Back when I was working in retail gardening, I confess to telling a little white lie: if I thought someone was likely to douse their peonies with heavy doses of insecticide to rid them of the ants that always appear before (and perhaps just shortly after bloom time as the above photo shows), I would say, “Oh no, please don’t do that. The ants are eating the sap so the peonies can open.”
It worked like a charm and my favorite pollinators, the ants, were spared.
Of course we know that the peonies don’t need the ants to “eat the sap” for them to open. It’s more of a symbiotic relationship, akin to the way that the ants pollinate things–although this isn’t a true pollinator relationship.
What is happening here is that the ants are attracted to the peonies sugary sap. In the process, they keep other predators at bay–things like aphids, which are prevalent in this early spring, and thrips, which affect so many of our ornamental flowers. Ants might even be thought of as the peonies own natural insecticide.
You can read more about this beneficial relationship here at this fact sheet from the University of Missouri.
But of course no one wants to bring ants into the house if you want to enjoy peonies as a cut flower. There are a couple of ways to solve for this. First, cut the peonies in the evening, or first thing in the morning and leave the cut flowers in a cool place (a shed or garage) for several hours so that the ants, if any, can leave the flowers.
If you cut the flowers at this stage–or slightly larger–you can gently shake or wash the ants off to know that you have removed them all. That way there’s no guessing. Make sure that there’s enough color showing in the bud that the flower will open. This bud is just a little bit too small yet.
Finally, I am sure that most of you won’t get to the point where you’ll rejoice when you see ants in the garden as I do. But if you see them on your peonies, thank them. They are protecting them from other insects pests–so you don’t have to!
And once the blooms are fully open, they move on. So once again, no insecticide needed. It doesn’t get any better than that!
Gardendaze 
Ants also cultivate aphid on roses, citrus and a few other plants. That is why we apply tanglefoot to fruit trees. Without the ants to cultivate and protect them, aphid do not proliferate, and are controlled by predatory insects that fly in.
Oh yes, but that’s more of a “farming” relationship, isn’t it? I often have ants on my citrus–I have posted about it here more than once. I have even had hornets on them (talk about a relationship you don’t want to encourage!)
My understanding of the ant/peony relationship is that the peonies actually shut down their sugar production once the flowers are fully open because there’s no more need of protection. That’s what my MSU link says. And that’s right, I think. I can’t say that I see ants on just the foliage.
Or maybe at that point, they’re too busy with our other plants!
Karla