Weeds are often noticed when they bloom, but by then, they have competed with desirable plants and can be difficult to control since they are well established. Weeds such as henbit (Lamium amplexicaule), purple deadnettle (Lamium purpureum), chickweed (Stellaria media) and hairy bittercress (Cardamine hirsuta) germinate in fall and early spring. As the soil starts to warm up, they grow rapidly and produce conspicuous blooms.
It is prudent to execute control measures before such weeds germinate. Applying a suitable mulch is a common practice to reduce weed germination by blocking sunlight. Mulches also serve a physical barrier that affects seed germination, conserves soil moisture and reduces the incidence of soil-borne diseases.
Mulches to control weeds in the landscape include landscape fabric, wood chips and shredded wood. Landscape fabric is effective in perennial plantings, especially if they are installed prior to establishing a flowerbed then covered by a desirable medium. Aged wood chips are better than fresh chips to reduce tie-up of soil nitrogen required for plant growth. If fresh chips are used, apply a layer of compost beneath the mulch.
Temporary mulch materials may include black plastic, straw over newspaper and dry grass clippings. Avoid using pine needles since they may affect plant growth. Also, avoid certain colored wood mulches as they can injure certain plants. If using lawn clippings, make sure that the lawn was not treated with herbicides or fertilizer-based herbicides. No more than 4 to 6 inches of mulch is needed in landscape plantings. A 6-mil black plastic is more effective than thin plastic, and landscape pins may be used to secure it to the soil.
Applying mulch before weed seeds germinate is critical for successful weed control. If weed seedlings have already emerged, a light cultivation or hoeing may be necessary to kill emerged weeds prior to applying the mulch. Certain pre-emergent herbicides, such as Snapshot or Pennant Magnum, may be used for control in landscape plantings. If using herbicides, please refer to the label to ensure that they are safe for the ornamentals used and apply them at the labelled rates and timings to avoid plant injury.