Common ragweed

Images above: Left: Common ragweed seedling (Antonio DiTommaso, Cornell University). Right: Common ragweed young plant (Antonio DiTommaso, Cornell University).

Identification

Other common names:  Roman wormwood, hog-weed, bitter-weed, wild tansy, mayweed, hay-fever weed, blackweed, Roman wormweed, annual ragweed, short ragweed, small ragweed

Family:  aster family, Asteraceae

Habit:  Erect, branched, summer annual herb.

Description:  Seedling stems are purple-spotted to purple.  Cotyledons are dark green, paddle shaped, thick, 0.2 -0.4” (0.5-1 cm) long.  Purple flecking possible along edges. Young leaves are green, deeply divided into lobes with slightly pointed to rounded tips; leaves are hairy above and densely hairy below.  First four pairs of true leaves are opposite but later leaves are alternate.  Mature plants are 1-3 ft (30-90 cm) tall, erect, branched, with a shrub-like appearance.  Stems are densely covered with rough, 0.1” (0.25 cm) hairs.  Leaves are 2-4” (5-10 cm) long, with or without hairs on all surfaces.  Leaves fern-like, deeply divided into many lobes or composed of many smaller, strongly divided and lobed leaflets.  Leaf stalks more pronounced on lower portion of plant.  Roots are fibrous and shallow, often close to the soil surface.  Flowers are green, 0.1” (0.25 cm) wide, and arranged in clusters of male and female flowers on each plant.  Male flowers are produced in spikes at branch tips of upper branches, whereas female flowers occur singly or in clusters in lower leaf axils.  Fruits are top-shaped, woody, brown with spots or streaks, 0.14” (0.3 cm) long by 0.1” (0.25) wide.  Ridges on side extend over the top, resulting in a crown-like appearance.  A longer beak rises out from the center of the crown.  Seeds are brown, 0.1” (0.3 cm) long by 0.08” (0.2 cm) wide and disperse in the hard fruits.

Similar species:  Seedlings of giant ragweed (Ambrosia trifida L.), mayweed chamomile (Anthemis cotula L.), and corn chamomile (Anthemis arvensis L.) are similar to common ragweed.  Giant ragweed cotyledons are larger, measuring 0.75-1.75” (1.9-4.4 cm) long by 0.5” (1.3 cm) wide, and young leaves are either unlobed or more commonly divided into three lobes.  The chamomile species first leaves are more finely lobed or divided, have longer stalks and lack dense hairs on the underside.  Western ragweed (Ambrosia psilostachya DC.) is similar to common ragweed, however western ragweed is a perennial with a taproot.  The leaves of western ragweed are lance-shaped, and less finely divided than those of common ragweed.  Mugwort (Artemisia vulgaris L.) has similar foliage to that of common ragweed but has silvery-white hairs on the undersides of the leaves.  Also, mugwort is a perennial with branching rhizomes.

Management

Common ragweed seeds germinate primarily in the spring.  If the seeds are near the soil surface, delaying tillage as long as possible allows more to germinate before planting and decreases density in the crop (Barnes et al. 2017).  Similarly, if the seeds are mixed through the soil profile, a lag between tillage and seedbed preparation allows many to be eliminated before planting (Myers et al. 2005, Sweet et al. 1978).  Thus, rotations that include late planted crops tend to decrease the species provided weed control in the crop is adequate.  Since common ragweed does not set seed until late in the summer, early harvested crops like winter grains or short cycle spring planted vegetables provide an opportunity to prevent seed production and break the life cycle.  Because the seeds are highly persistent in the soil, however, you may need several years of good management before the population is well controlled.

Tine weeders and other in-row cultivators will kill many seedlings before they emerge, but fewer after they emerge because the seedlings grow quickly.  After emergence, aim to bury the fast growing seedlings before they begin to elongate.  In a year when prolonged rains prevented tine weeding of soybeans until four weeks after planting (early second trifoliate growth stage), we controlled a dense stand of common ragweed by aggressive tine weeding followed immediately by inter-row cultivation that threw about 1” (2.5 cm) of soil into the row (Mohler, personal observation).

Straw mulch is less effective for suppressing common ragweed than for other annual weeds, but can still be useful if applied in especially heavy layers or if a paper barrier is used under the straw.  An early-planted winter annual cover crop that establishes a complete leaf canopy by early spring will suppress ragweed establishment, whereas, a late-planted cover crop with an incomplete canopy will permit establishment of a competitive ragweed population in no-till planted crops (Nord et al. 2012).  Also, interseeded red clover in wheat can moderately suppress common ragweed (Mutch et al. 2003).  We have found that mowing the clover at a height of about 5” (12 cm) after common ragweed has flowered but before seeds form effectively prevents seed set because the clover rebounds from mowing more rapidly than the ragweed (Mohler, personal observation).

Because the seeds are so persistent in the soil, prompt clean-up of fields after harvest is recommended.  Rogue large individuals out of vegetable crops. Electrocution with a Weed ZapperTM controlled most plants that were reproductive and growing above a crop canopy, while it reduced viability of the majority of seeds present during treatment (Schreier et al. 2022). Seeds can be captured when harvesting corn or soybeans, but over 50% will have shattered by this time (Schwartz-Lazaro et al. 2021, Simard et al. 2020).  If seeds are produced, leave them on the soil surface until spring to maximize seed predation.  

Ecology

Origin and distribution:  Common ragweed is native to central and eastern United States and southern Canada, but in forested regions it was probably rare and restricted to disturbed, open areas prior to the advent of Native American agriculture (Bassett and Crompton 1975).  The species has been introduced to locations in Europe, Asia and South America.  (Sweet et al. 1978)

Seed weight:  2.4 mg (without the fruit coat), 4.4 mg (with the fruit coat), 1.7-3.7 mg (Buttenschøn et al. 2010), 2.4 mg (Shergill et al. 2020), 4.0 mg (Gaba et al. 2019), 4.4 mg (Stoller and Wax 1973), 4.6 mg (Gardarin et al. 2010).  Seeds harvested in France ranged from 1.2 to 7.7 mg and had similar germination rates regardless of seed weight (Guillemin and Chauvel 2011).

Dormancy and germination:  Common ragweed seeds are dormant when shed in fall (Baskin and Baskin 1980, Bazzaz 1970, Samimy and Khan 1983).  The seeds require several weeks of chilling at 41 °F (5 °C) for germination (Baskin and Baskin 1980, Bazzaz 1970).  Light (Bazzaz 1970) and alternating temperatures in the range of 50-86 °F (10-30 °C) increase germination of seeds after chilling (Baskin and Baskin 1980, Sweet et al. 1978).  Optimum alternating day/night temperatures required for germination were 77/68 to 95/86 °F (25/20 to 35/30 °C) (Strestha et al. 1999).  Seeds that do not germinate under dark, low temperature early spring conditions (as would occur when buried in soil) and are subsequently exposed to increasingly warm late spring soil temperatures become dormant and again require a chilling period for germination (Baskin and Baskin 1980, Bazzaz 1970).  Seeds chilled in the absence of oxygen remain dormant (Brennan et al. 1978), suggesting that seeds buried at soil depths that restrict oxygen would remain dormant despite cold winter temperatures.  A range of concentrations of N-P-K nutrients had no effect on germination (Parrish and Bazzaz 1982).  The presence of established plants can reduce germination and emergence of common ragweed, presumably through impact on the temperature and light environment of seeds (Fenesi et al. 2014).

Seed longevity:  Seeds can persist for up to 40 years in the soil, but most die earlier (Kivilaan and Bandurski 1981, Toole and Brown 1946).  Viability was 85% after 20 years of burial (Buttenschøn et al. 2010).  In shorter experiments covering two to three years, the annual seed mortality rate was 7% (Gardarin et al. 2010) and 12% (Stoller and Wax 1974).

Season of emergence:  Emergence occurs mainly in spring with declining emergence rates in early summer.  If a seedbed is prepared in early spring, most emergence will have occurred by late spring.  It is typically one of the earliest emerging weeds and has a relatively short duration of emergence (Barnes et al. 2017, Doll 2007, Guillemin et al. 2013, Myers et al. 2004, Stoller and Wax 1973, Werle et al. 2014).

Emergence depth:  Seedlings emerge best from seeds at or very near the soil surface.  Nearly all seedlings emerge from the top inch of soil (Sweet et al. 1978), though a few can establish from larger seeds as deep as 3.1” (8 cm), and no seedlings can emerge from 4” (10 cm) (Guillemin and Chauvel 2011, Stoller and Wax 1973).  

Photosynthetic pathway:  C3.

Sensitivity to frost:  Common ragweed is killed by frost (Sweet et al. 1978).

Drought tolerance:  The species is drought tolerant (Buttenschøn et al. 2010, Onen et al. 2017).  Extreme drought limits above and belowground growth, but the ratio of roots to shoots is not affected (Onen et al. 2017).  Photosynthesis declines with high water stress, but remarkably rapid recovery is observed when water stress is relieved (Bazzaz 1974).  Despite tolerance to short term drought conditions, common ragweed generally thrives best in environments with high annual rainfall, and specifically with high October rainfall (Qin et al. 2014) that presumably favors seed development and maturation.

Mycorrhiza:  Common ragweed is considered a strong mycorrhizal host (Vatovec et al. 2005).

Response to fertility:  Common ragweed is highly tolerant of infertile soils but it also responds to fertility (Parrish and Bazzaz 1982).  On infertile sites, plants < 4” (10 cm) tall may mature, whereas on highly fertile sites, plants may reach 6 ft (1.8 m).  The species is known to be a strong accumulator of N, P and K and many micronutrients, particularly zinc (Bassett and Crompton 1975, Parrish and Bazzaz 1982).

Soil physical requirements:  Common ragweed tolerates a wide range of soil textures and drainage conditions (Bassett and Crompton 1975, Onen et al. 2017, Sweet et al. 1978).  It is highly tolerant of extreme soil compaction.  Common ragweed tolerates moderate soil salinity by decreasing allocation to root relative to shoot growth (Onen et al. 2017).    Salt tolerant populations occur on roadsides that receive deicing salt during the winter (DiTommaso 2004).

Response to shade:  Common ragweed tolerates moderate shade (Sweet et al. 1978), but photosynthesis progressively declines and growth is stunted as shade increases (Bazzaz 1974).  Light reduction of 62% only reduced growth rate under cool conditions (63/45 °F = 17/7 °C), but not under warm conditions of 73/55 to 84/66 °F (23/13 to 29/19 °C) (Deen et al. 1998).  This species increases height and partitions more biomass to stems and less to leaves as competition with adjacent plants increases (Barnes et al. 2019).  It is one of the few annual weeds that regularly matures (always at small size) in hay meadows and goldenrod dominated fields.

Sensitivity to disturbance:  Seedlings are fragile and easily broken, and they quickly dry when uprooted.  However, because common ragweed rapidly produces a long taproot, it is more difficult to kill in the seedling stage than are many other small-seeded annuals.  Medium sized to large plants will re-root readily in moist soil.  Common ragweed has greater capacity for regrowth following repeated cuttings than other upright annual broadleaf weeds (Meiss et al. 2008).  Repeated mowing of the plants at 8, 16 or 32” (20, 40 or 80 cm) causes some mortality and reduces size at the end of the season, but does not prevent reproduction (Pattacchini et al. 2011).  One mowing in mid-summer may reduce biomass substantially (Mutch et al. 2003), but plants generally have a high capacity for regrowth by lateral stems which can produce viable seeds (Bassett and Crompton 1975, Buttenschøn et al. 2010).  No-till management of cover crops with a roller-crimper does not kill established common ragweed plants (Nord et al. 2012).

Time from emergence to reproduction:  Common ragweed flowers 2-5 months after emergence (Bassett and Crompton 1975, Doll 2002), generally from August to October in response to decreasing daylength (Bassett and Compton 1975, Buttenschøn et al. 2010, Sweet et al. 1978).  Flowering is initiated when daylength decreases to 14 hours per day or less (Deen et al. 1998).  Genotypes from northerly locations flower earlier than those from southerly locations (Bassett and Crompton 1975, Sweet et al. 1978).

Pollination:  Common ragweed has male and female flowers on the same plant, and can be either self- or cross-pollinated by wind (Buttenschøn et al. 2010, Simard and Benoit 2012).

Reproduction:  Common ragweed seeds mature as the plant senesces (Bassett and Crompton 1975).  Typical sized plants produce 3,000-60,000 seeds (Bassett and Crompton 1975, Sweet et al. 1978), but small, highly stressed individuals may produce as few as one or two seeds.  Plants in Quebec produced up to 18,000 seeds without competition and 3,000 seeds when grown with crops (Simard and Benoit 2012).  In other experiments in eastern Canada, common ragweed plants produced 200-2,000 seeds in corn and soybean, but 30,000 seeds when grown without crops (Simard et al. 2020).  Seed shattering is highly variable between sites and years, but, on average, greater than 50% of seeds shatter at the time of corn and soybean harvest, implying that seed capture at harvest would be insufficient to limit seedbank replenishment with these long-lived seeds (Schwartz-Lazaro et al. 2021, Simard et al. 2020).

Dispersal:  Common ragweed seeds are each tightly encased in a woody fruit that is the dispersal unit.  These have no adaptations for dispersal.  However, because they often reach high densities in soil, they are easily spread from one site to another in soil clinging to boots, tires, tillage equipment etc.  Common ragweed is one of the most prevalent species along field margins which can serve as a source for immigration into fields (Sosnoskie et al. 2007).  Seeds seem to be heat tolerant and may be spread in compost (Buttenschøn et al. 2010).  The seeds survive well in the digestive tracts of cows, sheep, horses etc., and manure is commonly contaminated with common ragweed seeds.  Horses are particularly fond of common ragweed and horse manure is especially likely to be contaminated.  Birds also disperse common ragweed.  Long distant dispersal can take place in contaminated crop seed, birdseed or hay (Chauvel 2006).  Hay transported from North America to feed horses during World War I is speculated to have contributed to the spread of this species in France (Chauvel 2006).  (Buttenschøn et al. 2010)

Common natural enemies:  Two native chrysomelid beetles, Ophraella communa and Zygogramma suturalis, feed on the foliage (Teshler et al. 2001).  Ophraella communa is most adapted for use as a biological control agent in subtropical climates (Zhou et al. 2010).  White rust (Albugo tragopogi) was isolated from local populations in Quebec and severely reduced growth, pollination, and reproduction of 14% of common ragweed plants tested (Hartmann and Watson 1980).

Palatability:  The seeds are edible but so small and difficult to extract from the hard fruit coat that their use as a grain is impractical.  Palatability to sheep was lower than that of many annual weeds despite having similar protein and nutrient levels (Marten and Andersen 1975). 

Notes: The pollen is extremely allergenic, and causes hay fever symptoms in 10% of the U.S.A. population (Simard and Benoit 2012).  Common ragweed plants can produce over a billion pollen grains per plant when growing with crops (Simard and Benoit 2012).  Some people also develop allergic skin reactions on contact with the foliage and pollen.  (Buttenschøn et al. 2010)

References:

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