One thing I have always wondered is if tree peony seedlings could gather up enough energy to produce even more leaves, or if what appears in the spring is everything that will appear for the year. Now that it has been nearly 4 months after the tree peony seedlings started to come up, some of them have started to grow more leaves. I think it depends on how good the soil is, the species, and perhaps if there is enough moisture.
Here are some of the ones putting on more leaves. The new summer leaves appear to be slightly larger than the ones appearing in the spring which means the plants have already gathered some energy.
It sometimes looks like they are sacrificing the old leaf for the new foliage as if it were going onto a new season.
This one is surprisingly putting on its 4th and 5th leaf simultaneously. This is a good sign that shows that this may be a vigorous grower.
Some of them don't show any new summer growth at all, but they could still activate at anytime if the plant decides that it needs more leaves for photosynthesis.
This one has held 3 leaves since spring and also shows no signs of producing any more for now, but the woody stem is already up to 1/4 inch in diameter meaning there is already plenty of starch stored in preparation to come out strong next year.
Here's a 2nd year seedling which was only a 1 leaf seedling last year, but it has 4 leaves, 2 of them which basically equate to 3 1st year seedlings. The leaf surface has increased by around 10X from last year so some of the stronger 1st year seedlings from this year should get much bigger than this 6 inch tall 2nd year seedling.