Combine the potatoes, carrots, celery, garlic and ginger in a slow cooker. Toss in half of the flour and season with salt and pepper. Scatter the bay leaves over the vegetables.
Season the pork generously with salt and pepper, sprinkle with the thyme and allspice and toss with the remaining flour to coat. Place the pork over the vegetables in the slow cooker. Add 2 cups water and the tomatoes, cover and cook on low 8 hours.
Discard the bay leaves. Remove the pork roast and slice or pull the meat off the bone into large pieces. Serve in bowls with the vegetables and broth.