Sisymbrium irio

S. irio is an annual or winter-annual, herbaceous, stiffly erect, tap-rooted plant. It first came to prominence as an invasive species, and gained its common name ‘London rocket’ when it became abundant after the Great Fire of London in 1666.

The well-known yellow flowered weed is native to southern Europe, North Africa and temperate Asia but has been carried by migrants to North America, Australasia and South Africa. In North America, S. irio is naturalised in south-western States and in Mexico, where it is often abundant, like elsewhere, in abandoned fields and other neglected areas, as well as in pastures, livestock watering sites, and open deserts. In Australia, it is described as a widespread weed of crops, pastures, roadsides, parks, gardens, lawns, footpaths, disturbed sites and waste areas. It is classed as invasive in Hawaii (PIER, 2014).

The plants produce a large number of seeds which can be introduced to countries accidentally as a contaminant in crop seed or deliberately by migrants.

S. irio is native to southern Europe, North Africa and temperate Asia but has been carried by migrants to North America, Australasia and South Africa, where it has naturalised and become a well-known, yellow-flowered weed of waste and neglected areas. Its transport to the far corners of the globe has either been by accidental movement of seeds (sometimes as a contaminant of crop seeds, agricultural produce, etc.) or by deliberate transport of seeds since the plant has been used for herbal medicines and food. 

The risk of introduction to the few countries where S. irio is neither native nor naturalised is quite high, either as seed deliberately taken there by migrants or accidentally as a contaminant in crop seed. Modern phytosanitary precautions ought to prevent such occurrences but regulations are not strongly enforced in all countries. 

In its native environment S. irio can be found in abandoned fields, waste places, roadsides and orchards. According to Wilken and Hannah (1998), it can be found in much the same disturbed habitats where it has become naturalised. In North America, for example, where it is found in western States and Mexico, it occurs in abandoned fields, waste places, vacant lots, off-highway vehicle staging areas, orchards, pastures, livestock watering sites, roadsides and open deserts (Parker 1972, Rollins, 1993; Wilken and Hannah, 1998; DeFalco and Brooks, 1999; Guertin, 2003). It tends to grow best where its seeds can germinate undisturbed by shade or vegetation and where plants can grow with limited competition.

In Australia, where it is widely naturalised in southern, central and eastern Australia, it inhabits pastures, roadsides, parks, gardens, lawns, footpaths, disturbed sites and waste areas and is also a widespread weed of crops (Weeds of Australia, 2013).

Marshall et al. (2000) report that invasion by S. irio competes with or displaces native annual plant species (in North America and possibly elsewhere).

Genetics

2n = 14, 28, 42, 56 (Khooshoo, 1955; Guertin, 2003), although Flora of North America (2013) states 2n=14. Khooshoo (1955) reported from India that: ‘S. irio Linn. is a polytypic winter annual which grows throughout the Panjab plains. It occurs in diploid, triploid, tetraploid, hexaploid and octoploid races. All these races can be considerably modified in response to varying amounts of moisture and sunlight.’

Reproductive Biology

The tap-rooted herbaceous plant reproduces entirely by seed. Wilken and Hannah (1998) suggest that S. irio, like other colonising mustards, is self-compatible and self-pollinated, as suggested by its small flowers. It has been estimated that a large plant can produce 9500 seeds or more (Guertin, 2003). Seeds are dispersed when the fruit splits, dropping the seeds to the ground beneath the parent plant.

Physiology and Phenology

In Australia, the seed germinates from autumn to winter and the plant grows without forming a rosette, branching freely from the base to form a bushy appearance. The flowers appear from the tops of the stems in late winter to early spring. As the fruits mature the stem elongates and new flowers are added to the cluster at the top (Herbiguide, 2013).

In California, Guertin (2003) reports that the seeds of S. irio germinate from October to March (autumn through to spring) and the plants mature from April to May. In contrast in Arizona, plants flower from December to May but in moist cultivated fields they can flower all summer (Parker, 1972; Guertin, 2003): usually plants disappear as temperatures rise.

In Pakistan, this species is known to flower between March and May (Flora of Pakistan, 2013).

Ray et al. (2005) studied the factors affecting the emergence of S. irio, one of the very few studies on its germination. They determined that 100 seeds weighed 8.5 mg ± 0.05 SE. Germination was tested at 5 ^oC, 15 ^oC and 25^oC and found to be best at 15^oC, although some seeds germinated at the other temperatures. When seeds were buried at different depths in the soil, emergence was greatest (almost 50%) for seed buried at 2 mm, followed by depths of 1 mm and 3 mm. The same authors tested the interactions between temperature and soil moisture requirements for germination and found that seeds emerged between temperatures of 5^oC and 30^oC but that maximum emergence was predicted to take place at soil moisture tensions of – 0.01 to 11.2MPa and between 15^oC and 20^oC. The authors suggested that their data could be useful in developing models of S. irio emergence and thus a predictive model of beet curly top virus occurrences; since the sporadic nature of the outbreaks of the virus in New Mexico may be related to the timing of S. irio emergence.

Although relatively little information seems to be available on the germination of S. irio, more is known about the germination of the closely related S. officinale, and this may give a guide to that of S. irio. In S. officinale both light and nitrate are needed for germination, with the nitrate supplied exogenously or present endogenously in the seed (Hilhorst and Karssen, 1988). Bouwmeester and Karssen (1993) found that buried seeds of S. officinale showed seasonal changes in dormancy, with dormancy being reduced in periods of low temperature and induced in periods of high temperature, with temperature apparently the only factor influencing these changes. However, light, nitrate or desiccation all stimulated germination of exhumed seeds. The roles of light and nitrate in the germination and dormancy of S. officinale seeds was also examined by Hillhorst (1990a; 1990b).

Longevity

The plant itself is an annual or winter annual without any means of vegetative reproduction. The seeds seem to be able to survive in low numbers in the seed bank. Gomaa (2012) found vegetative plants growing on reclaimed land in the Eastern Desert of Egypt and also recovered 7.3 seeds m^-2 from the top 5 cm of the soil. DiTomaso et al. (2013), report that the seeds can survive for up to about 10 years in the soil.

Associations

Like most plants inhabiting waste spaces, cultivated land, pastures and roadsides, S. irio is found growing in association with many other species. Such associations vary widely between countries and also between individual habitats.

Environmental Requirements

S. irio seems to be capable of growing under a wide range of conditions, provided that its seeds can germinate in full sun and without the hindrance of overlying vegetation.

Ray et al. (2005) reported that S. irio can be a primary overwintering host of the beet leafhopper (Circulifer tenellus) and of the beet curly top virus in southern New Mexico.

S. irio is capable of sustaining populations of the tarnished plant bug Lygus lineolaris, an increasingly important cotton pest, especially during the late season overwintering period of the pest (Esquivel and Mowery, 2007).

DiTomaso et al. (2013) noted that like other mustards S. irio can harbour (and be affected by) diseases and pests that attack related species in the same Brassicaceae family. 

Medicinal, pharmaceutical

• Traditional/folklore

S. irio is similar to other species of Sisymbrium such as S. officinale and S. orientale, but both species differ from S. irio in having the fruiting pedicels (the stalk below each individual fruit capsule) about the same width as the fruit capsule and a denser pubescence on the stems and leaves (Wilken and Hannah, 1998). S. officinale has short, appressed fruiting pedicels and beaked fruits that are 8-20 mm long, whilst S. orientale has a basal rosette of leaves, pubescent ovaries and young fruits and mature fruits that are 35-100 mm long. 

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Physical/Mechanical Control

Herbiguide (2013) recommends removing isolated plants (presumably before they set seed and thus multiply). DiTomaso et al. (2013) say that hand pulling is a viable control method if the population is small and isolated, but is time-consuming and is not recommended for large or widespread infestations. Cultivation is effective provided it is done before seed production. Apparently solarisation can be effective in areas with hot summers.

Biological Control

DiTomaso et al. (2013) say that no microbial pathogens or insect biocontrol agents are available for the control of S. irio.

Chemical Control

Herbiguide (2013) recommends the use of diflufenican for controlling S. irio, and this can be used selectively in lupins, clovers and some other crops. Glyphosate gives effective control and can to some extent reduce seed production but also kills all other vegetation it touches.

DiTomaso et al. (2013) suggest a range of herbicides for control of S. irio, some of which are selective to grasses whilst others will kill all green vegetation. Herbicides that can be used include 2,4-D, aminocyclopyrachlor + chlorsufuron, aminopyralid + metsulfuron, dicamba, glyphosate, chlorsulfuron, metsulfuron-mehtyl, sulfometuron, imazapic, imazapyr, propoxycarbazone-sodium, rimsulfuron, sulfosulfuron, and hexazinone. As with the use of any weed-killer, it is important to read the package carefully before starting any weed control operation as some require special precautions to be taken.

Control by Utilization

Grazing is an effective control method if it is early enough to prevent seed production. DiTomaso et al. (2013) recommend using sheep rather than cattle because sheep graze closer to the ground. They also point out that meat and milk can be tainted if cattle graze large quantities of mustard-type plants. 

Little published information surrounds seed germination, seed longevity and growth rates of the seedlings or the plants. Further studies on its impacts on crops or associated species are also needed. 

17/11/2013 Original text by:

Ian Popay, consultant, New Zealand, with the support of Landcare Research