To restore a dirty, tangled tail when it's too cold to break out the hose, turn instead to dry and spray-on grooming products, along with your own two hands.
First, pick out any burrs or debris by hand. (A quick spray of WD-40 lubricant will help even the most tangled burr slip free.) Then, apply a waterless shampoo according to the directions on the bottle. When you're finished cleaning, spritz the hair with silicone spray and separate the hairs a few at a time by hand or with a tail brush.
Dirty manes can be treated similarly if you have no plans to braid or band?most silicone sprays will make the hair too slick for either. If showing is on your schedule, simply brush areas of the mane and tail that will be styled, or try using a hot-toweling technique on the tailbone and crest for a deeper clean.