Native to the midwestern and eastern U.S., several species of annual ragweed (Ambrosia spp.) Have found their way west of the Rockies, including varieties that have developed resistance to common herbicides. Common ragweed (Ambrosia artemisiifolia) and associated species, banes of allergy sufferers, have different developmental stages. Many ragweed species germinate over prolonged, extended intervals, which makes them difficult control because neighboring plants can be at different phases of growth. The two primary phases of ragweed development, vegetative and reproductive, can be split further.
Ragweed seeds germinate anywhere as soon as the soil warms and moisture is present. In warm climates, that can be as early as February or March. The seedlings first appear as a set of oval, spoon-shaped cotelydons around 1/2 inch long. Clusters of seedlings often cover the soil surface in which seeds fell. Based on the species, one ragweed plant can produce 3,000 to 5,000 or more seeds. Ragweed seeds that were covered with disturbed dirt can continue to germinate throughout the growing season if moisture temperatures and conditions are appropriate. Annual ragweed plants have a long taproot and many fibrous roots, which makes them efficient at taking in water , even in sandy dirt.
First True Leaves
The ragweed’s first true leaves grow about five to seven days after germination when the seedlings are about 2-3 inches tall. These leaves are oval-shaped, simple leaves around 1 inch wide, arranged opposite each other on the stem. A second set of oval leaves rises above the first, also opposite each other on the stem but tilted in the initial pair.
Following the second set of oval accurate leaves, the very first set of lobed leaves seems. Ragweed leaves are almost always simple, and typically in pairs arranged on opposite sides of the stem. Average lobed leaves have stiff, short hairs covering their surface. Ragweed plants tend to develop over neighboring plants, competing for light. Giant ragweed plants (Ambrosia trifida) growing as annuals in the exact same seed head may reach only 3 or 4 feet tall when competing with low-growing plants, but they can bend over taller plants at 7 to 8 ft in their hunt for sunlight.
Ragweed plants are monoecious, meaning one plant has both male and female flowers. They could be in bloom for several months, from ancient adulthood until the end of the growing season. The male flowers are about the graceful spikes, known as racemes, that develop at the peak of the plant. The female flowers form in clusters on small stems just under the racemes. The male flowers on one plant can release as many as 10 million pollen grains. This enormous amount of pollen ensures greater genetic diversity — one reason for the fast spread of herbicide resistance through cross-pollination. Following the flowers are pollinated, ragweed seeds grow inside a pithy, protective husk. The seeds are distributed by hanging to sneakers, clothing, animals, and farm or gardening equipment.
Western ragweed (Ambrosia psilostachya), also a native perennial, rises in U.S. Department of Agriculture plant hardiness zones 6 through 10 and produces seeds and also spreads by rhizomes. It tolerates dry, sandy dirt and hot states, sometimes shedding its leaves during periods of stress, then recovering when conditions improve to rebloom and produce more seeds. Desert ragweed (Ambrosia dumosa) is another perennial species that grows in USDA zones 7 through 11. It is a little shrub about 2 feet tall.