Beware the bindweed, amaranth and waterhemp
by Courtney Llewellyn
Dr. Lynn Sosnoskie, assistant professor of horticulture at Cornell, focuses her work on biology-based research, crop injury and weed management. She’s worked in California, Washington, Wisconsin, Ohio, Georgia, Pennsylvania and most recently New York. Having seen the danger they pose in the West, she said of Palmer amaranth and waterhemp, “I am really, really, really terrified of them getting a foothold here.”
Sosnoskie presented “Herbicide Resistant Weeds and Perennials: What We Know Now and What We Need to Learn for the Future,” speaking on the damage she has seen the aforementioned weeds cause. One of the biggest takeaways from her lecture was the fact that with no weed control, corn growers could see yield losses of $26.7 billion – or 50% – of their crops (based on 2016 estimates).
These pests are a threat to sweet potatoes, tomatoes, melons and wine grapes as well, not just agronomic crops. And they are especially a concern for specialty crops because those growers tend to have less chemical opportunities to try to eliminate them.
Sosnoskie noted there are 510 unique cases of herbicide resistant weeds globally, with many having built up resistance to 23 of 26 herbicides (and the U.S. leads with the most resistant weeds). Palmer amaranth and waterhemp are large, competitive, and very herbicide resistant – and are native to North America. They are not invasive species. Sosnoskie saw them overtake fields along the West Coast and in the Midwest. That’s why she’s so concerned about them creeping into the Empire State.
Palmer amaranth has been reported in three counties, and waterhemp in 11 in New York State (mostly in the west to central regions). Sosnoskie and her team from Cornell will be studying known stands of the weeds this summer to monitor their speed of growth.
But what can growers do? The first step is positively identifying the weeds. There are nearly four dozen species of amaranth, and they tend to look alike early on. Palmer amaranth (Amaranthus palmeri, also known as pigweed or careless weed) has alternating leaves along the stem, with petioles longer than the leaf length. Waterhemp (Amaranthus tuberculatus) lacks hair on its stems and leaves, unlike others in the genus. Both can completely eliminate crop yields if they get established. Some plants can grow up to nine feet tall, and can even grow several inches in a day. Even worse, it’s been found that herbicide resistance is inheritable through pollen.
Depending on their emergence date, growers have very little time to control these two weeds. Sosnoskie said they need to be stopped early in the invasion process, and never allowed to reach reproductive maturity. “Do not let them flower. Do not let them seed,” she stated. She also listed the following tips: If possible, use residual herbicides. Deep tillage can help. Utilize cover crops. Clean equipment between fields – and harvest suspect fields last. Also, don’t forget about late season flowering and seed production.
“We need to try to gain every percentage of control we can,” Sosnoskie said.
Another perennial pest that never quite disappears is field bindweed. A vining weed with white and pink flowers, bindweed was native to the Mediterranean region. It was introduced into the U.S. in the mid-1700s, and had spread to the Pacific coast by early 1800s. It is difficult to control for three main reasons: it has huge root systems; its seeds can be viable for decades; and it can grow from seedling to vine in only three to four weeks after emergence.
Sosnoskie said it can be controlled with systemic herbicides as well as good timing (treating it at flowering, when its carbohydrates are moving underground). She noted that some pre-emergence herbicides do make a difference, but more integrated strategies are needed, including soil disturbance, shading – since bindweed hates shade, and cover cropping.