Deadheading clematis IMHO only aids in the health of the vine..why let it use it's energy to make seedheads if you don't plan on harvesting them..I see you are in zone 8...our longer growing season allows us to get 2 maybe 3 bloom cycles from our pruning group 3's( I've already pruned back several of my pruning group 3's/viticellas because they have finished blooming)..I hard prune them back to the 3rd leaf axil refertilize and they will grow back up and bloom again..
Your Pruning group 2's will have two bloom cycles..once in spring and again in late summer/fall...I even had clematis Florida "Alba Plena" blooming in January...most of mine can bloom off and on up till October easily...remember to fertilize in late winter/early spring which is about the same time as when the Forsythias are in bloom or for me in zone 8B around Valentine's Day..same time I prune me roses!! and then you fertilize again after the first initial bloom cycles is over and that's it for the year...I also top dress my flowerbeds with a couple inches of composted cowmanure/compost each March..this will give added OM to your beds and will help with needed nutrients...Hope this helps...Jeanne

Sissylougal, it really depends on the clematis you are talking about. Generally speaking, yes deadheading can prolong flower production in clematis. If I deadhead my Petit Faucon, it will flower from April until late August/early September. Other clematis you can deadhead and the plants may or may not rebloom. What I have found to be the best way to keep clematis in bloom is to cut the plants back by 1/2 to 2/3 after the first flush of flowers, water and fertilize the plant well, and it typically reblooms again in the fall.

Also a point to note, just because the flowers fade and fluffy plumes start forming does not mean that the clematis is going to seed. Some clematis are sterile and can never form seeds, for example Purpurea Plena Elegans. Others have eggs that are fertile but their pollen is sterile, such as Betty Corning. To see if the plant is actually going to seed, you have to look past the fluffy plumes and look at where the styles attach to the stem. If you start seeing roundish objects forming there, then the plant has been fertilized and is producing seed. If you see this happening, then definitely cut them off since seed production zaps a lot of strength out of the plant.

Some examples of fertilized seeds on the plant.

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Also, take note that seeds of different clematis are of different sizes.

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I have always deadheaded the flowers I could reach, but this year I guess I'll be doing a lot of cutting back too.

I've been such a wimp about this. I couldn't bear to cut all the leaf budded stems on the Hagley Hybrid, but once the first flush is over, it's down to 3 leaf axils on at least half of the stems & half off on another half of the remaining stems.

I did cut off about a third of the old wood on macropetalas Markham's Pink & Bluebird in order to remove outgrown trellises around May 1st, when I first saw the Forsythia flowers in other peoples yards. Mine is down hill in the field, so it is a few days later in blooming. It is so big I call it a Fivesythia. Thinking of planting a spare Multiblue near it to climb.

Marie in ME

Thanks so much! I've been reading on the forums and online a bit today and have learned a bit that is new info for me. I went to look at one of the seed heads I saw forming and it actually does have a few seeds forming and turning brown. That is one plant that I thought had bit the dust late in the fall, but it came back and was a very nice surprise to me! It's blooms have been stronger and richer than they were last season, even after a freak freeze that we had back in March!

Please continue to through your wisdom my way! I'm starting to become much more encouraged!

Sis