spacing looks perfect to me .. it is NOT an issue .. IMHO

your idea of the root system of a peony is not quite right.. you might want to google images of such ...

give the two a small does of balanced fert.. and 5 years.. and they will catch up ...

you are on the verge of loving them to death.. ignore them ...

ken

They don't look too close to me either. You can move the middle Peony, but don't do it until the fall. DON'T cut back the foliage - you will set the plant, not to mention you'll lose the blooms!

I usually give my Peonies a sprinkle of compost each year, but that 's all I do for fertilizer.

Ken, thanks. As always. Here is my question: these peonies are the same age and in the same soil as the above. They are positively thriving and about triple the size of the above three. Is that an indication that these aren't doing so well? Or do I need to just evaluate them all separately?
Also, this "peony expert" recommends three feet in between herbaceous peonies. Mine have more like six inches.
http://peonysenvy.com/planagarden.html#herb

IME if peonies are unhappy, they don't bloom. Yours look pretty healthy to me. I'd wait until they are actually blooming to make any decisions since there may be things that you haven't taken into account that are causing the plants to be different sizes. (If I remember correctly, this home and garden is new to you.) A few possibilities (and I am sure that there are others):
They may have slightly different amounts of water or sun. (Less sun might mean that the buds of the smaller peonies are smaller because they are delayed but won't stay smaller.)
They could be different varieties of peonies with differing plant sizes.
One set might be older than the other.

I think that 3' between plants is between the plant centers, not the edges. It's a guideline for when you plant them, not how far apart they stay.

the plant on the right is at least 2 feet across ... and the plant behind a foot ...

your plants.. ON CENTER.. as mentioned.. are NOT 6 inches apart ...

DO NOTHING!!!!!

plant uniformity.. on long lived perennials... is not consistent .... some years are great.. some years.. not ... frost\/freeze can affect one.. and not the one next to it.. etc .... who knows why ...

you are welcome ....

ken

Here is a link that might be useful: link

I agree. They look great and not too close together. Just be patient with them and give them time.

Peonies are long-lived perennials & experience the same vagaries of weather as the rest of us--they have good years and bad years. My brother planted a row of them in a garden he started here 25+ years ago. All six are thriving but in my experience they have both good and not-so-good years. What they appear to need more than anything else is room to grow, good soil and patience while they mature.

I've planted many more peonies since I moved here and year after year they appear to be thriving. I just enjoy them no matter how they perform.

Your photo tells me you have three peonies planted close together but doesn't tell me they're the same cultivar which begs the question, does the middle plant have the same characteristics as those on either side? I'll grant you my own plants have far more real estate than yours but that isn't necessarily a peony-specific requirement.

While the middle plant doesn't appear as robust as those either side of it, perhaps it's not as well established for whatever reason and needs more time to settle in to its location. I honestly wouldn't worry about it unless it shows signs of disease or insect infestation. Perhaps all it needs is a chance to 'grow up' to reach its full potential.

Thank you so much for the opinions. If memory serves they are all the same cultivar. Yes, this is a new house but I was here to see them bloom last year. Last year they all did well- and thrived about the same. This year the middle one is suffering for some reason.
Even from center, they are about a foot apart, maximum.
I'll let item go this season and see what happens. It will be a little experiment.