Good answer, but I'll raise the same question I raised with StoneyB. Overgrown doesn't relate just to height or growing "over". It also relates to density, characteristics of maturity (like becoming woody), plants growing into each other (lack of separation, transmission of pests and diseases), one plant outdoing another (supplanting or choking it), etc. Your answer defines "lawn" as the grass and weeds as something else. (cont'd)

But lawn also refers generically to the carpet of mixed short vegetation (grass and weeds), that surrounds your house. In that context, it isn't weeds overgrowning lawn, it's overgrowth altering the ratio of weeds to grass in the composition of the lawn. So weeding is how you readjust the mix when the lawn becomes overgrown.

@fixer1234 My first thought is that the OP mostly needs help understanding the grammatical pressures from established usage of "overgrown" that give the "has" and "is" versions of the sentence their different meanings. Bringing in weed/grass ratio would add undue complexity. But your objection still deserves an answer! I'll let it slosh around in my brain and see if anything comes up.

@fixer1234 OK, here's what came up. Oddly enough, I wasn't thinking of "lawn" as limited to the grass and excluding the weeds. I was thinking "lawn" more as the place. The word "lawn" (like any word) is flexibly ambiguous: it will "glom on" to whatever you want to point out or imagine in the situation that corresponds passably well to its core meaning of a faux-savannah kept artificially short and neat by regular, conspicuously time-consuming maintenance ("mowing"). It can refer to the grass exclusive of weeds—"weeds have invaded my lawn!"—or even to the location bereft of any plants as yet—…

@fixer1234 …"The house is almost up. Now I'm going to put sod into the lawn [referring to the region of uncultivated dirt in front of the house]." IOW, the word "lawn" points, as if you pointed with your finger. Did you point at the place or just at the grass or at all the plant life whether grass or weed? A person would reasonably understand you only by considering what you were saying about it. The same applies to "overgrown". What could you be pointing to that answers to notions of "growing over" and/or "growing too much"? Since you think the lawn needs weeding, probably the weeds. ;) …

@fixer1234 …Having established that, there's no need to form any more-precise interpretation or definition of "lawn". Weeds have been growing out of control in that grassy, often-mown region, they're getting unsightly, they need to go—got it. Does that answer your objection?—"These words swim in a slurry of useful vagueness, where your question about whether 'lawn' includes or excludes the weeds is neither asked nor answered."

Geez, I thought you'd slosh better than that. :-) Where I was going was that, to me, "my lawn has overgrown and needs weeding" could be correct in terms of grammar and definition (and certainly applies to my own lawn). I didn't want your answer to be biased to a particular usage or misleading by omission. However, that would be a less common usage, it's not like the fate of the world hangs on this nuance, and getting into the weeds of this discussion (sorry, couldn't resist), is probably of limited incremental value to the OP. So, good enough for government work.

@BenKovitz, there's more? I already conceded. :-)

@fixer1234 I started this chat room only by mistake, but I'll still say: Good work, fixer! (If I may call you that.) You caught something subtle that is quite easy to miss, and that illustrates how English grammar really works. I hope you'll add your point to your answer.

@BenKovitz, OK, I added it for completeness. BTW, you often comment on how questions that appear trivial can turn out to be complex. A lot of questions, like this one, look like they can be fully answered with a few sentences. By the time the comments and answers are complete, there is an amazing and unexpected amount of detail. I just discovered that with a question on sports.

First impression, it looked like the OP hadn't bothered to look up the definition of "sports". The range of subsequent discussion was mind-boggling, and people are still posting nuances.

@BenKovitz, in case you want to add an answer: english.stackexchange.com/questions/392338/…