Which perennial grass weed grows in clumps and can reach heights of up to 3 feet?

Dallasgrass is a perennial grass weed known for its ability to grow in clumps and reach heights of up to 3 feet. This characteristic clumping growth habit allows it to form dense patches, which can outcompete desirable turf species for resources. It thrives in diverse conditions, often on poorly drained soils, and can be problematic in lawns and ornamental settings due to its aggressive growth and tendency to spread through underground rhizomes.

In contrast, Bahia grass is primarily a turf grass rather than a weed and is typically managed for its ecological benefits in landscapes. Wild garlic is a bulbous perennial that does not resemble grass in growth habits and is more distinguished by its foliage and flowering characteristics. Fall armyworms, on the other hand, are not grasses at all but rather caterpillars that can cause damage to turf and ornamental plants but do not exhibit the clumping growth form associated with grass weeds. Thus, Dallasgrass is the accurate choice as it embodies the features outlined in the question.