Hello. I planted two sunglow dwarf sunflowers in mid May. The first plant has a ton of flowers and buds, but the open sunflowers (opened within the last two weeks) do not have petals. Today, the second plant's bud opened and has petals. What is wrong with my first plant and what can I do to fix it?

They look so startled! I have no idea why it's petalless or what can be done about it, however.

I doubt that anything could be done. The genetic abnormality that might have caused this crazy thing to happen occurred during the year it bloomed.

Weird. Couldn't find anything about that online except for a nearly identical photo and question on Reddit a year ago. No one there had a clue, either. I agree with the above that it's likely genetic. Could be interesting to harvest some seeds and plant them next year to see what emerges!

I had an echinacea grown from seed this year that did the same thing.
Seems like I've seen petalless sunflower seed offered...
Can't find it now on google, but other petalless sunflower posted...

Well I have hundreds of Echinacea here, maybe 8 varieties. In the summer of 2022, I had dozens of what I thought were petal less flowers. Turned out I had a bad earwig infestation!! In 2023 and this year, no such problem.

At my house, it was a single plant with every flower no petals.
The other plants all had petals on all flowers.

At my house, the seedling with no petals was from seed collected from Echinacea paradoxa... While the seedlings from purpurea were normal.

There are small but poorly developed petals that are more easily seen on the upper part of the flower, and the flowers below. The earwig theory is a good one, except I don't see any insect damage on the petals. I think ity is a genetic mutation also.

There are Series: called Sun-Fill™ in purple and green shade bracts, they are for cut flower type, some of them could also have small petals
I think this one sunflower has similar mutation in the making, but with a dwarf growth habit suitable for garden or pot plant. Would be nice to isolate and propagate that

So what we're seeing from the petalless sunflowers and the Sun-Fill blooms are actually sepals?

Since the sunflower is a composite flower, the term in this case I think is "phyllaries" a leaf-like bracts surrounding florets of asteraceae blooms. Together those phyllaries are called "Involucral bracts"

The Sepals (collectively called Calyx) in asteraceae is located at each individual floret and usually modified into scales, bristles, sometimes feathery, or like, awns, or even absent. Next when seed developed it will turn in-to "Pappus" often functioned as dispersal mechanism.
So, in asteraceae we usually do not see sepals much, it is small and hidden in flower, and it protective functions been taken by bracts (phyllary),until seed develop, like in dandelion we notice feathery parachute like pappus which actually was a sepals.
This terms sometimes made us gardener kept in busy thinking, and some of the above I wrote could be wrong, since my brain also love to mixed them up

Tofi is exactly right. Asteraceae is an odd bunch, and often the easily distinguished characteristic between similar species are the make up of the phyllaries.