Because many of my clients are busy people hoping to create beautiful yet low maintenance gardens, I am often asked about the use of mulch to suppress weeds. Often the questions is asked as we stand in the garden, staring down at bunches of degrading landscape fabric poking up through a scatter of wood chips. “What,” they ask, “should be done with this?”
Very good question. Let’s start by talking about mulching in general.
Mulching is one of a variety of methods for maintaining a healthy garden. A mulch is any material applied to the soil surface for protection or improvement of the area. Mulching is as old as nature itself: fallen leaves, pine needles, twigs, pieces of bark, spent flower blossoms, fallen fruit and other organic material fall to the ground, and that is mulch.
The English word mulch is probably derived from the German word molsch, meaning soft, beginning to decay. It no doubt referred to early gardeners’ use of straw, leaves, and loose earth spread on the ground to protect the roots of newly planted trees and shrubs.
Mulching has many benefits for the garden:
◦ Mulch prevents loss of water from the soil due to evaporation.
◦ Mulch reduces the growth of weeds, when the mulch material itself is weed-free and applied deeply enough to prevent weed germination and growth.
◦ Mulch keeps the soil cooler in the summer and warmer in the winter, maintaining a more even soil temperature.
◦ Mulch prevents soil splashing during rains, which not only stops erosion but keeps soil-borne diseases from splashing up onto the plants.
◦ Organic mulch can improve the soil structure. As the mulch decays, the material becomes topsoil. Decaying mulch also adds nutrients to the soil.
◦ Mulch prevents crusting of the soil surface, thus improving the absorption and movement of water into the soil.
◦ Mulch helps prevent soil compaction.
◦ Mulch can add to the beauty of the landscape by providing a cover of uniform color and interesting texture to the surface.
There are basically two types of mulches: organic and inorganic. Both types may have their place in the garden.An organic mulch is a mulch made of natural organic substances such as bark, wood chips, leaves, pine needles, or grass clippings. They decompose over time and need to be replaced regularly. As they decompose, organic mulches improve the condition of the soil. They also attract beneficial wildlife such as insects and the birds that eat them.
Inorganic mulches include gravel, pebbles, black plastic and landscape fabrics. They are longer lasting than organic mulches, but do not improve the soil condition and do not attract beneficial wildlife.
Organic Mulch Materials:
Grass Clippings The best use for grass clippings is to leave them on the lawn. Grass clippings will decompose rapidly, adding nutrients back into the soil. A two-inch layer of grass clippings provides weed control if they are not full of weed seeds. It is best to build up the layer gradually using dry grass, not fresh clippings, to prevent the formation of a solid mat. Be careful not to use clippings from lawns that have been treated with herbicides.
Leaves A 2- to 3- inch layer of leaves provides good weed control. It is best to shred the leaves coarsely, using a shredder or your lawn mower. Whole leaves have a tendency to blow away, while finely shredded leaves do not allow water to penetrate. Oak and beech leaves help to acidify the soil for acid-loving plants. Leaves are usually easy to get, attractive as a mulch, and they will improve the soil once they decompose. After the leaves decompose, dig them into the soil and add a new layer of mulch on top.
Pine Bark or wood chips A 2- to 3- inch layer of pine bark is good for weed control. Pine bark makes an attractive, usually dark-colored mulch. It can be purchased in various particle sizes. If you need a large quantity of mulch, you can often have one of the local tree care companies dump a load of chips in front of your home for free – just give them a call and inquire.
Pine Needles A 2- inch layer of pine needles makes an excellent mulch for acid-loving trees and shrubs. This mulch is very attractive and allows water to penetrate easily.
Ground Cover Many perennial ground cover plants, such as ivy, periwinkle, pachysandra, mondo grass and liriope, will cover the soil and act as a mulch. I highly recommend using ground cover as a mulch material: it is organic, attractive, effective and doesn’t blow around or decompose. In combination with other organic mulching materials, perennial ground cover improves the condition of the soil over time.
Inorganic Mulch Materials:
Gravel, Pebbles and Crushed Stone A 1- inch layer of small rocks will provide good weed control. Do not use them around acid-loving plants since the rocks may add alkaline elements and minerals to the soil. These materials reflect solar radiation and can create a very hot landscape environment during the summer months.
Black or clear Plastic Black polyethylene film is very effective in preventing weed growth, however it is not good for the overall health of your garden. Use it to kill weeds prior to planting; if you cover the weed infested area for 3-4 months with black plastic, many of the weeds will be killed due to lack of light and excessive heat. Clear plastic can also be used – this process is called solarization and heats the area to a temperature that kills off the seeds in the topsoil. When used as mulch, plastic impedes drainage, as well as the proper exchange of gasses between soil and atmosphere.
Landscape Cloth or Woven Ground Cloth Materials woven of fabric, plastic or paper are available in various lengths and widths. The materials are treated to resist decomposition. Unlike plastic films, woven materials allow water and air to move through them. They are very effective in controlling most weeds, although some grasses may grow up through the holes in the fabric, and over time a layer of topsoil forms above and the weeds grow there. This is the stuff I often find poking out between the wood chips – not a very attractive solution. While not as damaging as plastic film, I do not recommend landscape cloth as a mulch.
How often mulch needs to be replenished depends on the mulching material. Grass clippings and leaves decompose very fast and need to be replenished frequently. Inorganic mulches such as gravel and pebbles rarely need replenishing. As the plants grow and fill in the bed areas, less and less mulch is needed.
The amount of mulch to apply depends on the texture and density of the mulch material. Many wood and bark mulches are composed of fine particles and should not be more than 2 to 3 inches deep. Excessive amounts of these fine-textured mulches can suffocate plant roots, resulting in yellowing of the leaves and poor growth.
Coarse-textured mulches such as pine bark nuggets allow good air movement through them and can be as deep as 4 inches.
Mulches composed of grass clippings or shredded leaves should never be deeper than 2 inches, because these materials tend to mat together, restricting the water and air supply to plant roots.