I have some Mare's Tail also and even here it tends to look great when coming up in the Spring and now in July (and we are not that hot) some of it is turning brown. I planted it in a big pot of kitty litter. Maybe it needs lots of fertilizer as I got mine from a really mucked up, stinky ditch near where we go camping. It was kind of shaded in the ditch so maybe it does not like full sun. I just don't know.
Here it is on the left w/Parrots Feather on the right in a separate pot. Mare's Tail always comes back nice in the Spring.
Was out helping moving cows today and pushed them up 1600 ft elevation gain, saw some mares tail in a small pond there. Mine don't look so good after transplanting, hope they do ok, but now I know where there is lots more.