By Linda McDonnell, CSU Extension-Denver Master Gardener since 2013
Mulch makes an impact in so many ways – from water conservation to weed suppression, improved plant health to enhanced visual appeal – mulch seemingly has superpowers.
There are two general types of mulch – organic and inorganic. Organic mulches include bark chips, straw, grass clippings, and dried leaves; gravel and small rocks are inorganic choices. Which mulch to choose? The best advice is to match the mulch material to its intended use. Let’s take a look at some common mulch applications.
Shrubs, Trees and Perennial Beds
Three to four inches of bark mulch laid directly on the soil helps maintain moisture, reduce water use, and lower the soil temperature. Skip the landscape cloth and plastic barriers, which when left in place for multiple years, will impede water penetration, limit essential oxygen exchange, and inhibit root development.
Leave a few inches of bare soil around the base of the plant (more for large shrubs and trees) to allow the plant to absorb moisture. Avoid mounding mulch around the trunk of trees – a “volcano” of mulch will hinder healthy root development. Do not mulch over the root ball of a newly planted tree.
When adding plants to the bed, rake the mulch away from the space before digging to avoid mixing the bark into the planting hole. Bark chips break down in soil and as they decompose, can rob soil of essential nitrogen.
How much mulch do you need? A two cubic yard bag of mulch provides three inches of coverage over 216 square feet. This online calculator is handy for determining the quantity needed based on depth and coverage area. It calculates the number of bags or bulk quantity needed, or at least gives a good estimate.
Pea gravel (stones with diameters of less than one half inch) are highly effective mulches. Three inch deep coverage provides weed control and even more effective moisture infiltration than bark chips. During cold months, gravel mulch’s warming effect can increase biological activity down to one foot below ground, resulting in healthier, more resilient plants. Pea gravel is frequently the mulch of choice for xeric plants because it offers excellent drainage.
As with bark mulches, do not use landscape cloth or black plastic beneath pea gravel and move the mulch away from the crown of plants.
According to CSU, “In general, mulching minimizes evaporation of water from the soil surface, reducing irrigation needs by around fifty percent. It helps stabilize soil moisture levels, thereby improving vegetable quality and encouraging the beneficial activity of organisms.”
Mulching also helps reduce soil compaction, can add organic matter to the soil, controls weeds, and modulates temperature extremes. Bark chips are not recommended around vegetable plants but are useful as a garden path.
Tomatoes, peppers, eggplants, and vines such as cucumbers, summer and winter squash, pumpkins, and numerous melons benefit from a layer of black plastic placed on the soil early in the season. The plastic warms the cool soil, allowing for earlier crop growth. Be sure to remove the plastic in the fall to preserve soil health.
Grass clippings from lawns that are untreated by herbicides or pesticides make excellent vegetable garden mulch. Build up the mulch coverage by adding up to a quarter-inch of clippings, allow them to dry, and then repeat with another layer. This layering process prevents the grass from forming a thick, impenetrable mat which restricts the plant’s moisture absorption.
At the end of the season, turn the grass into the soil; it will break down and add organic matter.
This CSU publication contains more options for mulching a vegetable garden.
Turf Alternatives and Garden Containers
Many are rethinking conventional bluegrass lawns and opting to use mulch to replace portions of turf. Large rocks, pea gravel, and bark mulch are popular landscape accents, paths, or borders that reduce water use and create an interesting, practical aesthetic.
Ornamental garden containers can be topped off with a layer of mulch to help retain moisture and add a finished look to the planter.
For a rewarding and plentiful garden, don’t skimp on mulch this season!