Is Mulch the Missing Ingredient in Your Garden?

Claire Splan

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So, you’ve cleaned up your garden and put in some new plants and watered and fertilized and you’re basically done for the season, right?

Wrong!

See all that bare soil in between your plants? That’s a problem. That’s a big problem because it will lead to multiple other problems. Serious problems like an invasion of weeds, evaporation of precious moisture, and more. But the solution to those problems lies in one little word:

Mulch.

What is mulch?

Mulch is a general term for a material that is spread over open ground and around plants to protect and insulate the soil. There are many different types of mulches and they each have their pros and cons.

Visit any garden center or nursery and you’ll quickly discover that there are numerous options when it comes to choosing a mulch. Before making your selection, consider what you want mulch to do. A good mulch should do all of the following:

  • Minimize evaporation in the soil
  • Suppress weeds
  • Moderate soil temperatures
  • Reduce soil erosion
  • Improve a garden’s appearance
  • Be readily available at a reasonable cost

In addition, organic mulches will improve the soil structure and add nutrients to the soil as they decompose.

Following are 11 choices for garden mulch. A few are harder to come by in some regions, but most are commonly available.

Bark or wood chips

Among the most commonly used mulches, bark and wood chips are easily available in bags from garden centers but can also be obtained (sometimes for free) from tree services that are happy to unload truckfuls of chips of trees they’ve pruned or cut down rather than pay to dump them. Wood chips can be slow to deteriorate, which means you won’t have to replace it as often. They can bind up nitrogen in the soil as they do, which robs nearby plants of needed nutrition, but that is just a temporary situation and generally doesn’t affect the overall health of the plants. Some bark chips may be toxic to plants if they are too fresh, so it may be advisable to let fresh chips age for a few months before spreading them.

Straw

Straw mulch can be hard to find in urban and suburban areas but is readily available in rural areas. Rice straw adds nutrients to the soil and doesn’t have seeds, but it deteriorates quickly. Wheat or oat straw lasts longer but you have to deal with the seeds first by soaking the straw and allowing the seeds to sprout and die before spreading it. Straw is not expensive and is good at holding moisture in the soil but allowing air to pass through.

Alfalfa hay or pellets

This is a more expensive option but a good source of nitrogen for the soil. It is seedless and long-lasting compared to rice straw. Since it is pelletized and bagged, it is easier to transport than bales of straw.

Shredded redwood “hair”

Shredded redwood can make a very attractive mulch, but it’s lightweight and can blow around. Also, weed seeds and other debris can get caught in the finely shredded fibers, making it messy.

Cocoa bean hulls

This mulch gained a lot of popularity primarily for its fragrance. It smells like chocolate. The downside is the fragrance doesn’t last long, it’s expensive, and it’s lightweight enough to blow away. It can also be toxic to dogs if they should eat it.

Grape seed mulch or compost

Grape seed compost makes a really dark (almost black) mulch that is visually appealing, long lasting, and will feed the soil as it decomposes. It’s not as readily available as other mulches but nurseries and soil suppliers may be able to hook you up. Depending on the source, you may need to screen or filter it before spreading it out to remove bits of vine and other debris.

Yard waste (grass clippings or leaves)

Using your own yard waste as a mulch is one cost-saving option. Grass clippings and dead leaves make an effective, if not the prettiest, mulch, and will feed the soil as they decompose. It’s recommended that you run the waste through a chipper first.

Gravel

Used as mulch, gravel’s greatest benefit is that it doesn’t decompose. It’s heavy enough not to blow away, but it does tend to migrate and you’ll find pieces of gravel popping up all over the place, including in the lawn where it can be caught by the lawnmower and thrown, potentially causing injury. Realistically, gravel only works as mulch in areas where you won’t be doing any future digging, so that limits its use to walkways, driveways, and permanently planted shrub beds.

Sawdust

Sawdust can be effective at lowering the pH level of soil as it decomposes so it is sometimes used as a mulch for acid-loving plants. It’s not very long-lasting as a mulch, however, and it’s also important that the sawdust not include particles from chemically treated woods if you are going to use it around edible plants.

Pine needles

If you have a good source for pine needles, you have a gardener’s goldmine. Pine needles make a terrific, nutrient-rich mulch that helps to acidify the soil, so they’re particularly good to use around edibles. They’re long-lasting and don’t compact too much, so they allow for water and oxygen to easily pass through.

Cardboard

If you are looking for a weed-suppressing material to put down underneath a layer of mulch, cardboard is your best bet. It is effective at blocking sunlight from getting to weeds and weed seeds below, but it allows water to get through. It is recommended as an effective way to kill a lawn that you want to reseed or replace with other landscaping. You can use cut-up pieces of cardboard boxes (overlapping them for complete coverage) or you can purchase large rolls of cardboard that you can lay out in long strips and cover with wood chips or bark.

Ground coverings to avoid

Following are two ground covers that you’ll find in nursery centers, though I don’t know why because they are terrible options. Avoid these at all costs!

Landscape fabric

Frequently used underneath other mulch materials, landscape fabric is purported to suppress weeds while allowing water to penetrate through to the soil. The fact is, however, that it does a poor job of both. Weed seeds can get caught in the weave of the fabric and germinate and most fabrics allow little if any water to permeate.

Some professional-grade landscape fabrics may do a better job than the fabrics commonly sold in garden centers, but before you lay landscape fabric in your garden, it’s a good idea to test it to see if it is water-permeable. Cut a square of the fabric and drape it over the top of a water glass, pushing it down to form a well in the center. Secure it in place with a rubber band. Then pour water into the well and see how long it takes to pass through the fabric into the glass. With some fabrics, you may be surprised to see the water just sit there and never drip through. That’s not going to be helpful in your garden.

Black plastic

Although it’s effective at suppressing weeds, black plastic is a poor choice of soil cover in home gardens. It prevents water from penetrating and can trap heat in the soil, creating a “solarizing” effect that kills even the good bacteria other organisms needed to maintain the health of the soil. There are times when solarizing is needed to kill pathogens or really noxious weeds, but as a rule it's too much of a "scorched earth" treatment.

Now, go get that bare soil covered!

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Writer/editor. Author of "California Fruit & Vegetable Gardening" and "California Month-by-Month Gardening."

California State
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