Garmin GPS in rural Tuscany

Not all Italian addresses are as strange as the agriturismo, which are farmhouses in the middle of nowhere. Most other addresses are regular streets with street number. But even if you enter that agriturismo address you mentioned in Google Map it will take you right there exactly on the roof of the agriturismo, so at least you now know that Google knows what is doing. However it is true that the GPS navigator may not respond as intended to some addresses sometimes (only Google is that good, I guess). So it's a good idea to test and save your agriturismo address from home. Just insert the Europe Micro-Card in, enter the address of your accommodations, then zoom in and see if it's close to what Google Maps shows. If yes, save it on the favorites before you go. Do the same with all the accommodations or addresses you need.
It's also a good idea to pack a regular paper map (you can buy one there). Maps are better for planning purposes and as a back up. Sometimes the navigator sends you through strange itineraries (also in the US). When you plan your trip, use the map and mark the towns you will be passing through. Italian road signs tend to indicate directions using the next town names rather than highway numbers.
However even if all you find is Montalcino, the town, I'm sure everybody there will know where the Piombaia agriturismo is. Just don't get there at night when everybody is asleep and nobody is around to ask. Don't worry, tourists have been going to Tuscany's agriturismos long before GPS existed and nobody got lost.

I wouldn't expect an isolated farm to have a street number, just the name of the farm. And, they may not be on a GPS database.

I found their website fairly easily, It does have a directions (click here), and a map (click here).

There are a lot of wineries in Pennsylvania and New York's Hudson River Valley that produce useless or misleading GPS directions. When you're lucky, the websites give you an alternate piece of information to input, like the actual coordinates. Another approach, maybe not as useful for an agroturismo is to look on Wikipedia, "the source of all wisdom." In addition to term-paper plagiarism options, many entries include geo coordinates in a box towards the top right of the Wikipedia page.

Another approach could be to search for the place (at home, before leaving) on Google Maps, until you see enough landmarks to feel that you found the place, and then have Google Maps give you the actual coordinates.

Finally, you could email the place and ask them for the coordinates. But I've tried that, and even companies that answer all their email don't always know their coordinates!
Addendum: As a micro-planner, be sure to check that the cigarette lighter fuse is good when you pick up the car. Once I didn't, and the Garmin died 45 minutes later, and 30 minutes away from Hertz.

Yes. Please check the cig lighter. Also I noted that the lighter charger in my Garmin doesn't fit well in some European cars (nearly all of them actually). Primarily because I think their cigarette lighters in subcompacts are not as deep. As a result, any time I hit a bump, the lighter popped out. I now always carry with me a piece of wire (the one you twist to close a garbage bag) and stick it there to keep the charger in place. The other solution is to tell your spouse to hold it in place with her hand during the entire trip (my wife didn't like that too much, hence my wire solution). I also have a charger for the Garmin that I can use it at home, in case the lighter doesn't work. I bought that at Radio Shack when my rental car had a defective lighter in Hawai'i.

E-mail the agriturismo and ask for GPS coordinates. Also print out a copy of the directions from their website, just in case the GPS fails you for one reason or another. And as a backup, it's good if you have a cell phone that works in Italy so you can call them if you need help. If you want to know why I'm giving this advice, here is my experience with an agriturismo in Sicily: https://community.ricksteves.com/travel-forum/italy-reviews/sicily-caltanisetta-central-sicily-agrigurismo

What Chris F found looks really good.

I always like to have a mental picture of where I am going before I let the sat nav loose on it.

Have a great holiday - it looks really nice there.

Be sure to post a trip report when you get back...