Pictures of the yellowing of the plant & leaves may provide the specifics we need.

You may never know so I would suggest it is best to plant a new 1. Late fall usually november is the best time to plant natives like fremontodendron. It is just as likely that the fremontodendron died from watering as from roundup.

our flannel bushes (also planted in the last year) go through periods where their leaves turn yellow and they don't seem to be doing great, then they come back.

give it a couple of weeks.

also, i've read that they really don't want any summer water (which is confusing when you're in the first year trying to get them established!).

I killed three Fremontodendrons before I heeded the good advice of Barbara Coe, now horticulture director of Luther Burbank Gardens. Plant it in the fall as bahia suggests and DO NOT WATER in the following spring. It is difficult to not water when you see the plant wilt and look like it is dying, but if you do it will die as mine did three times. The whole plant will look really bad for about 10 days, when you notice it is coming out of it. Once you have passed this point you are home free, as long as you do not ever water it again. Al