Mechanical Weed Control

By Hand

Hand-pulling, cutting or hoeing low density infestations of annual weeds from desert landscapes can be effective. However, disturbing the soil in the process of hoeing or removing weeds creates an ideal environment for seed germination and weed establishment. Mixing the soil may bring more weed seeds to the surface where they can germinate and the disturbed soil surface can trap weed seeds that are washed or blown in from adjacent areas. In desert areas or pea graveled sites, any sharp instrument like a hoe or knife can be used to control annual weeds. These tools should be sharpened frequently. In areas with larger rock, weeds should be hand-pulled or controlled with herbicides. Annual weeds should be removed before they produce seeds that will result in more weeds in future years. Grass (monocot) weed seeds survive 1 to 3 years. Broadleaf (dicot) weed seeds can survive 5 to 10 years but some broadleaf weeds produce large seeds with thick seed coats that can survive up to 50 years or more (e.g., little mallow, field bindweed, and morningglory).

Perennial weeds require repeated removal of above-ground portions of the plant to exhaust the underground food reserves. Following hoeing or close mowing, perennial plants use stored food reserves to regenerate shoots and foliage. When these shoots reach sufficient size, they replenish the underground food reserves. Thus, to eradicate perennial weeds mechanically, the foliage must be destroyed every two to four weeks for one or two years. If the foliage is allowed to regrow, then perennial weeds recover and eradication is delayed. After five or six timely hoeings, perennial weed density will decline but mechanical control must be continued until the infestations are eradicated. Perennial weeds are more difficult to control due to their ability to generate new growth from rhizomes, stolons or tubers and the large size of the underground root structures makes it very difficult or impossible to eradicate them by digging up the underground portions of the plants.

Physical Barriers and Mulches

Using plastic tarp and landscape fabrics to prevent weed growth in rock-covered areas is not always practical. Plastic tarps and landscape fabrics can be used effectively in small areas where perennial weeds aren't present. Perennial weeds can grow through most plastic barriers and all mulches. Eventually plastic degrades and becomes unsightly. Plastic also reduces the performance of preemergence herbicides by preventing good soil contact and restricts the use of rainfall and irrigation water by ornamental trees and shrubs.

A landscape mulch is a layer of either inorganic or organic material placed on the soil surface. When the mulch layer is sufficiently deep, weed emergence is greatly reduced but not entirely eliminated. Where herbicide use is restricted by the presence of sensitive ornamental plants, the combination of a mulch and hand pulling of small weeds before they flower and produce seed can be a very effective weed control strategy. In addition to weed control, other potential benefits of a mulch include: improved appearance of the landscape, water conservation through reduced evaporation from the soil surface, reduced soil erosion by wind and water, and reduced crusting of the soil surface. For organic mulches, potential benefits also include increased water absorption, percolation and aeration as the soil structure is improved over time by the addition of organic matter. Several potential problems can result from using a mulch: weed seeds may be introduced into the landscape with organic mulches, mulch materials on slopes may be moved from their location by runoff water, organic mulches may induce nitrogen deficiency in ornamental plants, and excessively deep organic mulches around woody plants can facilitate the growth of pathogens that cause stem rots and restrict water infiltration into the soil.

The most commonly used mulches in Arizona landscapes are rock and shredded or chipped wood and bark. Inorganic mulches such as crushed granite, gravel, or various size categories of rock provide effective long-term cover and weed control if applied sufficiently deep (one to two inches deep depending on particle size). Organic mulches such as chipped wood and bark provide the greatest benefit to plant growth through the improvement of soil structure but are the most expensive to maintain because they last only one to three years. Organic mulches are generally applied two to three inches deep to prevent weed growth. Fine textured organic mulches are not effective for weed control because they provide an ideal medium for weed seed germination. Larger size organic mulches gradually breakdown into smaller particles and over time also become a good medium for seed germination. Small and fine textured rock or other inorganic mulches can accumulate enough dust and fine wind blown dirt over time that they too support seed germination. In inorganic mulches, preemergence herbicides can be used to prevent weed germination and emerged weeds can be hand-pulled or controlled with postemergence herbicides. Preemergence herbicides are ineffective in organic mulches because they are bound to the organic material in the mulch. Thus, only post-emergence herbicides or hand weeding can be used to control emerged weeds in organic mulches.

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