Properly managed ground covers can also promote water penetration rather than runoff, and provide a firm surface for walking or driving. Eliminating all weeds may also eliminate native predators. Predators feed on insect and mite pests, but most of them can also feed on, or may even require, pollen and nectar to complete their life cycle. By having some flowering weeds and grasses in the ground cover mix or field borders, predators will be more likely to stay in the trees to feed on pests. The best pollen nectar sources are large open flowers like wild carrot, yarrow, and buckwheat.
Green lacewings require grass pollen to reproduce.
The goal of ground cover management is to promote the growth of native plants as a ground cover to protect the soil, promote good tree root growth, and provide a habitat for natural insect and mite predators without hurting tree growth and quality. How competitive a weed is with a Christmas tree is often determined more by its rooting than its aboveground growth habits. In general, plants with thick, fibrous root systems, like cool-season grasses, will exclude and outgrow Fraser fir roots from growing and are, therefore, highly competitive. A weed like wild carrot that only has a deep taproot is much less of a problem. Other plants, such as wild violet, produce prolific rhizomes that can once again exclude fir roots.