How Deep Should Mulch Be?

Best depth of mulch depends on the plant. Find out what's right for whatever you're growing, plus get more expert mulching advice.

Easy Exterior Updates: Plants and Pinestraw

Easy Exterior Updates: Plants and Pinestraw

Pine straw is a popular mulch choice in regions of the country where pine trees are prevalent. The right choice for your yard or garden — and how deep it should be — will depend on what you're growing as well as on local resources.

Photo by: Flynnside Out Productions

Flynnside Out Productions

Pine straw is a popular mulch choice in regions of the country where pine trees are prevalent. The right choice for your yard or garden — and how deep it should be — will depend on what you're growing as well as on local resources.

Mulch keeps the soil in a garden bed cooler and moister during dry spells in summer, improves soil structure over time, and helps keep unwanted plants – those things we call often call “weeds” – from taking over your perennial, annual and vegetable beds.

But how much mulch is enough? And is it possible to use too much? As a general rule, one to three inches of mulch spread over the soil will do a good job of keeping garden beds protected and holding weeds at bay. But there are details to consider for specific plantings.

What to Mulch

  • Perennials and annuals: A two-inch layer of mulch can keep perennials and flowering annuals content during the growing season. As it decomposes, you may need to add another inch or so to replenish the bed.
  • Roses: These beauties need plenty of water, so a thick blanket of mulch – three to four inches – will help retain moisture in the soil. Make sure the mulch is not piled up around the base of the plant.
  • Bulbs and tubers: It’s not necessary to mulch bulb beds at planting time. If mulch is used, the depth of the mulch should be figured in to the planting depth of the bulb. Tubers that are planted in a shallow bed, such as bearded irises, should not be covered with heavy mulch.
  • Vegetables: A layer of pine straw, wheat straw or grass clippings or compost between the rows in your raised or in-ground beds can discourage the unwanted weeds that compete with your produce for water and nutrients.

Mulching Trees and Shrubs

When you plant new trees or shrubs, a deep layer of mulch can help get these larger landscape features off to a good start.

What you don’t want is a “mulch volcano,” a thick mound piled high up against the trunk of the tree or shrub. This heavy, moist mountain of mulch covering the base of the tree causes the outer bark to decay and leaves the tree vulnerable to disease. It also smothers the trees roots, and over time, the roots begin to grow into the mulch where they are more exposed and vulnerable to damage and disease.

To mulch a newly-planted tree or shrub, place a two- to four-inch layer of mulch around the tree out to the tree’s drip line, then use a rake to pull the mulch away from the trunk so that you can see the root flare, the point where the tree trunk meets the roots. This creates a “saucer” around the tree that helps direct water to the tree’s roots.

Types of Mulch

  • Shredded bark (hardwood or pine bark) is a popular choice. Buy by the bag at home stores or nurseries, or buy in bulk from local suppliers. Bark nuggets are larger and don’t decompose as quickly.
  • Pine straw or wheat straw are easy solutions in many situations. Wheat straw – not hay, which contains seeds that will sprout in your well-tended soil – is especially good in vegetable beds. Place it over a layer of newspapers for extra weed suppression.
  • Shredded leaves and grass clippings, if your landscape has trees and lawn areas, are handy, easy to use and free!

Mulch Products to (Mostly) Avoid

  • Stones, gravel or volcanic rock will not have to be replenished each year, but since they don’t break down, they don’t enrich the soil. Remember, too, that stones can hold heat, which could turn up the soil’s temperature more than the plants’ roots can stand. They may be a good choice for cactus gardens.
  • Rubber mulch, made from ground-up tires, is a good way to recycle old tires, but it won’t do anything to enrich the soil in your garden beds. On the other hand, it may be a good choice to use under a children’s play area.
  • Landscape plastic and fabric makes sense in certain conditions – under gravel paths, for example, to keep gravel from quickly working its way into the soil. But these ground covers can prevent water from reaching the roots of plants. And while they will suppress weeds, they may also be detrimental to the plants you favor.

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