Although many gardeners believe cedar mulch delays or prevents plant growth, studies at Washington State University show that, unlike the black walnut (Juglans nigra), cedar mulches do not harm plants. Quite the opposite: Cedar mulches benefit gardens in multiple ways, and are often superior to other types of mulch.
Cedar mulch, like other wood mulches, creates a barrier slowing soil moisture loss. By keeping the moisture where plants can use it, cedar mulch improves plant growth and vegetable or fruit production. Many plants suffer from wildly swinging soil moisture; for example, tomatoes crack and show signs of rot. Other plants drop buds or young fruit when the soil dries. Cedar mulch also cuts down on your water bill, since it slows evaporation and stretches the time between watering.
The fibrous structure of cedar mulch interlocks, preventing sunlight from reaching weed seeds and deterring weeds from sprouting. The interlocking mulch also stays in place during windy or wet weather and cuts down on erosion. The result is a neater, more attractive garden and reduced work for the gardener. Cedar's naturally slow decomposition rate allows it to remain attractive longer than other wood mulches.
Cedar mulches keep the underlying soil relatively cool, even on triple-digit temperature days. A study by the Colorado State University Extension comparing various natural and artificial mulches showed cedar mulch kept soil temperatures at an even 85 degrees F, even when the ambient temperature was 104 degrees F. The plant's leaf temperature was 98 degrees F, the coolest of the mulches tested. Insulating plant roots from temperature extremes keeps the plant healthy, reduces wilting or leaf scorch and prevents winter freeze-thaw cycles from heaving up bulbs and roots.
Cedar oil repels various ant species along with other insects. According to the group Texans for Alternative Pesticides, the evaporating oil even acts as an ant mound fumigant on hot days. It may also repel roaches, slugs and snails. However, some cedar mulches do not contain enough cedar oil to produce these results. Check the label to make sure the manufacturer has not stripped the oil from the mulch.