Astute readers pointed out several morphological adaptations found in drought-tolerant turf weeds: fleshy taproots, reflective leaf surfaces, etc. What we can’t see is what many of these plants do physiologically – and that’s photosynthesize using a biochemical pathway that temperate turfgrasses don’t possess.
This pathway, called C4 photosynthesis, contains some extra preliminary steps not found in plants using traditional (C3) photosynthesis. The downside: it takes more solar energy for the plant to photosynthesize. The upside: these extra steps allow the plant to "fix" carbon (transforming it from gas to solid) faster, especially when it’s sunny, warm, and droughty. Practically speaking, this means that C4 plants do not have to keep their stomata open as long and they conserve water more efficiently than C3 plants.
So in the summer – when it’s hot, sunny and dry – the C4 plants in your lawn are operating under optimal conditions, while the C3 grasses go dormant. The tables turn when the seasons do: cool, moist conditions favor traditional photosynthesis, and the C4 plants are overtaken by the turfgrasses.