Generally where are you, zone or nearest large city? When does it get sun?

I don't grow this particular clematis, but some of my other white non-clem flowers (especially doubles which don't seem to lose petals as quickly) turn brown when it's hot before they get around to dropping petals.

This is a relatively common phenomena with white flowered plants that are not self-cleaning - that is, drop their petals readily once the bloom cycle is over. And actually, it is not restricted to white flowers.......it just shows up more noticeably :-)

As babs notes, doubled or multi-petaled flowers seem to be more inclined to this as they tend to hold their petals more. It certainly won't hurt to deadhead the spent flowers if the coloring is not to your liking. And since the Duchess is a rebloomer, removing or pruning back a portion of the growth that supports the current flowering will encourage a better late season bloom. The later flowering will likely not produce many doubles, however.

Thank you for the replies.:)

I'm in Iowa, zone 5. The vine gets morning sun, midday shade, and then dappled afternoon shade, due to an oddly protruding tree branch. While it has been unusually hot the past couple of days, the flowers started to discolor while temps were still mild. I will go ahead and deadhead and hope for a rebloom. I was surprised not to see it mentioned anywhere that this can have a tendency towards this discoloration.

I have been trying to reduce maintenance for my front yard, mostly by eliminating plants that spread too quickly. But I can see it also has me turning a more critical eye to other plants. I may end up transplanting this Clem to the back---an area where I'd love some more clematis anyways. :)

I purchased some no Id Clem's and am enjoying seeing their new blossoms.

Great at time to be out in the garden!