Alliaria petiolata

Alliaria petiolata is an obligate biennial herb of the mustard family (Brassicaceae). Seedlings emerge in spring and form basal rosettes by midsummer. Immature plants overwinter as basal rosettes. In the spring of the second year the rosettes (now adult plants) produce flower stalks, set seed, and subsequently die.

Basal leaves are dark-green and kidney-shaped with scalloped edges, 6-10 cm diameter. Stem leaves are alternate, sharply-toothed, triangular or deltoid, and average 3-8 cm long and wide, gradually reducing in size towards the top of the stem. All leaves have pubescent petioles 1-5+ cm long. New leaves produce a distinct garlic odor when crushed. The fragrance fades as leaves age, and is virtually non-existent by fall.

Plants usually produce a single unbranched or few-branched flower stalk, although robust plants have been recorded with up to 12 separate flowering stalks. Flowers are produced in spring (usually May) in terminal racemes, and occasionally in short axillary racemes. Some plants produce additional axillary racemes in mid-summer. Flowers are typical of the mustard family, consisting of four white petals that narrow abruptly at the base, and 6 stamens, two short and four long. Flowers average 6-7mm in diameter, with petals 3-6mm long. Fruits are linear siliques, 2.5-6cm long and 2mm wide, held erect on short (5mm), stout, widely divergent pedicels. Individual plants produce an average of 22 siliques (range 2 to 422; Nuzzo unpublished). Siliques contain an average of 16 seeds (range 3- 28; Nuzzo unpublished), arranged alternately on both sides of a papery sinus. Seeds are black, cylindrical (3mm x 1mm) and transversely ridged, and range in weight from 1.62-2.84mg.

Adult plants range in height from 0.05m to 1.5m, and average 1.0m, at the time of flowering. As plants of all sizes are found in the same cluster, plant height is likely a response to competition rather than genetically determined.

Immature plants can be confused with other rosette forming species, especially violets (Viola sp.), white avens (Geum canadense), and Cardamine sp. Alliaria petiolata can be distinguished from these plants by the strong garlic odor in spring and summer. In fall and winter Alliaria can be distinguished by examining the root system. Alliaria has a slender, white, taproot, with a distinctive "s" curve at the top of the root, just below the root crown (Nuzzo, personal observation). Axillary buds are produced at the root crown and along the upper part of the "s".

Chromosome number of 2n=36 has been recorded for European material, and 2n=24 for North American and European material (Cavers et al. 1979).

Excellent illustrations are contained in Cavers et al. (1979). Descriptive characteristics derived from Cavers et al. (1979) and Gleason and Cronquist (1991) except where otherwise noted. There is one other species in this genus (Gleason and Cronquist 1991).

In its native Europe Alliaria is an edge species, growing in hedges and fencerows (Fitter et al. 1974, Martin 1982) and in open woods (Wilmanns and Bogenrieder 1988). Alliaria is disturbance adapted, and is frequently a component of ruderal communities (Swies and Kucharczyk 1982), including open, highly disturbed forests (Klauck 1986).

In North America Alliaria invades wet to dry-mesic deciduous forest (Cavers et al. 1979, Nuzzo 1992a, 1993a), and also occurs in the partial shade characteristic of oak savanna, forest edges, hedgerows, shaded roadsides, and urban areas, and occasionally in full sun (Nuzzo 1991a). Alliaria is rarely found under coniferous trees in the Midwest, but has been reported from under seven species of coniferous trees in Ontario (Cavers et al. 1979). Alliaria grows on sand, loam, and clay soils, and on both limestone and sandstone substrates, but has been observed only once growing on a drained peat soil, and does not occur on muck soils. Alliaria frequently grows in well-fertilized sites (Cavers et al. 1979), and is described as a nitrophile by Passarge (1976) and Wilmanns and Bogenrieder (1988). In Europe, Alliaria increased in cover with deposition of air-borne industrial emissions, which increased soil nitrogen, nitrate, phosphorous and pH (Wilmanns et al. 1986, Wilmanns and Bogenrieder 1988).

Alliaria is common in river-associated habitat, particularly in the Northeast (Nuzzo 1993a). It may preferentially invade drier forest communities in the Midwest than it does in the northeast (Nuzzo 1993a). This is supported by the higher presence along railroads in the Midwest (Nuzzo 1993a), which are generally indicative of drier habitats. Byers and Quinn (1987) reported that Alliaria, once considered a plant of floodplains and moist woods in New Jersey, had become common in a wider range of habitats. In the Great Plains Alliaria is most frequently recorded from moist, usually riverine, habitat and waste ground (Kansas and Oklahoma), while on the eastern edge of the Rocky Mountains Alliaria has been recorded along hiking trails (Utah), and on hotel grounds and around a beaver pond (Colorado).

Alliaria seeds germinate in early spring, beginning in late February or early March, and concluding by mid May in northern states and Canada (Cavers et al. 1979, Kelley et al. 1991, Roberts and Boddrell 1983). In northern Illinois, germination coincides with emergence of spring beauty (Claytonia virginica) and false mermaid weed (Floerkea proserpinacoides).

Seedling density in heavily infested forests was recorded at 5,080/m2 at the cotyledon stage, and 2,235/m2 at the 2-3 leaf stage, in Illinois (Nuzzo unpublished), and approximated at 20,000/m2 in Ohio (Trimbur 1973). Seedlings undergo high mortality, declining by 41% (Trimbur 1973) to >50% (Cavers et al. 1979) by late spring.

By June seedlings develop the characteristic rosette of first year plants. First year rosettes are sensitive to summer drought (Byers 1988) and approximately 95% die by fall (Nuzzo 1993b). By mid-fall rosettes average 4-10 cm diameter and are dark green to purplish in color (range 1-15 cm). The rosettes continue to grow in winter during snow-free periods when temperatures are above freezing (Cavers et al. 1979).

Natural mortality continues through winter: in northern Illinois rosette density in November averaged 186.4/m2 (range 50-466/m2), and declined significantly to an average of 39.9/m2 (range 4-102/m2) by the following spring (Nuzzo 1993b). Rosette density varies between sites and years; mean densities range from 30/m2 to 80/m2 (Nuzzo 1991a), and reach a high of >450 adult plants/m2 (Nuzzo 1993b). Over-winter mortality is only slightly density-dependent: 9% of the variation between fall and spring densities was due to initial density in fall (Nuzzo 1993b). Total survival rate from seedling to adult stage varies from 1% (Nuzzo 1993b) to 2-4% (Cavers et al. 1979).

Alliaria is an obligate biennial: all plants that survive the winter produce flowers, regardless of size, and subsequently die (Cavers et al. 1979, Byers and Quinn 1988, Bloom et al. 1990). Plants only 5cm tall, with 3-4 leaves, have been observed with flowers and seeds. The majority of plants are taller, averaging 0.7 to 1.0 meters when in flower. Flower stalks begin to elongate in March or April, and flowers open early April through May. This is some 6-10 weeks after new seedlings germinate; in established populations generations overlap, and two cohorts can be seen from March through July. Alliaria flowers can be self-or cross-pollinated (Cavers et al. 1979, Babonjo et al. 1990). Syrphid flies, midges and bees visit flowers and may effect pollination (Cavers et al. 1979). Whether in-bred or out- bred, Alliaria plants maintain substantial genetic variation within populations (Byers 1988).

Plants usually produce 1-2 flowering stems, although a single individual may produce up to 12 separate stems. Damage to the primary flower stem stimulates growth of additional stems (Cavers et al. 1979) from axillary buds at the stem base and along the root crown, although such damage is not a prerequisite for development of multi-stem plants (Nuzzo personal observation). Some plants continue to produce flowers through August in small axillary inflorescences. Large plants produce flowers earlier and for a longer time period, and consequently produce significantly more seeds, than plants with small rosettes (Byers and Quinn 1988).

Seeds develop in a linear silique, with siliques forming on the lower part of the inflorescence while flowers are still opening on the upper part. Seeds ripen and disperse between mid-June and late September (Cavers et al. 1979, Kelley et al. 1991). Alliaria produces an average of 16.4 (+ 3.0) seeds/silique (range 3 to 28), and 21.8 (+ 22.5) siliques/plant (range 2 to 422; Nuzzo unpublished, Cavers et al. 1979). Actual production varies significantly within and between communities, with plants in drier communities tending to produce fewer seeds than plants in mesic and wet communities (Byers 1988). Plants produce an average of 360.5 seeds, ranging from 194.3 in mesic sand forest to 608.2 in mesic floodplain forest (Nuzzo unpublished). Maximum production per plant is estimated at 7,900 seeds on a plant with 12 stems, while minimum production is 14 seeds on a plant with 2 siliques (Nuzzo unpublished). Seed production is density dependent, with plants producing fewer seeds as density increases (Trimbur 1973). However, total seed production increases with increasing density (Trimbur 1973). In Illinois, seed production within dense patches of Alliaria ranged from 3,607/m2 to >22,000/m2 (Nuzzo unpublished), while in Ohio Trimbur (1973) reported 19,060 to 38,025 seeds/m2, and in Ontario Cavers et al. (1979) estimated average production at 19,800 to 107,580 seeds/m2.

Seeds are dormant at maturity and require 50 to 100 days of cold stratification to come out of dormancy (Byers 1988, Lhotska 1975, Baskin and Baskin 1992). Dormancy period lasts eight months in southern locales (Baskin and Baskin 1992, Byers 1988) and 22 months in northern areas (Cavers et al. 1979). Alliaria seeds may break dormancy more rapidly when exposed to low temperatures that fluctuate around freezing (0.5 to 10 C, as occurs in central states such as Kentucky) than under a constant temperature regime well below freezing (as occurs in northern states and Canada). This is likely a physiological rather than genetic response, as Ontario seed germinated at 20% and 50% in 3 months when moist stratified at 5 and 2 degrees C, respectively (Cavers et al. 1979).

Unlike some forest crucifers that fail to germinate under leaf cover, Alliaria seeds germinate in both light and dark after dormancy is broken (Bloom et al. 1990, Byers 1988). Light alone will not stimulate germination during cold stratification (Byers 1988). Germination rates of 12-100% have been reported (Baskin and Baskin 1992, Byers 1988, Cavers et al. 1979), but vary greatly within and between populations and habitats (Byers 1988, Cavers et al. 1979). Interestingly, substrate affects germination rate: Baskin and Baskin (1992) reported lower germination on sand substrates than on soil, as seeds on sand failed to afterripen (possibly due to water relations at the seed:soil interface). The majority of seeds germinate as soon as dormancy is broken (Roberts and Boddrell 1983, Baskin and Baskin 1992). A small percentage of seed remains viable in the seed bank for up to four years (Roberts and Boddrell 1983, Baskin and Baskin 1992).

Byers (1988) determined that seeds were concentrated in the upper 5cm of soil, and that three of four populations maintained a seed bank after germination. The fourth population, located in a floodplain, lacked a seedbank due to flooding and scouring of the surface, but was expected to gain new seeds during flood deposition.

Alliaria spreads exclusively by seed (Cavers et al. 1979). Seeds typically fall within a few meters radius of the plant. Wind dispersal is limited, and seeds purportedly do not float well, although seeds readily attach to moist surfaces (Cavers et al. 1979). Anthropogenic distribution is the primary dispersal mechanism (Lhotska 1975, Nuzzo 1992b, 1993a). Seeds are transported by natural area visitors on boots and in pant cuffs, pockets and hair, and by roadside mowing, automobiles and trains (Nuzzo 1992b). Seeds are widely dispersed in floodwaters. Seeds may be dispersed by rodents or birds; isolated plants are frequently found at the bases of large trees in forest interiors. Seeds may possibly be distributed directly or indirectly by white- tailed deer (Odocoileus virginianus).

In southern locales Alliaria populations are even-aged, alternating annually between immature plants and adult plants (Baskin and Baskin 1992), probably due to the 8 month seed dormancy. In northern climates Alliaria populations can be even-aged in early stages of invasion, and then become multi-aged as the seed bank builds up. Dormant seeds may have different longevity rates in northern and southern locales, but no work has been conducted to test this.

Alliaria is frequently overlooked at low density levels. In many sites Alliaria can be present for a number of years before appearing to "explode" in favorable years. Once Alliaria reaches this level of infestation control is difficult to achieve. At any given site Alliaria frequency and cover fluctuate annually, reflecting the biennial nature of the plant. These annual fluctuations are deceptive, as Alliaria consistently occurs with increasing frequency through time, on average doubling in four years (Nuzzo 1992a). The greatest increases in presence occur in sites subjected to large-scale natural disturbances. One site, flooded in mid- summer, experienced a 241% increase in frequency two years later (Nuzzo 1992a). In a site hit by a severe windstorm that blew down overstory trees, Alliaria frequency increased 1000% during the same time period (Nuzzo 1992a).

Alliaria is rarely if ever browsed by deer or other large herbivores. Alliaria is only rarely observed with obvious signs of insect herbivory in the U.S., although it is a preferred host plant for Pieridae butterflies in Europe (Forsberg and Wiklund 1989, Courtney and Duggan 1983, Remorov 1987, Kuijken 1987), and is utilized by a European curculionid weevil (Ceutorhynchus constrictus; Nielsen et al. 1989). In the Netherlands Alliaria is targeted by the orange- tip butterfly Anthocharis cardamines (Pieridae) when the preferred host species Arabis glabra is unavailable (Kuijken 1987). In eastern Europe Alliaria is utilized by butterflies that feed on commercial crucifers (Remorov 1987) and thus may be a threat to commercial production of cabbage. However, macerates of Alliaria leaves sprayed on cauliflower deterred oviposition by the garden pebble moth (Jones and Finch 1987). Alliaria is a preferred host of the monophagous weevil Ceutorhynchus constrictus (Nielsen er al. 1989). The general lack of insect utilization in North America may be due to Pieridae preference for open habitat (Alliaria usually occurs in forested habitat), general lack of natural enemies in the US, and Alliaria's cool season growth habit: Plants undergo rapid growth at low temperatures in late fall or early spring when insects are not very active (Cavers personal communication 1989). Cavers observed plants in Britain and Europe with greater obvious insect damage to leaves than in Ontario, suggesting that natural enemies may have some impact on the plant (Cavers personal communication 1989). Pieridae butterflies are common in North America, and Alliaria may be used by Pieridae species when the preferred host plant is unavailable.

In Ontario an unidentified virus (or several viruses) has been observed to kill flowering plants and prevent them from ripening viable seeds (Cavers personal communication 1989). Alliaria is frequently infected with a strain of turnip mosaic virus (TuMV-Al) in both Ontario and Europe, with infected plants developing a mosaic leaf pattern (Stobbs and Van Schagen 1987). The virus does not affect total seed production or seed germination, but does reduce diameter of individual seeds and average silique length (Stobbs and Van Schagen 1987). Although closely related to TuMV-Br, a virus that infects crops Brassicaceae, the two viruses are mutually exclusive: the Alliaria virus is not transmissible to commercial Brassicaceae species, specifically rutabagas and canola, nor does TuMV-Br infect Alliaria (Stobbs and Van Schagen 1987). In Europe Alliaria is a host plant for a number of viruses, including cucumber mosaic virus (CMV) and turnip mosaic virus (TuMV), that infect commercially propagated crucifers (Polak 1985). Alliaria is host for an isolate of turnip yellow mosaic virus (TYMV-A) that induces systemic infection in broccoli, turnip, and other crucifers grown in Europe (Pelikanova et al. 1990). This was the first finding of TYMV-A virus in wild growing vegetation in the former Czechoslovakia (Pelikanova 1990).

Alliaria was historically eaten as a potherb, particularly in winter and early spring when few greens were unavailable (Georgia 1920). There is no direct evidence that Alliaria was specifically imported for garden or medicinal use, although Fernald et al. (1958) state that this "old fashioned garden plant...has spread somewhat to roadsides and borders of groves", and cite earlier authors who describe the use of Alliaria as a salad plant. Zennie and Ogzewalla (1977) promote eating Alliaria for it's high Vitamin A content (8,600 units/100g in young leaves, 19,000 in basal leaves) and Vitamin C content (190mg/100g in young leaves), both substantially higher than levels in commercially grown fruits and vegetables.