We evaluated invasive exotic weeds and plant species diversity in relation to depth to groundwater (DTW) and microtopography in areas with DTW from 0·3–4 m in Owens Valley, California. Transects dominated by common plant species of the area were read at 1-cm intervals, and species cover was obtained at different scales: 1-m transect portions (microsites), whole transects (68 m average length), and the whole study area. Species richness and the Shannon–Wiener diversity index were obtained in microsites. DTW and microtopographical variation (soil-surface relative elevations) were jointly measured along the vegetation transects. Found in 34% of the sampled transects, the annual Bassia hyssopifolia was the most common exotic weed of the area. Its cover in the whole study area was only 0·9%, but it was the second most dominant species (19% cover) in microsites. B. hyssopifolia coexisted mainly with herbaceous species, typically in places with shallow groundwater (average <1·5 m) and microtopographical depressions. However, B. hyssopifolia cover was not affected by variations in DTW or relative elevation. In contrast, B. hyssopifolia cover was negatively associated with perennial cover and species richness. Species diversity (Shannon–Wiener) slightly decreased with the increase in DTW but not with changes in microtopography. In contrast, species richness clearly varied with the identity of the dominant species in microsites. Our study suggests that the variation in community-intrinsic factors such as competition and diversity may play a greater role in the growth of invasive plants than the variation in physical factors such as groundwater and microtopography. Copyright © 2011 John Wiley & Sons, Ltd.