouse or fully-enclosed structures

Landscape

prodiamine

Barricade® 4FL, 65 WG

3

S

YES

YES

NO

YES

flumioxazin

Broadstar™ 0.25G

SureGuard® 51WDG

14

S–C

YES

YES3

YES

YES3

NO

NO

YES

YES4

oxadiazon

Ronstar® 2G

14

S

YES

YES

NO

YES

dimethenamid-p

Tower® 6EC

15

C

YES

YES

NO

YES

isoxaben

Gallery® 75DF, 4.16SC

21

C

YES

YES

NO

YES

indaziflam

Specticle® 0.622 FLO

Specticle® 0.0224G

Marengo® 0.622SC

Marengo® 0.0224G

29

S–C

NO

NO

NO5

YES

NO

NO

YES

YES

NO

NO

YES6

NO

YES

YES

NO

NO

pendimethalin + dimethenamid-p

FreeHand® 1.75G

3 + 15

S–C

YES

YES

NO

YES

trifluralin + isoxaben

Snapshot® 2.5TG

3 + 21

S

YES

YES

NO

YES

prodiamine + isoxaben

Gemini™ 3.7SC

3 + 21

C

YES

YES

NO

NO

oxyfluorfen + oryzalin

Rout® 3G

14 + 3

C

YES

YES

NO

YES

oxyfluorfen + pendimethalin

OH2® 3G

14 + 3

C

YES

YES

NO

YES

1Herbicide groups are based according to primary sites of action and can be used to select herbicides that have differing sites of action (Mallory-Smith and Retzinger 2003) so as to minimize the potential for the development of herbicide resistant weeds.

2P = poor control; S = suppression, C = good control based on product labels or limited experimental data evaluating the highest recommended label rate.

3Can only be used in selected conifer and deciduous tree species. Check manufacturer's label for a complete list of species and recommended application methods.

4Can be applied as a directed application around established woody landscape ornamentals.

5Marengo 0.622SC can be used in pot-in-pot container ornamentals as a directed application only. Specticle™ is the same active ingredient but labeled for use in landscapes.

6Labeled for use on greenhouse floors prior to plant production. Plants can be placed back inside greenhouse 24 hrs after application.

Footnotes

1.

This document is ENH1271, one of a series of the Environmental Horticulture Department, UF/IFAS Extension. Original publication date November 2016. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Allison Bechtloff, graduate student, Environmental Horticulture Department; Shawn Steed, multi-county environmental horticulture production Extension agent III, UF/IFAS Extension Hillsborough County; Chris Marble, assistant professor, UF/IFAS Mid-Florida Research and Education Center; and Nathan S. Boyd, associate professor, UF/IFAS Gulf Coast REC; UF/IFAS Extension, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.