Wind

[CRNDLSM]

Acorns collecting
Reflecting tree branch dances
Sticking together

[TranquillityBase]

Acorns collecting
Reflecting tree branch shadows
Sticking together

"reflecting" seems the wrong word for the effect of shadows on an object.
then again, it's an interesting contradiction.
so I'm ambivalent.

TqB

[CRNDLSM]

It's the shadow play I'm seeing as a mirror , I edited it

[busker]

Acorns collecting
Reflecting tree branch dances
together apart

Maybe ‘gather’ instead of ‘collecting’?
‘Reflecting tree branch dances” doesn’t sound right. You mean shadows, but shadows aren’t reflections by definition. Would it work by inverting the order and having the tree branches shadow dance on a screen of acorns or something like that?

‘Together apart’ can be the title. I don’t think abstraction works well in short poems like this one

[Kynaston Levitt]

Acorns collecting
Reflecting tree branch shadows
Sticking together

The collecting/reflecting rhyme sounds out of place, to me—as well as using 3 present continuous verbs. I appreciate you are going for the 5-7-5 approach but you may consider changing "reflecting" to "reflect".
Other than that, I actually don't mind the contradiction of shadows being reflected. Normatively speaking it's hardly even a contradiction. And the shadows of tree branches sticking together is a beautifully simple articulation of a mute observation I am sure we've all made. I've no doubt that from now on whenever I witness this phenomenon I will recall this line.

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 you changed it completely. Oh well, fair enough. Serves me right for not learning to type with more than one finger.

[Lizzie]

Acorns collecting -- I like collecting better than reflecting here, and adding reflecting in the next line works much better. Seems like the main idea is acorns huddling up to protect against inclement weather -- it's adorable and original. Makes you care about the acorns. 
Reflecting tree branch dances -- keeps it from being too sad
together apart -- I have to be honest: this sounds like it's trying too hard to be clever. The juxtaposition isn't particularly fresh or surprising. 

I much prefer the last line in the first draft. Even though the phrase is commonplace, it was used in an original way. 

Makes me pine for fall..... (since oak for fall isn't really a thing)

[CRNDLSM]

Thank you, I put the last line back, I think you all see what I see and some more so, spontaneous you know

[Mark A Becker]

Makes me pine for fall..... (since oak for fall isn't really a thing)

That's really funny Lizzie!  oaking for fall might be a thing though