Too Long Lasting
Because of certain chemicals in cedar, it does not break down quickly. The disadvantage of this is that cedar mulch may start looking bad, but not rot and blend into the soil. You may need to remove last year's mulch for a fresh coat.
Ties Up Nitrogen
Because cedar is so long lasting, it does not break down quickly to provide nutrients to the soil. Instead, it may tie up nitrogen in the soil layer just below the mulch, restricting the nutrients available to plantings.
Possible Toxicity to Plants
While the data is not definitive, many believe that the chemicals in the cedar that keep it from rotting may also inhibit growth of other plants, particularly seedlings. This is good if the other plants are weeds, but not so good if they are the flowers you planted.
May Need Aeration
Because cedar bark mulch is of a finer and stringier texture than other wood mulches, it may become compacted and prevent sufficient water and nutrients from reaching the soil below. You may need to aerate it to avoid this problem.
More Expensive
Cedar bark mulch is generally more expensive than some other wood mulches, such as hardwood mulch.