Cons of Grass Clipping Mulch in a Vegetable Garden

Inhibit Water Penetration

• Because grass clippings are fine textured, they can mat together, especially when fresh and green. Piling grass-clipping mulch thicker than an inch prevents water and oxygen from penetrating the soil, according to the University of Missouri Cooperative Extension. When using grass-clipping mulch in the vegetable garden, spread less than 1 inch thick and let it dry before applying another layer.

Excessive Heat

• Fresh, green grass clippings are high in nitrogen, which makes the clippings heat up as they decompose. As the decomposing grass heats up, it draws in oxygen from the surrounding area. In a compost pile, this helps raise the temperature, killing off weed seeds and speeding decomposition. As mulch in the vegetable garden, the hot decomposition process of fresh grass clippings robs young plants of oxygen and heats the soil's surface, drying it out or overheating plant roots and stems. Mixing green grass clippings with other dry materials, such as dry leaves or wood shavings, increases oxygen availability, advises the University of California at Davis.

Odor

• Thick mats of fresh, green grass precluding sufficient oxygen penetration begin decomposing anaerobically. That process can produce foul odors. Letting thin layers of grass clipping dry before using as mulch or mixing fresh grass clippings with dry materials adds oxygen and makes for a more pleasant looking, smelling mulch.

Herbicides

• Lawn care often includes spraying with weed-killing herbicides or other lawn chemicals, such as insecticides. Using chemical-laden grass clippings in your vegetable garden may harm your vegetable plants, advises the University of California at Davis. Most lawn chemicals break down if the grass clippings are well composted before use in the garden, but these chemicals are likely to still be active if you use fresh lawn clippings as mulch. A few herbicides even survive the composting process, and long-lasting insecticides may kill beneficial insects are necessary for pollinating your vegetable crops. Know what chemicals have been used on your lawn before using the clippings as mulch around your vegetables.