Spartium junceum (Spanish broom)

Genetics: The chromosome number reported for S. junceum varies from 2n = 48, 2n = 52, 2n = 54 to 2n = 56 (Afzal-Rafii et al., 1986). Reproductive Biology: S. junceum is a fast-growing, perennial shrub that can live for up to 30 years. Its flowers are hermaphroditic, zygomorphic, yellow in colour and borne in terminal clusters or racemes. It is predominantly a xenogamous (outcrossed) species and its flowers are pollinated by insects, particularly bees (López et al., 1999; Zouhar, 2005). S. junceum begins to produce seeds when plants are two to three years old. This species is a prolific seeder with the potential to produce between 7000 and 10,000 seeds per plant in just one season (Zouhar, 2005; Silva et al., 2008). Under natural conditions, germination rates are approximately 70%. Scarification treatments, such as hot water immersion, dry heat, sulfuric acid treatment and water soak, effectively break seed dormancy and increase germination rates to between 78% and 92% (Travlos et al., 2007). This species produces large seed banks and seeds can remain viable in the soil for up to 30 years (DiTomaso and Kyser, 2013; Geerts et al., 2013; USDA-NRCS, 2017). Physiology and Phenology: S. junceum has a range of xerophytic adaptations. The stem is adapted to reduce the effects of overheating through the profiled positioning of the vegetative parts of the plant. The species has small leaves, which it loses before summer to reduce transpiration and to increase its tolerance to drought stress. The grass-shaped stem helps to reduce the total exposed area of the plant, whereas the root is well developed and ramified. The leaves of S. junceum have thick cuticles with a waxy layer and the plant produces oils that reduce evapotranspiration, which also reduces the temperature of the plant's microenvironment (Bezić et al., 2003).In California, shoots of S. junceum begin to grow in late winter and early spring. They elongate quickly and produce leaves with long internodes by March, with the most rapid growth occurring in May. Leaf longevity is four months or less, although stem photosynthesis occurs all year. Flowers are produced in May and pods mature in late June and early July (Nilsen et al., 1993; Zouhar, 2005). In South Africa, it has been recorded flowering from August to November, while in Lebanon it flowers from May to August (Invasive Species South Africa, 2017). In Europe, S. junceum has been recorded in flower from June to September, and in fruit from August to October (PFAF, 2017). AssociationsS. junceum has a symbiotic relationship with soil bacteria that form nodules on its roots and fix atmospheric nitrogen (PFAF, 2017). Environmental RequirementsS. junceum can grow on poor, dry and stony limestone soils. It is well adapted to rocky or sandy soils, clays, loams and sandy loams with a pH in the range of 5.5 to 7.5; it is also adapted to soils with high salt concentrations. This species has morphological adaptations to xerophytic conditions that allow it to endure severe drought. It thrives in full sun and can tolerate urban pollution, salt-laden winds near the coast and temperatures as low as -10°C (Bezić et al., 2003; Zouhar, 2005; Silva et al., 2008; DiTomaso and Kyser, 2013; Geerts et al., 2013).