Much ado about mulch

Sure, it’s not the most glamorous of garden topics, but mulch is something that can be an important tool for every gardener. The benefits of mulching are numerous, but there are some things you should know to most effectively use mulch.

My colleague and fellow garden professor ( Dr. Linda-Chalker Scott, from Washington State University, is definitely a maven of mulch. It is one of the topics she loves to research and to share.

Today I thought I would share some of her findings and information regarding mulch as I too share a belief in its importance in the garden.

Many gardeners use mulch in the landscape to improve aesthetics — making the garden look neat and clean while reducing the labor of keeping everything weeded. Whether we are talking landscape beds, trees and shrubs or the vegetable garden, mulch has many benefits in the garden: improving soil texture, water retention and reducing spread of pathogens.

The key to success is picking the right mulch for the job and applying it correctly. There are several practices that have become common that aren’t the best.

If you are applying mulch to your landscape beds or ornamental plants, you’ll want to stick with a good wood chip mulch or bark mulch. Many people think they need to apply landscape fabric or a weed barrier with the mulch, but this ends up being more of a problem.

The fabric can reduce water and air passage to the soil and will support weeds that grow on top of them. Plus, as mulch breaks down, it turns into valuable compost that can add to soil nutrients. Fabric can also be hard to remove when you change your mind.

I recently learned this lesson when I ripped some landscape fabric up to replant my front landscape — the stuff had broken down and came up in ripped shreds rather than one piece. I wasn’t able to get it all up.

There’s also a new practice emerging called sheet mulching where you apply newspaper or cardboard below the mulch. These can smother out weeds, but can also create a barrier if they don’t break down quickly. The trick is to make an initial mulch application thick enough to smother out weeds (say 4 inches or so).

If you have time before you plan to plant, Chalker-Scott’s research shows that applying a thick layer of wood chips (8-12 inches) can drastically smother out problem perennial weeds. You can plant in the mulch after it has broken down and lost some of the thickness.

Mulch made of shredded tires has been emerging on the markets and has attracted homeowners because it doesn’t break down. The problem with this is that it has also been show to leach chemicals into the soil.

Unfortunately, schools haven’t gotten that memo, as they have replaced wood mulch with rubber mulch on playgrounds because it is “safer.” Also, if the mulch sinks into the soil, it will be really tough to get it back out and could be stuck there for a really long time. And there are concerns that the rubber mulch may be a fire hazard (more so than wood chips).

I will caution you that if wood mulches stay overly wet for a long time, you will see fungal bodies start to develop. Mulch is always in the process of composting, and fungus is the main life form that does it. When conditions are right, you may see mushrooms form. They are harmless, but to limit them, reduce the depth of mulch to help it dry out faster.

One problem fungus is called artillery fungus, which, to reproduce, shoots packets of black, tarlike spores. They generally aim their artillery spores toward the sunlight. This means that white siding on houses and white cars can become easy targets for the spores. Unfortunately, they do not easily come off and will stain surfaces.

Some people find that the only way to deal with the damage is to replace siding or paint the car, but beware: Insurance companies don’t usually cover this damage. There really isn’t a good control besides regular removal and replacement of mulch.

In the vegetable garden, wood chips are not the best idea, as you will be replanting annually and they can get in the way. Plus, wood chips tie up nitrogen from the soil in the area immediately below the mulch. This isn’t a problem for perennials, trees or shrubs, since they are generally deep-rooted. But vegetable plants usually have shallow roots, so heavy mulching with wood chips may reduce the nutrients available to these plants.

Instead, use straw or even newspaper (shredded or in sheets) to mulch in the vegetable garden. It will greatly reduce weeds and keep diseases like early and late blight in tomatoes from splashing onto the bottom leaves of plants where they will begin their infection.

When applying mulch to trees and shrubs, you’ll want to keep it well away (at least a few inches) from trunks. Piling the mulch right up against the trunk can create a dark, wet area that can damage the plant and make it easier for diseases to infect the plant. Even professional landscapers don’t get this. I cringe every time I see what we in the industry call a “tree volcano.”

There are several ways you can find good mulch for the landscape. The most common way is to purchase mulch in bags or in bulk from a local garden center. Bags are convenient, but often cost more. If you are lucky, you may be able to talk to a local arborist or tree cutting company and ask for their wood chips. You’ll want to pile them up and age them for a while before using them. You’ll also want to make sure that they don’t contain any black walnut trees, which release a chemical called juglone that kills other plants.

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