Regional Gardening Reports :: National Gardening Association

In the Garden:
Pacific Northwest
April, 2011
Regional Report

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Well mulched ornamental and vegetable beds rarely develop weed problems.


Nearly every gardening Web site and magazine extols the virtues of mulch, but what exactly is it? And why is it considered as important to gardening as watering, fertilizing, planting, and pruning?
In general, mulch is a material applied to the surface of the soil to help slow evaporation, moderate soil temperatures, suppress weeds, and prevent soil erosion. Mulches can also help maintain the physical structure of the soil.

Types of Mulch
Almost anything that you can spread over the soil will serve as a mulch. It can be inorganic, like stones, plastic film, or shredded rubber, or it can be organi, like leaves, pine needles, bark chips, straw, or compost. Because organic mulches actually improve the soil and inorganic mulches do not, I prefer to use organic materials in my garden.

How Mulch Works
Mulch slows the loss of soil moisture by blocking sunlight and by insulating the soil. When mulch keeps the soil temperature from rising above the air temperature, it limits the amount of water lost through evaporation. Under this blanket of insulation, soil temperatures are buffered from large daily swings of heat and cold. Studies show that extremes in heat and cold can retard root growth. Because of this, maintaining a fairly constant soil temperature is beneficial to plants.

A layer of mulch can save you from spending much of your gardening time weeding. Since many weed seeds need light to germinate, and a thick layer of mulch prevents light from reaching the soil, weed seeds usually won't sprout. If any weeds do manage to sprout, they're easy to pull because their roots will be in the mulch rather than anchored in the ground. And, in their struggle to push their roots through the mulch to reach the soil, they are usually spindly and weak. This makes them that much easier to pull.

A protective layer of mulch can prevent soil erosion by keeping rain droplets and water from the sprinkler from hitting bare soil. If the rain or water from the sprinkler is forceful, it can dislodge individual soil particles, which can wash away. Mulch will break up those droplets, keeping soil particles from running down slopes and mud from splattering onto plant leaves and walkways. When mulch slows the intensity of the flow, water has more of a chance to soak down into the soil.

Organic mulches can improve the fertility and the physical structure of soil as they decompose. When organic mulches make direct contact with the soil, they release their nutrients, along with chemical compounds that help individual soil particles stick together. This allows the soil to drain well and yet hold adequate amounts of moisture and oxygen -- both important to healthy root growth.

Applying Mulch
Mulch is relatively easy to apply. I use a wheelbarrow to transport both bagged and loose mulch materials to the various beds in my garden. I open the bags, dump them into the wheelbarrow, then use a shovel to scoop the material from the wheelbarrow into piles between my plants. I usually spread mulch with a leaf rake, but I've also been known to crawl around on my hands and knees to work it into tight places between plants.

As a rule, loose mulches should be laid more thickly than finer-textured ones. A two inch deep layer of screened bark chips should be adequate, but you'll need 3 to 4 inches of pine needles or 2 to 3 inches of wood chips to provide the same protection.

It's important that the mulch not be packed against the stems of your plants or the trunks of your trees. If it comes in close contact, it can hold too much water against the stems, causing rot. To avoid potential problems, I keep mulch materials an inch or two away from the stems of my plants and the trunks of my trees.

Organic mulches require occasional replenishment as they break down. Compost, shredded leaves, and grass clippings may have to be replenished once or twice a year; more durable materials such as pine needles, wood chips, or bark can go two or three years without a fresh layer.

With the softer materials, I tend to dig the old mulch into the soil before adding a fresh layer. With the harder materials I generally rake the mulched area once or twice a year to "fluff" it up and then add a new layer each spring.

Mulch is so beneficial to the soil and the plants that spreading it gets top priority on my list of gardening chores. Do your garden a favor and put it at the top of your list, too!

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