The spots of Spotted Spurge (Euphorbia maculata)
About Fall time is when the "spots" in Spotted Spurge (E. maculata) start to disappear. Since "spotted" is actually in the name, there are typically a lot of misidentifications during this time of year. Euphorbia maculata isn't the only species that loses its "spot" at this time of year. Any species that has produced one will lose it during the fall as plants start to shut down. This also happens when plants are in too much shade and become etiolated.
This is also a good time to discuss the reliability of the spot in general. Ultimately, it cannot be relied upon for identification of a species. The presence or absence of the spot is completely variable and I have seen countless E. maculata plants without spots for seemingly no environmental reason whatsoever. However, the presence of a spot can be useful for excluding other species. Most species in the United States do not appear to have the ability to produce the spot (e.g., E. prostrata).
For more information on how to identify the species (including how it differs from E. prostrata) read here and possibly compare with some of the other species in The Weedy Species of Sandmats (Euphorbia sect. Anisophyllum) in Texas. I have not put together a guide for the common weedy species for the entire US, but the Texas guide covers all the major weedy species found throughout the US except in Arizona, New Mexico, Florida, and possibly California.
