Spring has seen an explosion of weeds in the Southwest following a winter of above-average rainfall. There is a group of weeds visible right now that have similar carrot-like foliage but are actually very different weeds. All of them can tolerate low mowing and they can be seen in tees, fairways and even greens. Here is a guide to identifying and controlling these carrot-top weeds:
Swinecress (Coronopus didymus) – Swinecress is in the mustard family. It has finely divided feathery leaves that are alternate on the stem and a strong tap root. The flowers are a tiny cluster of green balls in the leaf axils. Swinecress has an extended germination period from September through June and persists in the shade. Crushing the leaves and flowers produces a skunky smell. The key to identification is the tap root, the green ball flowers and the skunky smell.
Southern Brassbuttons (Cotula australis) – Southern brassbuttons is in the sunflower family. It has light green feathery leaves that are toothed and covered in fine hairs. The flowers are pale yellow disks, 0.125-0.25 inch in diameter; there are no petals on the flower. Southern brassbuttons has a weak tap root that is often multibranched. It is visible from winter through spring and there is no skunky smell.
Spurweed or Lawn Burweed (Soliva sessilis) – Spurweed is in the sunflower family. It has feathery dark green leaves that are opposite on the stem and finely hairy. The roots are fibrous. The flower is a green ball that forms in the leaf axils that becomes a spiny bur about 0.25 inch in diameter. Spurweed is visible spring through summer. The key to identification is the fibrous root system and the spiny bur.
Wild Celery (Apium leptophyllum) – Wild celery is in the carrot family. It has feathery dark green leaves that are much divided with branched stems. Wild celery produces a tiny cluster of white flowers on short stalks. It has a strong tap root. Wild celery is visible winter through spring, mostly in dormant bermudagrass. The key to identification is the tap root, divided stems and tiny white flowers.
Pineapple Weed (Matricaria matricariodes) – Pineapple weed is in the sunflower family. The leaves are feathery, bushy, hairless and light green in color. It has a shallow tap root with secondary fibrous roots. Pineapple weed has a yellow cone flower that forms at the tip of the stem and has a sweet pineapple smell when crushed. Pineapple weed is visible winter through spring. The key to identification is the sweet-smelling, cone-shaped flower.
So how do you control these weeds? The best control is achieved with a mid-October application of a preemergence herbicide. Good control can be achieved in both warm-season and cool-season grasses with preemergence herbicides containing isoxaben or dithiopyr. Isoxaben is especially good for preventing swinecress. Postemergence control can be achieved in most warm-season and cool-season turf species with herbicides containing clopyralid or three-way herbicide combinations containing 2,4-D, mecoprop and dicamba. Another option in cool-season grasses is triclopyr. Remember to always consult the herbicide label and apply all herbicides in accordance with label directions and state regulations.
West Region Agronomists:
Patrick J. Gross, regional director – pgross@usga.org
Larry W. Gilhuly, agronomist – lgilhuly@usga.org
Brian S. Whitlark, agronomist – bwhitlark@usga.org