GOLDENROD AND RAGWEED
Beth R. Jarvis
Goldenrod pollen is often blamed for causing hay fever; the true culprit is ragweed. Both plants bloom from late summer to early fall, but goldenrod, true to its name, produces masses of bright golden flowers on single-stemmed plants. Ragweeds have small, unremarkable green flowers that unleash copious amounts of pollen freely into the winds. Goldenrods produce far less pollen because they are both wind and insect-pollinated.
Hay fever sufferers often find relief during hay fever season in northeastern Minnesota as goldenrods but not ragweeds grow there.
Ragweed, Ambrosia species
Ambrosia, the name of the ragweed genus is defined as "food of the gods," yet ragweed is not used for food.
Ragweeds have two kinds of flowers. Pollen-producing male flowers form at the stem tips; female flowers form at the leaf bases and in the forks of upper stems. Leaves are generally alternately positioned on each stem, although they may be opposite closer to the ground. Ragweeds normally grow one to five feet tall and are found in waste places, such as roadsides. There are about 15 species of ragweed but three are most common in our state.
Common ragweed, (Ambrosia artemisiifolia) is a native annual that reproduces from seeds. This shallow-rooted plant grows one to four feet tall with hairy stems that hold deeply lobed, smooth leaves. Its foliage resembles artemisia, hence its species name. (Fig. 1)
Western ragweed (Ambrosia psilostachya), also called perennial ragweed, reproduces by means of creeping roots and seeds. Its tall, hairy stems form dense, bushy patches. (Fig. 2)
Giant ragweed (Ambrosia trifida) is commonly found in southern Minnesota. In ideal sites, this annual produces coarse, rough stems that can reach 15 feet in height. Its large, slightly hairy leaves grow almost a foot long. Each leaf has three or sometimes five, pointed lobes. Leaves are arranged opposite each other on the stem. (Fig. 3)
While giant ragweed grows well in waste sites, it does even better in cultivated sites. It's a particular pest of corn, soybeans and other field crops.
In gardens, ragweed can be contained by hoeing or hand-weeding young plants. In non-garden areas, a broadleaf herbicide mixture of 2,4-D (2,4-Dichlorophenoxyacetic acid) and MCPP (2,(2-Methyl-4-chlorophenoxy)propionic acid) may be used after the plants are actively growing in spring.
These herbicides are most effective when temperatures are between 60° and 80° F. Do not spray if temperatures are projected to exceed 85° within 48 hours. Choose a time when no rain is forecast for at least 24 and preferably 48 hours. To avoid herbicide drift, spray only when the air is still. Drift can harm or kill desirable broadleaf plants such as flowers, vegetables, trees and shrubs.
If you'd rather not spray, you can wipe herbicide onto individual plants. Tie a sponge to a stick then dip it into diluted herbicide or use a small paint brush to treat individual weeds. This method works particularly well with a non- selective herbicide containing glyphosate that will kill most green, growing plants.
Goldenrod, Solidago species
Over 60 species of goldenrod are found in northeast and north central North America. Forty-five species are found in Minnesota alone. Identification of a particular species is complicated because many species cross breed with each other.
The genus name Solidago is based on the Greek word "solidus," to make whole, referring to the plant's medicinal properties.
Depending on species, goldenrod may have either netted or parallel veined leaves. All are alternately positioned on the stems. There are five different flower cluster types. (Fig. 4)
Gray goldenrod (Solidago nemoralis), found throughout Minnesota, is the most common. This perennial reproduces both from seed and persistent roots. Its grayish stems grow six to 30 inches tall with tongue- shaped lower leaves and smaller, oblong upper leaves which are grayish and hairy. Flower heads are small, cylindrical and form slender, curving, one-sided clusters. Each cluster is tow to eight inches long. It is found in woods, especially on dry sites. (Fig. 5)
Rigid goldenrod (Solidago rigida) is a native perennial that reproduces from seeds and persistent roots. Its coarse, hairy stems often grow five feet tall. Its grayish leaves are stiff, thick, and hairy. Lower leaves have longer petioles than upper leaves which may be attached directly to the stem. It is found in dry, gravelly, open places of woods or prairies. (Fig. 6)
Canada goldenrod (Solidago canadensis) reproduces from seed and creeping rhizomes. The yellowish-green flower clusters are borne at the ends of the stems. This native goldenrod grows best in open sites under dry or moist conditions.
Native Americans reportedly used the roots of Canada goldenrod to heal burns. They treated fevers and snake bites with tea made form crushed flowers and chewed the roots to relieve a sore throat.
Showy goldenrods are commonly planted in European flower gardens. Here, they are rarely cultivated. Generally, they are not a weed in home landscapes as they are typically found along roadsides and in other uncultivated sites.