Giant Ragweed (Ambrosia trifida)

About Giant Ragweed:

The Giant Ragweed is a summer annual weed that reproduces through the germination of their seeds. It is known for being an extremely competitive weed that has been shown to reduce the yield in soybean field by about 30%. There are some human health concerns with the Giant Ragweed during August and September due to the fact that it contributes to hay fever.

Family: Asteraceae/ Compositae –Aster Family

Giant Ragweed Scouting and Prevention:

Giant Ragweed has a straight stem that can reach up to 4 m tall at full maturity with lots of branches and somewhat covered in hair. The leaves on the stem are typically opposite of one another, except on the rare occasion when they alternate one another on smaller branches during flowering season. The leaves are large, rounded in outline and have 3 to 5 lobes per leaf which can be coarsely toothed or smooth. The leaves near the top of the plant are smaller, typically not lobed and are rough like sandpaper. The Giant Ragweed produces flowers that bloom from August to October and are unisexual with male flower heads producing yellow pollen and female flower heads producing seeds. The seeds are about 5 to 10 mm long with many, prominent, lengthwise ridges with a blunt spine around the upper shoulder of the seed.

Common locations

• - Moist soil
• - Fertile loamy soil
• - Pastures
• - Cultivated fields
• - Soybean fields

Prevention

The easiest way to deal with Giant Ragweed is to make sure there are high standards in your fields, such as:

• - Making sure your variation of seeds are weed-free
• - That you are cultivating and planting at optimal times
• - Putting plant appropriate fertilizer on crops to promote fast and easy growth
• - Applying pre-emergent herbicide control, which is best done with Atrazine

Giant Ragweed Control:

Cultural Control

The best cultural control method for turf areas is to encourage wanted grass competition and reduce compaction. It is important that your lawn has good watering practices, and proper mowing and fertilization to maintain an actively growing, dense turf. It is also important that you reduce the compaction of the soil to encourage desirable turf growth. The best time to manage Giant Ragweed is in the late spring to early summer when the plant is actively growing, giving the turf a better chance of recovering areas that previously have had Giant Ragweed infestation. For fields and gardens, tillage can be an effective control method for seedlings due to the fact that they have early emergence. As the weeds get bigger, the harder it becomes to control Giant Ragweed with tillage.

Chemical Control

Depending on your location, Giant Ragweed can be resistant to glyphosate (WSSA group 9) or chlorimuron, cloransulam, imazethapyr and other WSSA group 2 herbicides. That being said, there are a few herbicides like Cobra herbicides, Fierce herbicide, and Gangster herbicides that have been shown to control Giant Ragweed in agricultural crops. As for turf and ornamental areas, BroadStar herbicide, Playload herbicide and SureGuard herbicide work well for those areas. The best time to achieve herbicide control over Giant Ragweed is in the spring.

Latin / Alternative Giant Ragweed names:

• - Ambrosia trifida
• - Great ragweed
• - Horseweed
• - Grande herbe ? poux,
• - Kinghead
• - Tall ragweed
• - Ambrosie trifide

Additional Giant Ragweed Resources