Rubber mulch distributors have taken great steps toward dyeing and shredding the rubber mulch so that it closely resembles the look of real mulch. However, one of the first problems noted about rubber mulch is its odor. Although the product is not usually malodorous, many users, including the South Carolina Department of Transportation, have noted that on hot, humid days the mulch can have a very strong odor.
Effect on Flowering Annuals
Rubber mulch creates another problem on hot days: heat reflection. In its study of the product, the South Carolina Department of Transportation noted that annual flowering plants in sunny areas did not survive because of the heat coming from the mulch. To be fair, heat retention is also a concern with traditional wood mulches. However, these traditional wood mulches do not necessarily reflect heat as much as they retain it, and proper application can help avoid heat-stressing plants.
One of the most traditional uses for mulch, apart from moisture and soil temperature control, is weed control. Rubber mulch does control weed growth, but only to a degree. In fact, comparison studies conducted at a number of institutions, including Washington State University, have found that rubber mulch is less effective at controlling weed growth than traditional wood mulch.
If you have ever noticed rubber mulch at your local garden center, you may have also noticed how costly it is. There are several reasons for this, but most important to consumers is that rubber mulch is supposed to last longer than traditional mulches. The South Carolina Department of Transportation noted that the mulch would have to last four to five years in order to be cost-effective. However, there are other cost concerns to keep in mind when deciding between the two. (See the article in the References section by Nature's Way Resources for a cost comparison.)
One of the biggest recorded problems with rubber mulch is that as it decomposes it releases chemicals into the ground that can pose a serious threat to the environment. One of the most cited studies was undertaken at Washington State University. It found that the leachate from car tires is deadly to aquatic organisms such as algae, snails and fish. Additionally, the study noted that the leachate could contain a high concentration of zinc, a heavy metal that can accumulate in many plant species and cause problems, including plant death.