It's difficult to describe why, but it doesn't sound right. I suppose it's because the word "lawn" brings to mind the image of something horizontal, i.e., the stretch of ground to which you are referring. Given the horizontal aspect the use of the adjective "tall" seems disorienting.
Better to say "the grass is tall", or "the lawn needs cutting/needs to be cut."
Grass can be tall in AE, but it wouldn't be the same thing as a lawn. Prairie grass is tall, and grass in a park, between the mown part and the treeline. A lawn can revert to a tall-grass state, but then we're likely to say it's "gone to seed."
And "tall grass" is a set phrase, meaning...uh, grass tall enough to lie down in and not be seen. Where are Jack and Jill? "Off in the tall grass" is another way of saying "don't ask."
"The lawn is long" sounds odd to me, but I can't figure out why. There are seed varieties for long and short-grassed lawns, after all. I would call a lawn in need of mowing "overgrown" or "getting a little thick." Or I'd say the grass (or lawn) was deep. Or shaggy.
To me a "long lawn" refers to the size and shape of the lawn itself, or its extent-- not the state of growth of the grass it's composed of. .
well, to respond to foxfirebrand, perhaps it feels weird because we use tall to describe height. However there is still nothing wrong with using tall to describe the lawn....in fact it could be quite usefull when one wants subtle emphasis or attention.
I think the lawn is referring to the whole area, though. If I heard, "the lawn is high", for example, I would be thinking of the location of the entire lawn being in an elevated place, not the grass that makes up the lawn.
"The lawn is tall" sounds strange to me, too. "The grass is tall" makes more sense, but it would take a long time for the grass in a lawn to grow enough to be called tall.
I'd suggest "the grass is high" or "the grass is overgrown".
This is just the time of year-- how did you know? Now that you've got me thinking of it, that might be easier than plucking them!
The irony is, geese themselves are the best lawn mowers in all of Creation. Keep your gaggle at just the right level, and the grass/lawn will never be high, long, deep, shaggy or overly fledged. But let them overpopulate, and we'll have to start a thread on what word to use for the baldness of lawns. .