Elaine Walker: Bark mulch vs. gravel: Which is best?

U OF I MASTER GARDENERAugust 15, 2013 

Someone asked me whether it was better to use bark mulch or gravel as a ground cover in the garden. I prefer bark mulch, but decided to do some research to find out which one is really better.

For starters, there’s a difference between bark chip mulch and wood chip mulch. And the size of the chip does matter.

Both bark chip mulch and wood chip mulch break down over time, adding nutrients to the soil. For that reason, both wood chips and bark chips need to be replenished every few years. Wood chips decompose faster than bark chips, so wood chips need to be replenished more often.

As long as bark or wood chips are spread on top of the soil, they won’t take up a significant amount of nitrogen as they decompose. But if either is mixed into the soil, it will decompose at a faster rate, which will take nitrogen from the soil.

The larger the chips, the deeper they need to be to do their job. Small chips or shreds need only be 1 to 2 inches deep to keep out weeds, hold moisture in the soil and reduce soil compaction. Large chips need to be 3 to 4 inches deep.

Landscape fabric is not needed when using bark or wood chips as a mulch. However, for areas with lots of weeds, a few layers of newspaper will help keep the weeds at bay.

Bark or wood chips are best for areas of the garden containing perennials. In vegetable gardens or other areas where annuals are replanted each spring, the use of grass clippings, straw or shredded leaves make more sense. They can be tilled into the soil at the end of the growing season where they’ll compost over winter.

Gravel is often used as a mulch. When gravel is used, it’s best to lay landscape fabric under the rocks so they don’t sink into the soil. They won’t break down and — if landscape fabric is used under them — won’t need to be replenished.

Gravel is best used in areas where heat-loving plants such as cacti and succulents are planted. Once the landscape fabric and gravel are laid down, it’s difficult to remove them to install additional plantings.

Gravel will warm the soil earlier in the spring, but plants fooled into thinking it’s later in the season may be adversely affected by a late frost. In the summer, gravel will heat the soil more than other mulches, which could require more water to keeps plants thriving. Gravel mulch will also heat the air surrounding it, so it’s best not to use gravel mulch around patios and other places where people gather. Gravel mulch adjacent to lawn areas could get picked up by lawn mowers, causing damage to both man and machine.

Bark or wood chips initially cost less than gravel and are much lighter weight. In the end, bark/wood chips may cost just as much since they have to be replenished.

If you have particular questions about gardening you’d like to see addressed in this column, send them to highprairielandscapedesign@yahoo.com.

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