Still don’t get 26ac; why is tree a theme? Oh yes, now I see, it is the theme of the puzzle!

Never have I finished a puzzle with less understanding of why!

It gives us the final ‘a’ in Augusta with august (majestic) coming first.

Of course, it wasn’t like that.
It took me a while to figure out what was going on.
The surface of 11ac suggested “Golf”, confirmed by the wording in 10ac. But my real way in was a misinterpretation of 26ac, which was TREE, but one that I saw as IRON (hidden in “environmentalist”). Given another ‘club’ in 23,20 and the theme seemed to be cracked.

Then I found MAGNOLIA, AZALEA and CAMELLIA – and I thought, maybe there’s some Flower Show starting today.
I had NANDINA and PAMPAS early on (from construction), but only after Googling the penny dropped.

So, a clever crossword?
Well, the idea was, even though I could not be bothered too much by golf (or rugby, which worked against me in 27d, the one I couldn’t parse apart from the cat-bit).

Gaff must have put a lot of thought in his puzzle, which I admire, but unfortunately there was too much iffiness to my taste.

Like in 1ac: ‘a bit of cash’ for half of cash?
And why the misleading ‘A’ in the clue for 11ac?
Why two definitions in both 21ac and 28ac? – for me it’s ‘padding’.
And indeed, Gaufrid, as you say, what about these anagrind/s in 29ac?
And Gaff is a bit minimalistic in 5d and 7d, in my opinion.
“with posh replacement” meaning “W replaces U”?
“starts lifting a slightly etc” for “starts OF lifting etc.”?
Perhaps, I am nitpicking, but there was a discrepancy between potential and work-out.

Unlike crypticsue, I didn’t give up – in fact I got them all right. (BTW, I am on the point of giving up Nimrod – why is he always so different?]

Thanks, Gaufrid for stepping in (the second day in a row, well well).
Thanks to Gaff, too, despite my critical notes.