Slag, waste from the steel-making process, contains large amounts of calcium, magnesium, iron and other heavy metals. Because of its composition, low pH and water retention ability, slag is considered an inhospitable environment to plants. Nevertheless, the spontaneously generated plant communities on slag are surprisingly diverse, but the assembly and structure of such communities is poorly studied. Previous studies have identified slow process of succession due to low growth rate and slow accumulation of topsoil. Using two former industrial sites on the South Side of Chicago, IL, I investigated whether slag communities display similar patterns. I removed all vegetation from plots on both slag and non-slag soil to test whether primary succession differed over one growing season (4 months). To directly assess plant growth, selected species were planted on both sites and harvested. I show that primary succession differed on slag and non-slag soils. The recruitment process on slag, measured by percent cover and number of recolonized species, was overall slower; however, the difference was not significant until 6-8 weeks of experiment, suggesting potential stage-dependent effect of slag on plant growth. Functional trait analysis found that graminoid and early successional species preferentially colonized slag plots. Slag plots recovered slower from disturbance, suggesting a slow succession process that would hinder natural recovery. However, slag also has the potential of hosting flora of analogous habitats native to the area, serving as plant refuge. Restoration effort should be informed by the low possibility of natural recovery, and its potential as native plant refuge.