(News article for June 2019)
Pigweed is a major nuisance for gardeners in the Florida parishes. Yesterday, a farmer told me that pigweed is one of his biggest challenges and it has to be “the worst weed in the world.” Another farmer was frustrated that pigweed is taking over his watermelon patch. Pigweed can be managed, but you must stay on top of it and use multiple approaches for good control.
There are many types of pigweed but the one I see most often is spiny amaranth, Amaranthus spinosus. I call it spiny pigweed. Spiny pigweed has a reddish stem which contains spines that will pierce your skin when grabbed. It is extremely prolific and very hard to control. One plant can produce over 100,000 seeds! Spiny pigweed is troublesome because the spines poke and scratch you as you work around it and also because the plant competes with vegetables for nutrients and water. It can grow to almost five feet tall and will shade out vegetables if you have a heavy infestation.
One of the best ways to prevent spiny pigweed is by using a pre-emergence herbicide containing the active ingredient trifluralin. Treflan is one of the well-known brands, Preen and Eliminator also make trifluralin products. Pre-emergent herbicides work by preventing weed seeds from germinating. Trifluralin can be incorporated into the soil before planting some vegetables. The most important thing to remember when using a pre-emergent herbicide is to read the label carefully! The label will give specific instructions on when and how to use the herbicide for the specific crops you are producing.
Mulch is another great preventative tool. I prefer to use plastic mulch or pine straw in a vegetable garden. Mulch will prevent weed seeds from germinating by blocking out the sun. It will also moderate soil temperatures and help the soil retain moisture.
Hand removal is another way to combat the weed. Ideally you should pull the weed when it is young before the taproot, seeds, and spines have begun to develop. Use thick, leather gloves to remove mature plants. Old fashioned cultivation with a hoe will work too. I always recommend removing the plant material from your garden and burning it. Certain types of pigweed can re-grow if a piece of the root touches the ground. Better safe than sorry.
Last but not least are post emergent herbicides. Post emergent herbicides kill the plant after the seeds have germinated and leaves begin to develop. One of the most effective post emergent herbicides for spiny pigweed is glyphosate, commonly known as Round Up. Use glyphosate carefully around vegetable plants because it will kill vegetables if it gets on the leaves or stems. I’ve seen people apply glyphosate with a paint brush, paint roller, or funnel attached to a spray nozzle. You can also apply using a cotton glove, provided you wear a latex glove underneath to protect your hand from the herbicide
It doesn’t take long for spiny pigweed to take over your entire garden. Stay on top of it and follow these management practices for best results. Good luck!
Jessie Hoover is a County Agent with the LSU AgCenter covering horticulture in East Feliciana, West Feliciana, St. Helena, and Tangipahoa parishes. For more information on these or related topics contact Jessie at 225-683-3101 or visit the LSU AgCenter website.
Spiny pigweed plant. Photo by Jessie Hoover.
Close up view of spiny pigweed spines. Photo by Jessie Hoover.