Start by chopping the rhubarb into 1/4" pieces—thicker, wider stalks should be halved lengthwise first. Sprinkle with about 1-1/2 to 2 tablespoons of the sugar and toss to combine in a bowl and set aside for 1 hour to macerate, softening it a bit and getting its juices flowing. If you haven't already, dice the butter and stash in the freezer to chill.
Preheat oven to 425°
When the rhubarb is finished macerating add the flour, remaining sugar, baking powder, salt and ground ginger to the bowl of a stand-up mixer (see note). Scatter in the butter and, with the paddle attachment, beat for just about 30 seconds to break the butter up so it's roughly larger than pea-sized.
Add the liquid from the rhubarb to the dry mix (don't get too hung up on this, getting out a sieve and the like—just use a spoon get out as much as you can. You'll probably end up with about 1.5 - 2 tablespoons of the liquid). Measure out 1/2 cup of cream into a liquid measuring cup. With the mixer on low add 1/2 cup of the cream and vanilla and beat for just a few seconds. Scrape the sides and bottom of the bowl with a rubber spatula. Tip in the rhubarb and beat again until just combined, a few seconds more only. If it hasn't all come together into a soft dough quite yet, add a bit more cream (up to 2 tablespoons plus 2 t-spoons more, so 2/3 cup in total).
Turn the dough out onto a lightly floured surface and form into a disc, pressing it down with the back of your hand until it's 9" in diameter. Cut the dough into 8 triangles and transfer to a parchment or silicone mat-lined baking sheet. Brush them with a little more cream and sprinkle evenly with turbinado sugar.
Bake at 425° for 20 - 22 minutes, until lightly browned on top and firm to the touch throughout.
While the scones bake, sift the powdered sugar into a large bowl and add the meringue powder (optional). Grate the ginger root over a plate or into a small bowl (NOT on a wood cutting board—it'll absorb the liquid too much!). Pick up the grated ginger in your hand and squeeze its juice into the powdered sugar, simultaneously whisking it. Splash in the vanilla and beat to combine, adding a bit of water splash by splash, as needed. You want it thin enough to drizzle evenly but stiff enough to cling to the scones.
When the scones are done baking, let them rest on the baking sheet for a few minutes before transferring to a wire cooling wrack to cool for just 5 - 10 minutes or so. Drizzle with the glaze and serve warm or at room temperature.
You can make them in advance and reheat them in the microwave for a few seconds if you want. They are best fresh and still slightly warm from the oven.
Mixer vs. Processor vs. By Hand: I find these easiest to make in a stand-up mixer (or a hand mixer would do) but you can manage with a food processor. With the latter, you need to stir in the rhubarb by hand so the processor doesn't puree the rhubarb into the dough. You can use a pastry cutter to break the butter into pieces or by hand, tossing your hands in the dry mix and butter as you rub the pads of your thumbs across your fingers to break up the butter. This takes forever, though, and can warm the butter too much.
The varying level of cream that scone recipes call for is due to flours ability to absorb liquid. The humidity in the air and the flour itself can cause it to absorb different levels. Most times that I make scones, though, I use about 1/2 cup plus maybe a splash more. Because the rhubarb here needs to be macerated a bit first—so, once baked, it's properly softened in the scones—it lets out some of its natural liquid. I usually end up with about 1-1/2 tablespoons of rhubarb juice, but if your rhubarb is particularly juicy start out with 1/3 cup of cream, adding as you go. You don't want a really lack, sloppy dough so its important to add the rhubarb liquid and then just enough cream for the dough to come together in a soft, supple dough.
The meringue powder in the glaze is optional but I love it because it helps the glaze to set up and dry a bit on the scones. If you can't find it at your local grocery store, check out restaurant supply stores or craft/hobby shops in the baking/cake decorating isles. Otherwise, you know you can buy everything online.