To answer the question, we must ask: are you watching the flowers undergo a change over time, or are you looking at them at the end of the period of change?
If you are sad as you watch the change taking place, you use the bare infinitive:
1. I'm sad to see the flowers wither away
2. I'm sad to see the flowers fade away
3. I'm sad to see the flowers shrivel up
These are actions.
If, on the other hand, you are talking about seeing the flowers at the end of the process, you say
1a. I'm sad to see the flowers withered away = ("the flowers are withered away")
2a. I'm sad to see the flowers faded away = ("the flowers are faded away")
3a. I'm sad to see the flowers shriveled up = ("the flowers are shriveled up")
Needless to say, these are change-of-state verbs. Sentences 1a, 2a, and 3a are similar to passive statives, in which there is no action but rather a resulting state.
Here are a few more similar utterances:
"” I'm glad to see your mother recovered from the flu
"” I'm happy to see our sales record improved
"” I'm angry to see your overall indebtedness increased
"” I'm happy to see our profits doubled so soon
"” I was surprised to see my watch stopped at 3:33 A.M.