If the distressed agave is at the end of the drip line, maybe you could reduce the volume to that line just a little bit? ...or couldn't you just pinch it off or something before it gets to that agave?
Also, is there any chance that the roadway I see next to it has been graded to drain to where the agave is -- maybe when there is enough water to overflow the concrete verge -- its a low curb -- your agave gets more water that way too? Maybe water sits there underneath the gravel....?
Because of the road, I doubt that you could do much in the way of changing the slope/grade -- my original quick fix idea was to dig a gravel filled ditch/moat/swale on the downslope side of the agave so excess water would drain away from it.
Can the agave be raised so it's planted on a mound? ( kind of a big project especially with all the rock/gravel mulch there that would have to be moved aside first -- would your landscaper accept that this is necessary to correct the situation? )
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