Yellow Starthistle Control Following Herbicide Applications at Two Stages of Plant Growth December 2000

A. Wayne Cooley former Area Extension Agent Tri River Area CSU Extension Montrose, Colorado

A. Clair Baldwin County Weed Administrator Eastern Montrose County Weed Commission Montrose, Colorado

Cooperators

Joe Scriffiny Montrose, Colorado

TABLE OF CONTENTS

Introduction

Yellow starthistle, Centaurea solstitialis L, is a plant of Old World origin that is believed to have arrived in California in the mid-1800's as a contaminant in alfalfa. It is a member of the sunflower family (Asteraceae) and belongs to the thistle tribe (Cynareae). Since its introduction, yellow starthistle has spread steadily and was estimated to inhabit about 8 million acres in California by 1985. In 1999 it was estimated that California had 22 million acres of yellow starthistle.

Yellow starthistle was discovered in July of 1999 in Montrose County on Buckhorn Road by Kathy See, Montrose County Noxious Weed Assistant. Subsequent investigations in 1999 and 2000 by Clair Baldwin, Montrose County Weed Administrator, Kathy See, Wayne Cooley, Area Extension Agent,(Soil and Crop Science), and other volunteers, have identified yellow starthistle on private lands in the general area of Buckhorn Road that exceeds 100 acres in area. The Eastern Montrose County Weed Commission; Clair Baldwin, Montrose County Weed Administrator; Eric Lane, State Noxious Weed Administrator, Wayne Cooley, CSU Extension; private landowners, and others in Montrose County are committed to eradicating yellow starthistle.

Objective

The objectives of these test plots were to determine the most effective herbicides and the best stage of plant growth for applying those herbicides to control yellow starthistle.

Materials and Methods

Herbicides were applied using a CO2 plot sprayer with a three nozzle boom (five foot spray swath) mounted on a Honda ATV. Nozzles were on 20 inch spacings, containing Tee Jet 8002 spray tips. Applications were made at four mph with a pressure of 28 psi at the CO2 tank. The sprayer was calibrated before the applications and delivered 11.6 gpa of solution at the above speed, pressure, nozzle spacing, and spray tips.

There were two stages of yellow starthistle tested and the herbicide treatments are listed in Table I. The first application was made on April 21, 2000 when the yellow starthistle ranged from 1.5 to 6 inch rosettes (average 3 inches). The plots were 5 feet by 200 feet and were not replicated. The conditions at application were partly cloudy, wind 0 to 2 mph, air temperature 48 F, R.H. 57 percent, soil temperature 40 F.

The application for the second stage of yellow starthistle was made on May 15, 2000 when the yellow starthistle ranged from two to eight inch rosettes (average five inches), with 10 percent of plants starting to bolt (sending up flower stalk). The conditions at application were cloudy, wind 0 mph, air temperature 55 F, R.H. 24 percent, soil temperature 45 F. The plots were 5 feet by 320 feet but the first 120 feet of the plots were not included in the evaluation due to no yellow starthistle present at application.

Evaluations were made by counting total number of live plants per plot (5 feet by 200 feet) on June 6, 2000 for the April 21, 2000 application and on June 23, 2000 for the May 15, 2000 application. Number of plants per plot was converted to percent control as a percentage of the untreated control and those results are reported in Table II.

Results and Discussion

Transline, Banvel + 2,4-D, and Curtail provided a 100 percent control of yellow starthistle when applied to rosettes on April 21, 2000 at the rates/A tested (Table II). Roundup resulted in 90 percent control of yellow starthistle and either killed or severely damaged all other vegetation present.

Banvel + 2,4-D, Redeem, and Curtail provided a 100 percent control of yellow starthistle when applied to rosettes on May 15, 2000 even though the rosettes were larger than the April 21, 2000 application. Transline provided good control of the rosettes, however the plants that were starting to bolt were not completely killed by the Transline.

Summary and Conclusions

Based on these test results, Transline, Banvel + 2,4-D, Redeem, and Curtail will provide excellent control of yellow starthistle when applied at the early rosette stage of development. Once the plants start initiating flower stalks, the control with Transline was reduced. There were not enough bolted plants in all plots at the May 15, 2000 application date to fully evaluate the other treatments. However, based on experience, annual plants that are treated prior to initiating flower stalks, are generally easier to kill.

Diagram: 2000 Yellow Starthistle Test-Joe Sciffany - Cooperator

2000 Yellow Starthisle Test Joe Scriffany - Cooperator See Treatment for Herbicides and Rates per Acre
2nd Stage, Applied May 15, 2000	1st Stage, Applied April 21, 2000
6	6
5	5
4	4
3	3
2	2
1	1

Table 1: Herbicides Applied to Yellow Starthistle

Table 2: Yellow Starthistle Control

Colorado State University, U.S. Department of Agriculture and Delta, Mesa, Montrose & Ouray Counties cooperating. Extension programs are available to all without discrimination. No endorsement of products is intended nor is criticism of products mentioned.

Page Maintained by Norraine Harvey, Administrative Technician
Colorado State University - Tri River Area
Mesa County Extension
2775 Highway 50, P.O. Box 20,000-5028
Grand Junction, CO 81502-5028