Scott,
No, your not crazy at all.
I don't know how much room you have, but technically, the mimosa tree is excellent shade for your hydrangias, dappled, which is what they love.
My mom planted a little Red cut leaf japanese maple on our front lawn when I was a kid, and the mimosa tree shaded it just right so the jap maple could survive the sun in summer.
I love those trees, you will probably get alot of nasty people on here in regards to the tree, but, in the scheme of things, do what you want.
By the way, does anyone in your neighborhood have one?
If you can't see any close by, it could be a locust tree, which looks Exactly like a mimosa, only minus the beautiful pink flowers in summer.
Let it grow. You'll see what it is eventually.

That's a coincidence! I moved three mimosa seedlings a few weeks ago to the back of my hydrangea bed, also to provide a little extra afternoon shade.

The old mimosa that these seedlings came from died last fall. (I think they are short-lived trees at best.)

I probably wouldn't have bought one because they are a bit like weeds. But I really like their summer show. The old one was already here when we bought the place 6 years ago. It was behind my flowerbeds in the edge of the woods and that eliminated the aggravation of most of its less desirable traits.

The new ones will have a similar place far enough back to keep the mess out of the lawn mower. I'm really anxious to see if they will turn out as pretty as I picture them in my mind.

Besides the slimy, sticky debris from the flowers that drop off after blooming, there are LOTS of seedlings that appear far and wide. Some people bitterly object to that; I see it as an opportunity to have more blooming trees without spending any money. Besides, the little ones are easy to identify and easy to pull (at least in my soil).

I agree with butterfly4u that the mimosa is a truly lovely summer-flowering tree. It is one of those traditional Southern exotics that has been grown and appreciated for many generations. I also agree with butterfly4u's advice that we can expect the Invasive Plant Kamikazes to swoop in, at any moment, with guns ablazin.' I say, with Lewis Grizzard, "Shoot low, Boys, they're ridin' Shetland ponies!"

Another attribute of the mimosa is its tendency "to rain" during the typically high heat and humidity of the Southern summer--literally to rain. On a hot day, you can stand underneath a mimosa; and the tree will rain on you. Its leaves transpire water droplets that fall down like rain. It's similar to the phenomenon called "the South Georgia Drip," in which tropical thunderheads that have rolled in from the Gulf randomly drop huge raindrops on an otherwise sunny day.

Scott, If your propery is bordering and ecological sensitive area you may want to pull it up. Otherwise, leave it and it should provide the perfect amount of shade for the hydrangeas. You will most likely have a yearly battle pulling up seedlings.
Years ago my neighbor planted a mimosa. Last weekend I got the chainsaw out and cut down 6 on my property and she asked if I could cut down all the ones at her house. I know I'll regret it, but I did leave one up by my driveway
because they are beautiful when in bloom.

Ok, I'll play the heavy. Mimosa is considered a "severe threat" level invasive plant in South Carolina. That is the highest level. You can do your own research by Googling "Albizia julibrissin", the scientific name for this plant.

I know there are plenty of people that consider that the barn door has been left open for too long on this plant, but I cannot pass by the chance to ask you to reconsider this one. Seeds are spread by wind and water so once yours gets mature enough to flower, you will be contributing to the problem.

Alternatives might be a Scarlet Oak, one of the fastest growing native oaks with beautiful fall color (Mimosa has no fall color). Redbud, especially the 'Forest Pansy' cultivar is a tree that is similarly shaped to mimosa and has nice spring blooms; a plus with the cultivar is that it does not seed much. Foliage color is nice during the summer, but it doesn't have much fall color. Another choice might be Sassafras, outstanding fall color with a nice loose shape.

Anyway, my two cents worth on this.

Here is a link that might be useful: List of all invasive plants in SC

Well thank you all. You each have some valid points and interesting advice. Horticulterally speaking, it sounds like we ran the politcal gammet from libs to conservatives to libertarians. Interesting! :)

I'm not sure how much I buy into the invasive part. I see them scattered around and they seem to be just that, scattered. But, I have never had one so I have no experience to draw from. Hence, my post. You want to talk about invasive, I grew up in South Florida and they have a tree there called the Brazilian Pepper tree and there are acres and acres where that tree took over everything. Worse than kudzoo. Or, how about Wisteria? Talk about invasive.

As it is, I don't even know if I have a Mimosa or a Locust tree. There aren't any flowering Mimosas around that I have seen but I haven't scaled fences to look in other peoples yards either. With that thought in mind, I will probably yank it out.

The alternative trees all sound interesting but I am working on a budget. Most of my plants are pass-alongs and few have been purchased, my dad taught me that.

Thank you all again. Lots to consider.

Scott

Mimosas are indeed invasive. When we bought our house, our 1/2 acre back yard was more than half populated with them (of the trees that were there; it was pretty full, too). We cut all but one on our property and were constantly amazed at how many came up from seed every year (and back again and again from the roots like hydras until we rounded up the stumps). The neighbor clear cut his backyard in 01 (not a good thing except for the mimosas), and so theirs were gone, and we are next to sewer easement and we cut all those out, too. The last one on our prop, an enormous one about 65' tall and with a 2 trunk base bigger than i could reach around, was probably the mother of all of the many ones, planted there in the mid 60's when the houses were built, is my guess. We decided to let it live its natural span, knowing it had to be old and not have too many more years. It finally succumbed to a lightening strike or disease or something and had 2 huge checks running down each trunk so we cut it this past summer (we measured it when we cut it to get the height). The hummers and butterflies missed it, and i, its color, but i will NEVER miss pulling all those seedlings. As it is, we'll be pulling them for a few yrs yet til they get exhausted. I would only ever recommend having one in the middle of an area that could be mowed regularly, and kept in full sun to limit the size, and even then would only plant the red cultivar which seems less vigorous. Personally, i'd never have one again, no matter how pretty.

Well, I went to my neighbors yesterday to cut down her mimosas. Ended up cutting at least 40 trees with trunk diameters from 2 to 14 inches, with at leat that many smaller plants that need to be killed. The one I left by my driveway will be cut next year after it blooms and before it set's seed. Next project is to get the Chinese Privit under control.

Did you round up the stumps after you cut? If not, they'll be baaaack...
For the virtual forest of small ones in the yrs after we tackled the big ones, i used pruners dipped in round up after each cut. Worked wonders!! It probably would work well for the sprouts of the ones you just cut when/if they start to sucker. If it were me, i'd get out there today and soak the heck out of the stumps/remaining bark while they still have the ability to wick it up. Maybe you'll avoid them coming back to life with 10 heads each.

Yea, I did paint the ones I could find with round up. There were so many I'm sure I missed a few and will find them in the spring when they re-sprout. That's a great idea dipping the prunners in round up. Thanks