"Oh! There are magic plants here!" she exclaims, approaching a section of mismatched flora edged with white stones. "I didn't realize there would be. I wonder where the gardener is, I wonder what they do?"
"Hello! Are you the gardener?"
"I am, Princess! What can I help you with?"
"I was just wondering what these plants do."
"Well, the tree fruits once a year, in the spring, and there's no fruit like it. Quite tasty. There's a bit of a party about the fruits when they come in. And that tuft of leaves there has a lot of roots and if you dig it up at all to see them it'll pull itself up and walk somewhere else and bury itself again. The other four, we haven't stumbled on it yet."
"...What, you have four magical plants growing here and you don't know what they do? Are they new, have you not got around to them yet?"
"No, Princess, they've all been here for years, the hanging moss the newest," says the gardener, politely puzzled.
"...I don't understand."
"No, Princess, except by accident and rumors and guessing I don't see how we could figure it out," says the gardener, puzzled.
"The flower the queen ate while she was pregnant? No one knew how it worked?"
"This was before I worked here, Princess, but my understanding is they had some idea it might be a healing plant..."
"...but they didn't know how to work it without killing it?"
"I don't believe so, Princess."
"But - but all you have to do is taste them."
Then scrambles backwards from it, alarmed.
"Did you ever try feeding a fruit to someone who wasn't expecting to have magical-tree-fruit in particular?" asks Rapunzel. "It's simple enough that it'd be easy to ignore the knowledge if you already had it even in vague terms."
"I suppose," marvels the gardener. He tastes a creeper and laughs.
He shows Rapunzel: see, it just isn't scary, it's fun and interesting and useful and now he can explain things so much better and by the way he loves her.
(That last part isn't quite intentional - he was using the plant in the way that doesn't hold anything back, to underscore his point, and, well. He loves her. It's part of every thought he has in which she is remotely involved. He doesn't especially regret sending it along, unless it turns out to embarrass her or something.)
"Sorry," says Rolan. (Slightly sorry, conditionally sorry - she's grinning, so he can't be all that sorry, her grin is just so delightful.) "Should I not?" (He is observing the gardener for reactions that might indicate incipient rumours about the princess and her Ambiguously Close Friend, ready to deflect suspicion with a suggestion that perhaps being thought-sent at by the flower can be a little overwhelming if sudden, but so far the gardener seems thoroughly occupied and not inclined to speculate about the details of this exchange.)
In that case he will continue. With details, even - he loves her and this is how: the delightfulness of her grin and the memory of hugging each other while they cried and the way he felt when he risked life and dealt death to save her from being kidnapped, the knowledge of future grief combined with the certainty that it was just worth it combined with that indescribable flying feeling he gets every time he takes a major risk on his life or freedom - and the things he's already told her about, with depth and colour now, the desire to share all his knowledge of the world with her and wrap her up in his arms to snuggle all her troubles away or just to feel the way he does when he hugs her, and to stay with her, to keep seeing her exist in her inimitable herselfness for the rest of his life - which could be quite a while, and isn't that an interesting new development, a long life has never been a tremendously high priority of his but he could see himself liking immortality if he got to spend all that time as Rapunzel's friend.
"Princess," says the gardener, when it's clear she's no longer occupied with having feelings about Rolan's thoughts, "I think you'll like the grass."
"...oh?" She bends and tastes the tip of a blade.
Then she bursts out laughing.
Within about thirty seconds the grass has animated sufficiently to undo her braid, while maintaining control of all seventy feet of hair in its unbraided state. When she doesn't get up right away, it starts re-braiding the hair, in a much more complicated pattern - it's divided the hair into a dozen parts and is simultaneously working on three different cylindrical braids of it. When it has finished these it twines all three together in a more standard braiding configuration and then wraps one of its blades around the end patiently until Rapunzel has it tied back up again.
"I can't pick how it does my hair, but it will sure do my hair. It'd weave threads too, if you gave it those, or anything else you can twist around in patterns."
"All these plants are so cute," says Rolan. "Rapunzel, would you like me to borrow the thinky one, for - convenient explanations?" (His hand is still close enough to the plant to count, and so he can clarify that 'convenient explanations' definitely also includes future instances of explaining his love in such detail that she nearly falls over from happiness, because that was amazing. Unless she would rather he not, in which case he can regretfully but earnestly try to tone it down.)
"Er, I'd want to ask their majesties to be sure, as everything in the garden's theirs," says the gardener. "I don't object for myself, if that's what you're asking."
"All right."
For bonus, the original intended use case of the plant: he watches the gardener go and sends Rapunzel his mostly-wordless assessments of what the man thinks of them - looks like there are no untoward suspicions going on, although he does seem to think they're both very cute, and that whatever Rolan is thinking at her must be terribly hilarious.
(The part of him that's always keeping half an eye out for trouble notes the movement of the guards, but they don't seem to want to interfere, so he supposes it's fine if they report back to Rapunzel's father that the princess and her Ambiguously Close Friend were giggle-hugging in the garden. Although it might lead to Cearl questioning him about his motives again. Eh, he'll deal.)
...He speculates upon the circumstances under which one might discover Rapunzel's planty qualities, realizes that he doesn't know what level of detail she might be comfortable with receiving of his Things-related thoughts, and drops the full send. But keeps up just the love part.
"It's the hanging moss one that does that, it wouldn't even need to be potted to be moved around," says Rapunzel earnestly. "And you already know about the fruit tree and the root, the gardener said. The bush sings, and the creeper will hold things you give it."
"Yes. Well, basically - plants don't do different things for different people, but they can give different instructions on how to make them do it. That's why the song for my hair is in Coronan and rhymes and so on, that's not the only possible way to work my hair, it's just the one Gothel had and by the time I was picking up any details about it myself I was already used to hers. But it definitely does what it told you it does, even if you got different implementation details."
"I'm not especially interested in your job, and even less so since getting it would traditionally involve you dying. I don't think a kingdom would be more responsibility than I can handle, but I do think it would be more than I want. I'd just as soon let you go on being king forever; I don't have any complaints about your approach."
The moss does not have any affordances whatsoever for lying.
Omission, sure, if it's used that way, but not lying.
He really loves her.
She might really be - safe. Maybe she can really trust him as much as she wants to. Maybe nothing bad will happen.
She's not completely... settled about it, but she can't think of an intellectual reason not to be. She's just been burned in the worst way.
But Gothel would've made up an excuse not to use the moss. It wouldn't have been hard, even, the way Rapunzel recoiled from it. Would have been plausible to pretend the same alarm.
And he didn't. And he loves her.