Surveys indicate this is the third most troublesome weed in peanuts.
By Eric P. Prostko and A. Stanley Culpepper, University of Georgia Professors and Extension Agronomists in Weed Science
Recent surveys in Georgia show that tropic croton (Croton glandulosus var. septentrionalis) is the third most troublesome weed in peanuts and fifth most troublesome weed in cotton. The increase in its prevalence in Georgia can be attributed to several factors including a change in herbicides, particularly the loss of dinoseb in 1986, and the fact that many standard herbicides currently used in peanut and cotton are ineffective on this weed. Also, mature seeds are forcibly discharged from the plant and are readily eaten by birds, which facilitate dispersal and spread.
Tropic croton, also known as northern croton, is a member of the Euphorbiaceae (Spurge) plant family. Other members of this family include such common weeds as hophornbeam copperleaf, spotted spurge and wild poinsettia. It is a summer annual that can grow up to 20 inches tall.
Tropic croton has oblong to egg-shaped leaves with sharply toothed margins. Separate male and female white flowers, approximately 0.4 inches long, occur in terminal, unbranched, elongated inflorescences called racemes.
One of the keys to identifying seedlings of this plant is the presence of star-shaped hairs on all plant parts. These hairs help distinguish tropic croton from other look-alike weeds such as prickly sida, velvetleaf and spurred anoda.
Research-based information on seed production, seed response to burial depth and soil pH extremes is not available.
Limited research has been conducted on the competitive effects of tropic croton in peanut. Preliminary full-season competition studies conducted in North Carolina have shown that tropic croton at a density of 32 plants per 20 feet of row has the potential to reduce peanut yields between 28 and 54 percent. Consequently, tropic croton is considered to be less competitive than many other troublesome broadleaf weeds of peanut including Florida beggarweed, sicklepod, bristly starbur and wild poinsettia.
No herbicide is labeled for use on tropic croton in peanut that provides full-season weed control with a single application. The best approach to controlling weeds in peanut is to use a combination of both soil-applied and postemergence herbicides.
Soil-applied herbicides labeled for use in peanut that have some residual activity on tropic croton include Strongarm (diclosulam) and Valor. Of these herbicides, Valor is the most effective. As with any soil-applied herbicide, moisture after application is critical for optimal performance. At least onehalf inch of rainfall or irrigation within seven to 10 days after application is required to activate soil-applied herbicides. However, rainfall or irrigation that occurs at peanut emergence after a Valor application will increase the potential for crop injury.
Postemergence options for tropic croton control include an “at-cracking” application of the combination of paraquat + Storm (acifluorfen + bentazon) or an early-postemergence application of Cobra (lactofen), Ultra Blazer (acifluorfen) or Storm. Paraquat is sold as Gramoxone SL, Firestorm or Parazone. Make these postemergence treatments when the tropic croton is in the cotyledon to two-leaf stage of growth. None of these postemergence treatments provide residual control of tropic croton.
Cadre (imazapic) does not provide effective control of tropic croton. However, research conducted in Georgia and South Carolina has shown that the addition of Ultra Blazer with Cadre greatly improves control of tropic croton compared to Cadre alone. The rate of Ultra Blazer used in this tankmix is dependent on the weed stage of growth at application. Control of small weeds of two inches or less can be improved with eight ounces per acre of Ultra Blazer while larger weeds will require at least 16 ounces per acre or more.
For more information on tropic croton in cotton, read UGA College of Agriculture and Environmental Sciences bulletin C-865 or go to extension.uga.edu/agriculture/crops/peanuts and click on Publications.