how to tell when sphagnum moss needs replacing

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It looks bad. Old sphagnum compacts down and the individual moss parts look broken and distorted. The moss gets a dark brown hue.

For plants that need to stay moist, sphagnum lasts only a year or two. Most people would repot a plant in sphagnum every 2 years at the most.

When Epidendrum polybulbon is happy it will grow so fast you need to repot every year.