To your slow cooker, add the turnips, parsnips, celery root, potatoes, and garlic. Sprinkle with salt and black pepper. Slow cook on high for 3-4 hours or low for 6-8 hours. Stir the potatoes every 1 or so if you are cooking on high. If your slow cooker gets too hot, it can burn the edges of the veggies. Once the veggies are tender, turn off the heat or turn to the warm setting. Add the sour cream, milk and desired amount of herb butter. I use a healthy amount, but reserved a couple of tablespoons to drizzle on top.