How is Morrowind's flora able to grow in ash?
Typically, pure ash is extremely alkaline (most plants prefer a neutral to acidic soil) , lacks nitrogen, and repels water but it can serve to add certain nutrients to soil if used in small amounts.
On Vvardenfell, however, plants have adapted to this ashy environment. Plants like marshmerrow and saltrice only grow near water or in the swampier/damper areas of the province. It is also how these plants get their nitrogen: directly from the water which has a high concentration of it as shallow coastal water and wetlands typically do.
Other plants, like trama root, hackle-lo and ash yams, are habituated to the dryness of the area. They don't require much water for long periods of time. Many of these plants have roots that are exposed to air which they can use to absorb water and nutrients directly. This of course is inefficient, which is why these plants are often tough, dry, and bitter in taste with the exception of hackle-lo which is grown in a blend of ashy soil and can be quite succulent.