Chamomiles

Images above: Upper left: Corn chamomile seedling (Joseph DiTomaso, University of California, Davis). Upper right: Corn chamomile foliage (Joseph DiTomaso, University of California, Davis). Bottom: Corn chamomile flower (Joseph DiTomaso, University of California, Davis).

Images above: Upper left: Mayweed chamomile seedling (Antonio DiTommaso, Cornell University). Upper right: Mayweed chamomile rosette (Scott Morris, Cornell University). Bottom: Mayweed chamomile clump of plants (Scott Morris, Cornell University).

Identification

Other common names:

• Corn chamomile:  field chamomile. 
• Mayweed chamomile:  mayweed, stinking chamomile, dog-fennel, stink-weed, dogs chamomile, dill-weed, stinking daisy, hogs fennel, fetid chamomile, stinking mayweed

Family:  Aster, Asteraceae

Habit:  Upright, branching winter or summer annuals with finely divided leaves.

Description:  Seedlings have thick, hairless, stalkless, oval shaped cotyledons.  The first two leaves are opposite, somewhat thick, and divided.  Subsequent leaves are thinner, lightly hairy, finely divided into leaflets with smaller lobes, and form a basal rosette.  Green leaves have a rounded pinnatifid shape; leaflets and lobes are oval or egg shaped. 

• Corn chamomile:  Cotyledons are light green and 0.1-0.2” (0.25-0.51 cm) long, in all other ways the seedling resembles mayweed chamomile.
• Mayweed chamomile:  Short-lived cotyledons are 0.3” (0.76 cm) long with obscure veins.  The base of the cotyledons is fused to form a fleshy cup.  The seedling stem is green to dull maroon. The first pair of true leaves is opposite with short surface hairs; all subsequent leaves are alternate.

Mature plant stems are upright or leaning; they branch just above the rosette, and take root at nodes when in contact with soil.  Leaves are club to egg shaped in outline, but are divided two to three times into finger-like leaflets and lobes.  Fibrous roots extend from short, broad taproots.

• Corn chamomile:  Stems are lightly hairy, green, red in strong light, and 8-20” (20.3-50.8 cm) tall.  Leaves are alternate, 0.8-2.4” (2.0-6.1 cm) long by 0.4-0.8” (1.0-2.0 cm) wide, and leaflet lobes are 0.03” (0.08 cm) wide.   Leaflets are less finely divided than mayweed chamomile.   Leaves are gray-green and covered in soft hairs.
• Mayweed chamomile:  Stems are nearly hairless, green, 4-24” (10.2-61.0 cm) tall, and highly branched.  Leaves are alternate, yellow-green, slightly hairy, 0.5-2.5” (1.27-6.35 cm) long by 0.25-1.25” (0.64-3.18 cm) wide, and very finely divided, with narrow leaflets and lobes.  Crushed leaves have a strong, unpleasant odor.

Flowerheads are daisy-like, and comprised of 10 to 16 small ray flowers with 0.25-0.5” (0.64-1.27 cm) long white petals surrounding a central disk of numerous tiny, yellow flowers.  Individual flowerheads are located on branch ends; 1 to 2 rows of small, tapered, hairy green bracts wrap the base of the flowerhead.    

Seeds are covered with a hard, tight shell of fruit tissue.   These seed units are light brown, narrow, 4-sided wedges, with 10 longitudinal ribs. 

• Corn chamomile:  Seeds are smooth, 0.06-0.1” (0.15-0.25 cm) long, and produced from both flower types.  Seeds from ray flowers are larger than seeds from disk-flowers.
• Mayweed chamomile:  Seeds are bumpy, 0.06” (0.15 cm) long, and produced only from yellow disk flowers.

Similar species: The best way to distinguish the chamomiles from closely related species is by scent, since the entire plant will exude a distinctive odor at all growth stages, when crushed.  Corn chamomile is lightly chamomile scented, while mayweed chamomile has a strong foul odor.  Scentless chamomile [Tripleurospermum perforatum (Mérat) M. Lainz, formerly Matricaria perforata Mérat] and yellow chamomile (Anthemis tinctoria L.) both lack noticeable odors.  Scentless chamomile has white flowers, and yellow chamomile has yellow flowers.  Pineapple-weed [Matricaria discoidea DC., formerly M. matricarioides (Less.) Porter] has a petal-less, cone shaped, yellow green flowerhead and, its foliage smells like a pineapple.

Management

Following sound general weed management principles should keep these species under control.  In particular, relieve soil compaction, and if mayweed chamomile is a problem, consider steps to improve soil drainage as well.  This will tip the competitive balance in favor of the crops and also improve the success of cultivation.  Timely rotary hoeing and tine weeding are important in both row and grain crops.  Cultivate row crops shallowly close to the row, and if the crop will tolerate hilling, cover the chamomile seedlings while they are still small.  Fertilize the crop for maximum vigor and competitiveness, but avoid over fertilization that will increase other weed species.  If the crop rotation allows, use a buckwheat cover crop to suppress corn chamomile emergence and growth before planting a fall crop (Kumar et al. 2008). 

In small grains, high density plantings and closely spaced rows will help suppress these weeds.  If necessary, consider cross seeding the grain to achieve a more uniform spacing.  Because these weed species are relatively short, long strawed grain varieties help reduce seed production.  If you have a severe, persistent problem, set the combine to cut high.  Then after harvest, chop the straw and chamomile into a wagon with a forage chopper.  Till the field before the chamomile flowers again, and dispose of the chopped material or compost it at high temperature.  Mowing will reduce seed production in hay and pasture crops, but plants can flower again below the cutting height, so a second mowing should take place just before secondary flowering begins.

Ecology

Origin and distribution:  Both species are native to Europe.  They occur throughout Europe, North Africa and the Middle East, and have been introduced into Australia, New Zealand, south Asia and southern South America (Kay 1971a,b).  Mayweed chamomile occurs throughout southern Canada and the U.S.A. including Alaska and Hawaii.  Corn chamomile occurs throughout the eastern and far western U.S.A and southern Canada, and sporadically in the mountain and Great Plains states south to Colorado and Nebraska (USDA Plants).

Seed weight:  Corn chamomile: 0.5 mg (disk flowers), 1.2 mg (ray flowers); mayweed chamomile, 0.4 mg (disk flowers), the ray flowers are sterile (EFBI).

Dormancy and germination:  The "seeds" of these species and other members of the sunflower family includes a shell (pericarp) which surrounds the seed proper.  This structure corresponds to the fleshy part of fruits in other families.  In most newly produced corn chamomile and mayweed chamomile seed, this shell is hard and tight around the seed and physically prevents growth of the embryo, even though it allows water uptake (Ellis and Ilnicki 1968, Gealy et al. 1985).  After several months to years, microorganisms in the soil break down this shell and allow seeds to germinate (Kay 1971a,b).  Smaller seeds germinate more readily than larger seeds because the pericarp of larger seeds completely encase the seeds whereas that of smaller seeds do not (Ellis and Ilnicki 1968, Ilnicki and Johnson 1959).  Light (Rashid et al. 2007) and strong diurnal alternation of temperatures from 68-86 °F (20-30 °C) day to 32 °F (0 °C) night promoted maximum germination of mayweed chamomile (Kay 1971a).  Under continuous temperature, mayweed chamomile germinated best at 68 °F (20 °C) in one experiment (Gealy et al. 1985) and 86 °F (30 °C) in another experiment (Squifer 1986), but germination was never as high as germination with alternating temperature or when the hard shell was removed.  Ammonium and nitrate stimulates germination of corn chamomile (Kumar et al. 2008) and mayweed chamomile (Rashid et al. 2007, Squifer 1986).  Optimum pH for mayweed chamomile germination is 3 to 6 and declines at increasing pH (Gealy et al. 1985).

Seed longevity:  In a seed burial experiment 89% of corn chamomile seeds survived 1 year and 47% survived 11 years [Salzmann (1954) in Kay 1971b].  In the same study, 63% of mayweed chamomile seeds survived 1 year and 6% survived 11 years [Salzmann (1954) in Kay 1971a].  In soil tilled three times per year, the number of mayweed chamomile seeds declined by 42-51% per year, and only 5% of seeds remained viable after 5 years (Roberts and Neilson 1981).

Season of emergence:  Both species emerge mainly in the fall and spring, with some sporadic emergence throughout the summer (Allie et al. 2005, Kay 1971a,b, Roberts and Neilson 1981).  In mild climates, these species also emerge in winter.

Emergence depth:  A depth of 1.2” (3 cm) was considered too deep for emergence (Kay 1971b), so these species presumably emerge from the upper 1” (2.5 cm) of soil.

Photosynthetic pathway:  C3

Sensitivity to frost:  Both species are frost-hardy and regularly grow as winter annuals in the northern U.S.A. (Kay 1971a,b).

Drought tolerance:  Corn chamomile is resistant to drought and prefers areas with relatively low summer humidity (Kay 1971b).  Mayweed chamomile is moderately resistant to drought, but is most prevalent in low, wet areas with high soil moisture for germination (Gealy et al. 1985, Kay 1971a).  Growth of mayweed chamomile was similar in wet and dry years (Ogg et al. 1993) and at high and moderately low moisture levels in pot experiments (Ogg et al. 1994, Squifer 1986).  Root development of mayweed chamomile is optimum at 50 °F (10 °C), and this trait may permit rapid root development during cool, early season conditions and allow greater access to soil moisture during drier conditions later in the season (Squifer 1986). 

Mycorrhiza:  Corn chamomile has been reported as mycorrhizal (Harley and Harley1987), but other reports indicate that neither this species nor mayweed chamomile is mycorrhizal (Kay 1971a,b, Squifer 1986).

Response to fertility:  Corn and mayweed chamomile can grow and flower at low levels of N and P (Kay 1971a,b).  They greatly increase in size and seed production with N, but competitive ability against cereal grains like barley decreases with increasing N (Kumar et al. 2008, Ogg et al. 1994).   Corn chamomile is moderately responsive to P (Ellis 1964).  Simultaneously low levels of Ca and K promote early flowering and increase the number of flower heads produced (Regan 1967).  Corn chamomile grows well on both calcareous soils of pH 7-8, and on moderately acidic soils of pH 5.6-6.6, provided N is readily available (Kay 1971b).  Mayweed chamomile grows well at soil pH ranging from 4.7 to 6.2 (Gealy et al. 1994).

Soil physical requirements:  Corn chamomile usually occurs on well drained soils.  It occurs on soil textures ranging from medium clay loam to sand (Kay 1971b).  Within fields, it is frequently most common on headlands, which indicates some tolerance for compaction.  Mayweed chamomile occurs on a wide range of soil textures, but is most common on clay and clay loam soils.  Its affinity for heavy soils is sometimes striking, with population density tapering off quickly at the boundary with lighter textured soils (Kay 1971a).  In contrast with corn chamomile, mayweed chamomile does well on soils with restricted drainage.

Response to shade:  In partial shade, mayweed chamomile grows more upright and produces additional leaves to capture light (Allaie et al. 2005).

Sensitivity to disturbance:  Both species grow new shoots quickly if the stems are cut, e.g. during cereal crop harvest.  Both species are moderately resistant to trampling.  Corn chamomile cannot regenerate if it is completely buried, but can regrow if only partially buried.  The plants can regenerate roots from the base of the shoot and, therefore, could re-root following hoeing or cultivation.  (Kay 1971a,b)

Time from emergence to reproduction:  Autumn emerging plants resume growth in early spring and begin flowering in late spring.  Spring emerging plants flower in mid-summer.  Flowering typically continues for two months or longer.  (Allaie et al. 2005, Erneberg 1999, Easterbrook and Tooley 1999, Kay 1971a,b).

Pollination:  Plants of both species are self-incompatible. Flowers are cross pollinated by hover flies and other insects.  (Kay 1971a,b)

Reproduction:  Average sized corn chamomile plants produce anywhere from 600-4200 seeds.  Small plants produce less than 20 seeds, but large plants can produce over 18,000 seeds.  Up to 10% of the seeds, however, lack an embryo and are non-viable (Kay 1971b).  Mayweed chamomile flower heads produce 50-75 seeds each.  Medium sized plants produce from 550-12,000 seeds (Allaie et al. 2005, Kay 1971a, Salisbury 1978).  Exceptionally large plants can produce up to 27,000 seeds (Kay 1971a).  Typically, 10-25% of seeds are non-viable, and more than 50% of seeds produced late in the season may be non-viable.

Dispersal:  Both species spread as contaminants of forage seed.  Plants can become incorporated into grain straw and spread the seeds when the straw is used for bedding or mulch (Mohler, personal observation).  Seeds also travel in soil clinging to farm implements, shoes and the feet of livestock.  Corn chamomile passes through the digestive tract of livestock and is spread with manure and when animals move.  Seeds also pass unharmed through some birds.  Mayweed chamomile may behave similarly.  (Kay 1971a,b)

Common natural enemies:  Corn chamomile is attacked by larvae of the moth Homoeosoma saxicola.  Mayweed chamomile growth and reproduction can be significantly reduced by various herbivores including aphids, spittlebugs, true bugs (Heteroptera), moths, slugs, and snails (Erneberg 1999).  Larvae of the beetle Apion sorbi attack the base of mayweed chamomile flower heads distorting them into a spherical shape.  Gray mold (Botrytis cinerea) can cause extensive damage to mayweed chamomile in wet conditions.

Palatability:  Livestock avoid eating both species. Consumption by dairy cattle taints milk.  (Kay 1971a,b)

References:

• Allaie, R. R., Z. Reshi, and B. A. Wafai.  2005.  Demographic plasticity in relation to growth and resource allocation pattern in Anthemis cotula – an alien invasive species in Kashmir Himalaya, India.  Applied Ecology and Environmental Research 4:63-74.
• Easterbrook, M. A., and J. A. Tooley.  1999.  Assessment of trap plants to regulate numbers of the European tarnished plant bug, Lygus rugulipennis on late-season strawberries.  Entomologia Experimentalis et Applicata 92:119-125.
• EFBI.  Ecological Flora of the British Isles.  http://www.ecoflora.co.uk/
• Ellis, J. F.  1964.  A life cycle study of corn chamomile (Anthemis arvensis).  Ph.D. thesis, Rutgers University, New Brunswick, NJ.
• Ellis, J. F., and R. D. Ilnicki.  1968.  Seed dormancy in corn chamomile.  Weed Science 16:111-113.
• Erneberg, M.  1999.  Effects of herbivory and competition on an introduced plant in decline.  Oecologia 118:203-209.
• Gealy, D. R., F. L. Young and L. A. Morrow.  1985.  Germination of mayweed (Anthemis cotula) achenes and seed.  Weed Science 33:69-73.
• Gealy, D. R., S. A. Squier, and A. G. Ogg, Jr.  1994.  Soil environment and temperature affect germination and seedling growth of mayweed chamomile (Anthemis cotula).  Weed Technology 8:668-672.
• Harley, J. L., and E. L. Harley.  1987.  A check-list of mycorrhiza in the British flora.  New Phytologist 105:1-102.
• Ilnicki, R. D., and M. W. Johnson.  1959.  Temperature, light, and seed size and their effects on germination of dog fennel.  Proceedings of the Northeastern Weed Control Conference 13:440.
• Kay, Q. O. N.  1971a.  Biological flora of the British Isles: Anthemis cotula L.  Journal of Ecology 59:623-636.
• Kay, Q. O. N.  1971b.  Biological Flora of the British Isles: Anthemis arvensis L.  Journal of Ecology 59:637-648.
• Kumar, V., D. C. Brainard, and R. R. Bellinder.  2008.  Suppression of Powell amaranth (Amaranthus powellii), shepherd's-purse (Capsella bursa-pastoris), and corn chamomile (Anthemis arvensis) by buckwheat residues: Role of nitrogen and fungal pathogens.  Weed Science 56:271-280.
• Ogg, A. G., Jr., R. H. Stephens, and D. R. Gealy.  1993.  Growth analysis of mayweed chamomile (Anthemis cotula) interference in peas (Pisum sativum).  Weed Science 41:394-402.
• Ogg, A. G., Jr., R. H. Stephens, and D. R. Gealy.  1994.  Interference between mayweed chamomile (Anthemis cotula) and pea (Pisum sativum) is affected by form of interference and soil moisture regime.  Weed Science 42:579-585.
• Rashid, I., Z. Reshi, R. R. Allaie, and B. A. Wafai.  2007.  Germination ecology of invasive alien Anthemis cotula helps it synchronize its successful recruitment with favourable habitat conditions.  Annals of Applied Biology 150:361-369.
• Regan, J. B.  1967.  Life cycle studies as related to germination and growth of corn chamomile (Anthemis arvensis L.) and its response to selected herbicide treatments.  Ph.D. thesis, Texas A. and M. University.
• Roberts, H. A., and J. E. Neilson.  1981.  Seed survival and periodicity of seedling emergence in twelve weedy species of Compositae.  Annals of Applied Biology 97:325-334.
• Salisbury, E.  1978.  A note on seed production and frequency.  Proceedings of the Royal Society of London. Series B, Biological Sciences 200:485-487.
• Squifer, S. A.  1986.  Photosynthetic productivity and germination responses of mayweed chamomile, Anthemis cotula L.  M.S. Thesis, Washington State University.  96 p.
• USDA Plants.  Natural Resources Conservation Service Plants Database.  http://plants.usda.gov