Thinopyrum is a genus of tall grasses with long-spiked seedheads whose thick, robust spikelets are spaced out along the stem (rachis). The spikelets of other wheat-like grasses (except commercial wheat) are much more delicate. The tall plant is quite distinctive with its thick stems and characteristic long, narrow seedheads.
A person can recognize the distinctive Thinopyrum genus from the roadside because of its tall,
narrow culm and seedheads. However, Arizona has two species in the genus, Thinopyrum intermedium and
Thinopyrum ponticum (tall wheatgrass). These are very similar and the sizes overlap, but T. ponticum
can grow to larger size. At maximum, T. intermedium is a substancial four feet tall, but T. ponticum
grows to 6.5 feet, taller than most people. So, if the plants are over four feet, it is T. ponticum.
Another difference is in the shape of the tops of the glumes. Ideally, in T. ponticum, the glume tops
are cut off squarely and evenly, while those of T. intermedium are oblique or pointy like in the last picture
above. Unfortunately, this is not always true. You might have to take apart a spikelet to check for this.