Spiney, or purple pigweed is a summer annual broadleaf weed that is very similar in appearance to other pigweeds but has spines along the stems. Spines are paired, occurring at the base of most of the leaf petioles. Commonly found in waste areas and disturbed soils, it grows erect from 1-5 feet in height. The lower stems are green in color. Flowers are small and green, in bristly terminal and axillary clusters. Prolific seed producers, pigweeds will produce up to 100,000 seeds per plant.
Seedlings are easily pulled or hoed from the garden. Older plants are more difficult to pull out and may scatter seeds in the process. Mulch will prevent seeds already in the soil from germinating.
When found in the lawn, pigweed can be spot-treated with herbicides containing MCPP, MCPA, and dicamba. In gardens, glyphosate (Round-up, Kleen-up) can be used, taking care not to get any of the chemical on desired plants. Pre-emergents containing oryzalin, pendimethalin, or trifluralin can be effective. Read labels on herbicides carefully and follow directions closely.
Photos: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010