The grass family, Poaceae, is among the largest of plant taxonomic groups and includes species known as lawn, bamboo, grain and ornamental grasses. Ornamental grasses display attractive leaves or seed plumes and gardeners use them just like perennials, providing color and textural interest in mixed plantings or in meadows. Grasses develop fibrous root systems that spread out to encounter water and nutrients. The size and depth of the root system varies among species as well as to the soil conditions in which they grow.
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Grasses do not develop carrotlike taproots, and if they do, it's only initially immediately after the seed germinates and tries to quickly establish. Dig up any grass and you encounter a fibrous, branching root system that spreads. Some species of grasses develop spreading, rootlike stems called rhizomes or stolons that move outward from the main plant. As the rhizomes or stolons spread, they sprout new plants on the soil surface, helping to create a thicket of plant clones. Although started from a rhizome or stolon, the new plant further develops a fibrous root system of its own to sustain the leafy growth it later produces.
Grasses native to or growing in soils that are naturally moist develop shallower root systems compared to grasses from drier regions or seasonally dry soils. If adequate moisture and nutrients exist in the top 18 inches of soil, there is no need for the grass plant to expend energy growing deeper roots to search for additional resources. Conversely, grasses naturally adapted to seasonal droughts or arid regions develop fibrous roots at many levels, including a few roots that grow several feet deep so that some moisture is always available to the plant. In cold or dry periods, the grasses become dormant, but the roots remain alive because of moisture and storage of carbohydrates deep below the soil surface. This is commonly seen in other grasses, such as those used in lawns. Drought- and heat-intolerant Kentucky bluegrass grows shallow roots, but the more drought-tolerant Bermuda grass drives some roots as deep as 7 feet to find water.
Ornamental grasses are generally easy to dig and transplant with success. Even though thin roots may spread widely or penetrate deeply in various areas of the soil, cutting the roots during digging is not detrimental to the plant's survival. On any ornamental grass, digging a root ball that is 12 to 18 inches deep retains the majority of the main core of roots. After transplanting, enough roots remain to sprout and regrow into the soil in the new location. Ornamental grasses transplant best when dug and planted during the spring into summer time frame, as the warming soil promotes and corresponds with the natural root-growing time in the plant's life cycle.
While digging deeply and preserving all the roots of an ornamental grass isn't necessary when transplanting, the density of roots can create digging challenges. A small grass such as blue fescue (Festuca ovina) creates a dense root clump, but is easy to dig around, lift and move. A larger-sized grass, such as a sugarcane (Saccharum spp.) or pampas grass (Cortaderia spp.), develops a dense thicket of roots and stem bases. Cutting the roots isn't difficult, but piercing into the soil among a dense cluster of roots and stems can make transplanting difficult. The larger mass of stems and roots naturally creates a larger and heavier root ball that must be lifted and moved.