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Giant ragweed is a large annual plant in the aster family, which has the scientific name Ambrosia trifida. It is generally considered a noxious weed and is very hard to control. In agricultural fields, it substantially reduces yields. Along with common ragweed, or A. artemisiifolia, it is the major source of hayfever in the fall, in most parts of North America. There are a few people that like to cultivate this plant in their gardens, however.
The published values for the height of the giant ragweed plant vary from 3 ft (0.9 m), to 12 ft (3.7 m), and even 17 ft (5.2 m) high. This difference in heights may have to do with whether the plant is growing in fertilized agricultural fields or in its native habitat. Giant ragweed leaves are quite distinctive and vary from the leaves of other ragweed plants. They have three to five lobes, with midribs radiating from one point. The leaves are generally 8 in (20.3 cm) across, and up to 12 in (30.5 cm) long.
The plants have a spike of greenish flowers that are about 3 to 6 in (7.6 to 15.2 cm) long. These grow from some of the upper stems. Some plants are male, while other plants are female. The plants are pollinated by the wind, which is unfortunate for allergy sufferers who are most affected by it. A single male plant is estimated to produce 10 million grains of pollen daily.
Its native habitat encompasses all light conditions, from full sun to some shade. Native areas include woodland borders, thickets, and meadows in woodland areas or near rivers. It is also often found in abandoned fields, vacant lots, and by the sides of roads. In the recent decades, it has started invading agricultural fields. This is especially a problem in the Eastern Corn Belt of the United States.
Several factors make this ragweed a fierce competitor among other weeds and crop plants. It emerges early in the season and grows quickly. Its large leaves allow it to compete for nutrients, light, and water during the growing season. Yields of corn and soybean have been reduced as much as 50% due to this weed.
The production of large seeds with a tough coat helps to spread the plant widely. Giant ragweed seeds can live in the soil for several years. A single plant in an agricultural field can produce thousands of seeds. Tilling the soil just drives them deeper.
One problem for both agricultural growers and home gardeners is that many of the giant ragweed plants have developed resistance to the common herbicide glyphosate. Many crops are genetically modified be resistant to this herbicide, so they can survive treatment with glyphosate to exterminate weeds. This does no good if the giant ragweed is also immune to the herbicide. There are very complicated herbicide mixtures recommended for commercial growers to control this weed.
For home gardeners attempting to control this plant, it is recommended to pull the giant ragweed. One may need to wear gloves, so as not to have an allergic reaction to the plant itself. If a gardener can identify the plant when it is small, mowing may be another way to control it.
Despite all the efforts to control giant ragweed, there are those who find it attractive and enjoy growing it in their gardens. Such gardeners should water the plant regularly, taking care not to over water it. It is listed as being drought tolerant, but other sources say it wilts when there is a substantial lack of water.