Weeds in landscapes

A mulch is any material placed on the soil to cover and protect it. Mulches suppress annual weeds by limiting light required for weed establishment. Many types of landscape mulches are available. The most common are bark and other wood products and black plastic or landscape fabric materials. Other products that are used include paper, yard compost, hulls from nuts (such as almonds) or cereals (rice), municipal composts, and stones.

Organic mulches

Organic mulches include wood chips, sawdust, yard waste (leaves, clippings, and wood products), and hardwood or softwood bark chips or nuggets. Bark chips are moderate-sized particles (1/4 to 1/2 inch) and have moderate to good ability to withstand decompostion, while bark nuggets are larger in size (1/2 to 1 1/2 inches) and have excellent stability over time. All of these can be used in landscape beds planted with herbaceous or woody ornamentals. Larger mulch pieces (greater than 1 1/2 inch) do not provide good weed control; the space between the pieces allows weeds to grow through.

The thickness or depth of a mulch necessary to adequately suppress weed growth depends on the mulch type and the weed pressure. The larger the particle size of the organic mulch, the greater the depth required to exclude all light from the soil surface. Coarse-textured mulches can be applied up to 4 inches deep and provide long-term weed control. Fine-textured mulches pack more tightly and should only be applied to a depth of about 2 inches. If the mulch is too decomposed, it is  a weed propagation medium rather than a means of prevention. Plan to periodically replenish organic mulches, regardless of particle size, because of decomposition, movement, or settling. If seedlings germinate in mulches, a light raking, hoeing, or hand-weeding will remove the young weeds.

Inorganic mulches

Including both natural and synthetic products, these mulches are generally more expensive and less widely used in the landscape. Natural inorganic mulches are stable over time and include materials such as sand, gravel, or pebbles. Most of these products are used in public and commercial plantings. If using a rock mulch, consider placing a landscape fabric underneath it. The fabric creates a layer between the mulch and soil, preventing rock pieces from sinking into the soil. The fabric prevents soil from moving above the rock layer, which would bring weed seed to the surface.

Black plastic (solid polyethylene) can be used underneath mulches to improve weed control. It provides excellent control of annual weeds and suppresses perennial weeds, but lacks porosity and restricts air and water movement. It also tends to tear and break apart rather quickly. For this reason, black plastic may not be the preferred long-term weed control method in landscape beds.

Synthetic mulches

Manufactured materials, called geotextiles or landscape fabrics, have been developed to replace black plastic in the landscape. Geotextiles are porous and allow water and air to pass through them, overcoming the major disadvantage of black plastic. Although these materials are relatively expensive and time-consuming to install, they become cost-effective if the planting is to remain in place for 4 or more years. Geotextiles are used mainly for long-term weed control in woody ornamental trees and shrubs. Geotextiles should not be used where the area is to be replanted periodically, such as in annual flower beds or in areas where the fabric would inhibit the rooting and spread of ground covers. Tree and shrub roots can penetrate the materials and if the material is removed, damage can occur to the plant’s root system. This might be a concern if a fabric has been in place longer than 5 years. One  geotextile fabric (BioBarrier) has an herbicide encapsulated in nodules on the fabric that reduces root penetration problems.

Placing a landscape fabric under mulch results in greater weed control than mulch used alone. There are differences in the weed-controlling ability among the geotextiles: fabrics that are thin, lightweight, or have an open mesh allow for greater weed penetration than more closely woven or nonwoven (spunbonded) fabrics.

To install a landscape fabric, you can plant first and then install the fabric afterwards using U-shaped nails to peg it down. After laying the cloth close to the ground, cut an “X” over the plant and pull it through the cloth. If laying down a fabric before planting, cut an “X” through the fabric and dig a planting hole. Avoid leaving soil from the planting hole on top of the fabric because this could put weed seeds above the material. Fold the “X” back down to keep the geotextile sheet as continuous as possible. Weeds will grow through any gap in the landscape fabric, so it is important to overlap pieces of fabric and tack them down tightly. Apply a shallow mulch layer (about 1 inch deep) to thoroughly cover the fabric and prevent photodegradation. If weeds grow into or through the geotextile, remove them when they are small to prevent them from creating holes in the fabric. Maintain a weed-free mulch layer on top of the fabric by hand-weeding or by applying herbicides. Use of a rock mulch above a landscape fabric can have greater weed control than fabric plus organic mulch combinations, but may stress plants in some situations. See Problems section following.

Special consideration should be given to the fact that yellow nutsedge grows through all geotextiles. However, some fabrics are better at suppressing yellow nutsedge than others. For more information, see Pest Notes: Nutsedge.

Problems with organic and natural inorganic mulches

There are several problems associated with the use of organic and inorganic mulches. Perennial weeds such as field bindweed and nutsedges often have sufficient root reserves to enable them to penetrate even thick layers of mulches. Some annual weeds will grow through mulches, while others may germinate on top of the mulch as it decomposes. Weeds that are a particular problem are those that have windborne seeds such as common groundsel, prickly lettuce, and common sowthistle. Applying mulches at depths of greater than 4 inches may injure plants by keeping the soil too wet and limiting oxygen to the plant’s roots. Disease incidence, such as root or stem rot, may increase when deep mulches are maintained.

When mulches are too fine, applied too thickly, or begin to decompose, they stay wet between rains and allow weeds to germinate and grow directly in the mulch. For best weed control, use a coarse-textured mulch with a low water-holding capacity. When used alone, mulches rarely provide 100% weed control. To improve the level of weed control, one can apply preemergent herbicides at the same time as the mulch. Supplemental hand-weeding or spot spraying may also be needed.

Avoid mulches with a pH less than 4 or that have an “off odor” such as ammonia, vinegar, or rotten egg smell. These mulches were stored incorrectly and contain chemical compounds that may injure plants, especially herbaceous plants.

If using a composted mulch, temperatures achieved during the composting process should have killed most weed seeds. However, if the compost was stored uncovered in the open, weed seeds may have been blown onto the mulch. Be sure the mulch is not contaminated with weed seeds or other propagules such as nutsedge tubers.

Rock mulches should always be used with a landscape fabric underneath. Avoid getting soil or other growing media between the rocks or else weeds will grow there. Removal of these weeds by hand weeding or hoeing is very difficult. White rocks are very reflective and this increased light can damage sensitive plants. Dark colored rocks will retain heat and may also cause plant stress.

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