Ragweed is a plant many people know by name because it's a prime source of fall allergies in North America. But not everyone knows exactly what the plant looks like. And it can be difficult to identify because it's fairly nondescript. There are no colorful berries on it, such as those on bittersweet nightshade, and even when it blooms, its flowers can only dream of having the character of dandelion flowers.
There are two main types of ragweed: common ragweed (Ambrosia artemisiifolia) and giant ragweed (Ambrosia trifida). While common ragweed often is easily confused with some other plants, such as mugwort, it has certain characteristics that clearly define it from its close relative, giant ragweed. Notably, common ragweed isn't always short (its size depends on growing conditions), but it can't match giant ragweed in stature. The latter can stand at a staggering 15 feet tall. In addition to this, you can learn to identify these plants by their leaves and blooms.
Common Ragweed Leaves
The leaf of the common ragweed plant is made up of multiple leaflets that have a fern-like appearance. The leaflets are essentially miniature versions of the whole leaf. Such leaves are sometimes referred to as "twice compound" or "double compound." The leaflets are roughly 6 inches long and 4 inches across.
Another identifying feature on common ragweed appears with the plant's initial leaves, those that first come out after the seed germinates. These leaves typically have some purple speckling on them. However, it's important to note that other weeds can display purple markings, so don't jump to any conclusions if you see leaves like this. For example, lamb's quarters (Chenopodium album) is another rather nondescript plant that is often found with some purple in its leaves.
Common Ragweed Blooms
The Ambrosia genus of which ragweed is a part belongs to the aster family. But common ragweed blooms are nothing like the colorful flowers of the New England aster, for example. Ragweed flowers are exceedingly inconspicuous. In fact, the casual observer might not even recognize them as flowers because they appear as small yellowish bumps.
Ragweed is monoecious, meaning a single plant contains both male and female flowers. The image below shows the flower spike of male common ragweed flowers. Using a magnifying glass, you would be able to detect the presence of five stamens on each of these male flowers. Ragweed's female flowers are even easier to overlook than its male flowers, as they are hidden in the upper leaf axils—the angles between the upper side of a leaf or stem and the supporting stem or branch.
The flowers are succeeded by fruits that are technically designated as "achene," a small, dry, single-seed fruit. Each fruit is brownish, and like the flowers, it also is inconspicuous.
Giant Ragweed Leaves
Giant ragweed bears "palmate" leaves. This means the leaf shape resembles that of the palm of a hand. There are two leaf varieties on giant ragweed: One type has five segments (more closely resembling a hand), and the other has three segments. So if you find a very tall weed with leaves like either of the ones pictured below, there's a good chance you've encountered giant ragweed.
The leaves usually have serrated edges, and their green stems are covered in tiny white hairs. Plus, the smaller leaves around the base of the plant often have hairs on their undersides. Overall, the leaves can be up to a foot long and 8 inches across.
Giant Ragweed Blooms
Giant ragweed flowers are quite similar to those of common ragweed. Many of the upper stems of the plant grow a flower spike that is cylindrical in shape and roughly 3 to 6 inches long. The plant also will often have slightly smaller flower spikes closer to its base.
Like common ragweed, the blooms appear like small bumps. Each flower is only about an eighth of an inch long. The flowers start out green but turn to a yellow-green and then a yellow-brown color as they mature.