To Mulch: Organic or inorganic?

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Tom Woods 

Master Gardener

Mulch choices can say as much about the gardener as they do about the mulch itself. A stony mulch of white marble tends to look formal, “imported,” expensive and hot. A mulch of lawn clippings suggests the gardener values easy access, recycling, organic approaches and saving money.

The most basic division among mulches is some are inorganic and some are organic, but some also are easier to find or apply. Some are semi-permanent, while others aren’t. As a group, garden and planting bed mulches are diverse, with widely varying pros and cons on everything from controlling soil erosion to attracting termites.

Weighing the inorganics

The inorganic options include some materials gardeners may not think of as mulches, such as black plastic sheeting and fiber mats. Even so, the more popular choices tend to look like or be rocks: gravel, pea gravel, pebbles, river rock, lava rock and chipped or shredded rubber tires. Depending on the site, these materials can look natural, unusual, exotic or near alien. What determines an option’s visual “tone” is often how it blends, complements or contrasts with the area’s native rocks and soil.

Because they last longer, inorganic mulches tend to cost more than the average organic product, but the rock-like types also trap empty spaces as they pile up in a layer, spaces that will gradually fill up with mud from below and with debris and seeds from above. At the least, you have to put porous weed-barrier fabric under them, to keep the rocks from slowly sinking into the ground. If they get kicked or cat-scratched out of place, the rock types also can be a mowing problem. And, if you ever change your mind, the fabric and inorganic mulch will be hard work to remove.

As do all mulches, the rock types can provide moist cover for insects, but if layered deeply enough, they help control weeds. They also can play mulch’s traditional role of evening out nature’s fluctuations in soil moisture and temperature, but perhaps not in ways homeowners expect.

For example, rocks and rubber can often lead to extra watering. Their heat-absorbing and heat-reflecting properties speed up evaporation. And, the heat itself can impact the health and sometimes the very survival of plant roots. Next to a house, rocks can actually affect home cooling costs. Shiny and/or dark materials tend to be the worst. Two particular types of inorganic mulch require extra investigation and thought before buying:

1) University research has found gravel can have a high pH level that can lead to chlorosis or iron deficiency in susceptible woody plants.

2) The rubber mulch that became popular on school playgrounds also earned kudos for recycling troublesome trash...used tires. But teachers soon learned that manufacturers often couldn’t remove all of the bits of steel from radial tires. They learned that when ignited by an open flame, rubber burns hotter than wood mulch, and the fire is harder to put out. They discovered sun-heated rubber smells and can trigger allergy-type reactions.

Assessing the organics

Any organic mulch is better for plants, soil and home air conditioning costs. Used correctly, the organics can do everything a good mulch is supposed to do. Because they’re organic, though, they’ll also gradually decompose, enriching the soil and improving its structure.

In general, the smaller and finer the pieces, the more often gardeners have to replenish organics, and the more easily the material can blow or wash out of place. Peat moss, for example, has such small, fine particles that it works better as a soil amendment than as a mulch. Cottonseed hulls, crushed leaves and sawdust can be hard to keep in place, although the latter two can compact, instead.

You have to be sure organic mulches aren’t contaminated by weed seeds, which can be the case with some straws and hays that haven’t aged for a several years. Composts can have that problem, too, if they aren’t fully “cooked.” These are materials vegetable gardeners want to plow in at the end of the season.

Many decorative types of mulch are a byproduct of trees. They may be shredded or chipped. Those with big, coarse pieces tend to last longest, as do those from such pest-resistant woods as cypress and cedar. If you use mulch that may contain non-resistant woods, you shouldn’t pile it deeply or plow it in later. You shouldn’t even think of placing in contact with your house.

Herbicide carryover can be a lawn-clippings mulch problem. With most types of weed killers, clippings from the fourth mowing after treatment will be safe to use. If, however, the lawn was treated with a product containing quinclorac (Drive), homeowners should just let the clippings drop back into the grass. The great thing about lawn clippings is they’re free, but you need to let them dry out for several days before using them as mulch. Wet clippings can form a mold so hard that water can’t pass through.

Shredded newspapers are another organic that’s basically free. And, leftover sections of newspaper, laid flat on the ground, can function for a while as a weed barrier “fabric.” To be safe, stick to newspapers without colored ink, especially around edibles. To make the freebees look more expensive, remember that you can always spread a thin layer of a better-looking mulch product over your newspaper sections or grass clippings mulch.