I can't seem to understand how to tell the difference between Coreopsis lanceolata and C. grandiflora. Maybe the answer is "neither" I sure hope these pictures will help with an ID. Thanks so much!

The key in the Flora of the Great Plains differentiates them this way:

3. Leaves best developed on lower half of the stem; naked peduncles prominent = C. lanceolata
3. Leaves well developed along the stem; peduncles less than 1/2 as long as leafy portion of stem = C. grandiflora

The key in Steyermark's Flora of Missouri says this:

4. Leaves confined to 1-3(-5) nodes mostly in the lower 1/2 of the plant = C. lanceolata
4. Leaves at (5) 6-12 nodes usually 2/3 or more of the length of the stems = C. grandiflora

Translated into English, they are both saying that the two species are difficult to tell apart with any certainty, but if there are relatively few leaves and they are confined to the lower part of the plant, it's probably Coreopsis lanceolata. If there are at least a half dozen leaves and they go most of the way up the flowering stem, it's probably Coreopsis grandiflora.

Based on the length of the peduncle (flowering stem) in your third picture, I'm guessing your plant is Coreopsis lanceolata. But, that's just a guess without being able to see the whole plant.

Thank you Kent. That is enormously helpful.

I'm starting to wonder if the original seeds (gift from a family member) were a mix of C. lanceolata and C. grandiflora or were possibly from an interspecific hybrid. I see now that the stem I photographed had a very long peduncle and a more compact foliage section than many of the other plants.

Several of the plants still out in the flower bed had 5 nodes (I didn’t find any that for sure had more than 5.) but much shorter peduncles.

I’ll refrain from labeling these with a species name and just stick with “Coreopsis sp.”

Thank you again so very much!