Tall Waterhemp Identification & Management

Common Name: Tall waterhemp 
Alternate Names: Common waterhemp, roughfruit amaranth 
Scientific Name: Amaranthus tuberculatus  
Legal Status: Prohibited- Eradicate

Tall waterhemp was added to the Maryland Noxious Weed Regulations in 2020. The Noxious Weed Law prohibits the import and transport of the noxious weed throughout the state and requires infested lands be managed for the eradication of the species.   

What is Tall Waterhemp?

Tall waterhemp is an aggressive, summer annual, broadleaf weed in the pigweed family, that has become a major challenge for farmers across the U.S. for much of the last decade. 

Historically, tall waterhemp has been problematic in disturbed habitats associated with waterways and floodplains. However, since the 1950's waterhemp has become more abundant in agricultural fields because it prefers well-drained, nutrient-rich soils, which are also preferred for crop production. 

Waterhemp is a prolific seed producer and reproduces via seed. Female plants may produce over 1 million seeds in one growing season. Waterhemp grows rapidly, up to one inch per day, under favorable conditions and can reproduce up to 3-4 times per growing season. Seeds are viable within a couple of weeks of forming.  It is dioecious, meaning male and female flowers occur on different plants and require cross-pollination. Seed may be dispersed by water, birds, machinery, contaminated manure, and by wind for short distances. 

In Maryland, tall waterhemp, like palmer amaranth, has widespread resistance to Group 9 herbicides (glyphosate) and Group 2-ALS herbicides. See management strategies.

Appearance

Waterhemp seedlings emerge in early summer and are light green to red-pink (fig a). Mature plants reach 2.5 to 8 feet tall. Stems are smooth, hairless and can be green, red, or red and green striped in color. The true leaves are oblong to lance-shaped, dark green, hairless, waxy appearance and shallowly notched at the tip (fig b). Unlike palmer amaranth, the stalks that join the leaves to the stem (petioles) are shorter than the leaf blades (fig c). Tiny flowers are tightly packed in small clusters in a spike-like arrangement at the tips of branching stems and arising from leaf axils. The smooth, long, and slender flower spikes can be 1-2 feet or more in length (fig d). Spikes can be densely packed, with numerous short branches, or they can be more loosely arranged with longer branches and gaps between the flower clusters. Female flowers do not contain sharp bracts, and clusters are more widely spaced compared to those of palmer amaranth. Seeds are small (0.8-1.0 mm diameter), black to dark red, and shiny. Waterhemp has a taproot with secondary fibrous roots.

Key Characteristics

These characteristics distinguish Tall waterhemp from Palmer amaranth, redroot pigweed and smooth pigweed:

• Flowers — Small, green flowers without petals clustered together on long (1-2 ft or more), slender, upright terminal spikes.
• Leaves — Smooth, glossy and narrow. Lance-shaped to oblong with a smooth or wavy margin. Shorter leaf petioles.
• Stems — Entirely smooth (hairless) stem that are green, red, or red-green stripes in color.
• Roots — Deep, thick white taproot with secondary fibrous roots.  Color can sometimes be light pink, but not dark pink like redroot pigweed.

Prevention

• Start with a weed-free seedbed through use of burndown herbicides or tillage. 
• Prevent seeds from entering the farm. Equipment is the most common method for moving seeds. Clean equipment thoroughly at the start and/or end of the season and between fields, using a leaf blower or compressed air. Sending one or two bales of clean straw through the combine can also help to dislodge seeds.
• Harvest infested fields last. Do not move equipment between infested and noninfested fields without cleaning the equipment first.
• Inspect your farm frequently during the growing season to ensure timely control. Early detection is key to prevent invasion.

Management

Managing tall waterhemp requires a multi-year integrated control strategy. With a relatively short-lived seedbank, eliminating seed production for 3 to 5 years will result in a dramatic reduction in population density of this weed.

• Chemical — Use a good burndown herbicide program or tillage. Apply residual herbicides with multiple effective modes-of-action at planting. Apply effective POST herbicides before weeds are 3-4” tall and include an additional residual herbicide in the tank to extend control later into the growing season. Additional POST herbicides may be required depending on infestation size. 

  Applicators should always follow the herbicide label, use full-labeled rates, and apply in a timely manner. 
  Below is contact information for specific herbicide recommendations as well as organic options.

• Mechanical — Tillage or cultivation may be effective if done timely and only on seedlings. Mature plants may re-root. Occasional moldboard plowing can be used to bury seeds beyond the germination zone. Hand-pulling waterhemp plants, especially before they go to seed, can be an effective method for controlling small infestations.

• Cultural — Planting a high residue cover crop and delaying cover crop termination as long as possible may help to outcompete emerging seedlings. Crop rotation (corn/soy/small grain/forage) can disrupt waterhemp's life cycle. Narrow spacing can allow for faster canopy development by limiting light exposure to the soil surface.

  Cutting or mowing alone is not the best management option for control, consider combining with a herbicide program.

Combining Weed Control Strategies Effectively

Palmer amaranth and waterhemp are herbicide-resistant weeds that can easily adapt to conventional spray programs. By combining different weed control strategies, these weeds are less likely to become more problematic as they cannot adapt to a constantly changing system.

Each practice listed has been assigned points based on the relative level of control obtained in university studies. The greater the number of points accumulated, the greater the likelihood of success of the management program. 

• 1 = Fair practice
• 2 = Good practice
• 3 = Excellent practice

It is recommended that conventional farming operations accumulate at least 8 points to control Palmer amaranth and waterhemp successfully. Organic operations should accumulate at least 6 points, including 2 different harvest weed seed control options. 

Adapted from Efficacy of Weed Management Tactics SPES-268N9 

Note: Land under the Conservation Reserve Program should consult with USDA Farm Service Agency on weed control and management.

Resources

For specific herbicide recommendations as well as organic options, contact University of Maryland Extension Weed Management Specialist, Kurt Vollmer: [email protected] or Ben Beale: [email protected]

MDA has entered into an agreement with many counties in the State to provide technical assistance to landowners. Many of the programs provide herbicide application to landowners on a fee-for-service basis. To learn whether your County has a Weed Control Program, visit plants and pests. 

For weed identification contact your local Extension office or your regions MDA Agricultural Inspector.