buddleia

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Is it normal for mature leaves on a buddleia to turn yellow and/or brown this time of year?  The new growth is nice and green and there is lots of it, but some of the other plants I have seen in my area (Davis, CA where it is 104 degrees F today) are not showing the same extent of yellowing that mine is.  Could it be that it is getting too much water (soaker hose at the base goes on 40 min every other day?) Perhaps not enough water, but it doesnt' seem likely given these plants are supposed to be quite drought tolerant?

Thanks in advance for any suggestions.