Short answer
Shrubs are woody plants: their stems are permanent, thicken over the years and typically branch near the base. Grasses, including ornamental grasses, are herbaceous plants in the grass family (Poaceae). They send up soft stems and narrow leaves each season from a crown at or near ground level, and they have no woody framework above ground.
Woody versus herbaceous
- Shrubs keep a woody skeleton year after year. Deciduous shrubs lose their leaves in winter but keep their branches, and evergreen shrubs keep leaves as well. New growth forms on old wood.
- Grasses grow new blades and flower stems from the crown each spring or summer. Warm-season grasses go dormant and turn straw-colored in winter, and the top growth is cut back so new growth can emerge.
Bamboo is a useful edge case. It is botanically a grass, but its stems are hard and woody-feeling and persist for years, so it behaves a bit like a shrub or small tree in the landscape (see growing bamboo in your yard).
Structure, growth and seasonal change
Shrubs give a garden permanent shape: a hedge, a rounded foundation plant or a flowering feature. Their form changes slowly. Grasses offer texture, motion and seasonal change, growing quickly, peaking in late summer or fall and fading to tan in winter. Grasses are also generally fine-textured, with narrow, linear leaves, while many shrubs have broader leaves or dense foliage.
Pruning and care compared
- Shrubs: pruning depends on species and bloom time. Cutting at the wrong time can remove flower buds, so check the particular plant. Shaping and thinning are usual.
- Grasses: most deciduous clumpers are cut to a few inches above the ground once a year in late winter or early spring. Evergreen grasses and sedges generally need only light tidying.
- Both can need division or renewal over time, though grass clumps are more often divided.
Grass versus grass-like
Some plants look like grasses but are not. A traditional garden rhyme says “sedges have edges, rushes are round, grasses have knees that bend to the ground”. Sedges (Carex) and rushes (Juncus) are often sold alongside ornamental grasses and used in similar ways. See grasses vs sedges for how to tell them apart.
Using both in a landscape
Pair them to get the best of each: shrubs for year-round structure and grasses for movement and softness. A fountain grass or feather reed grass in front of a boxwood, spirea or hydrangea is a typical combination. See pairing ornamental grasses with hydrangeas and flowering shrubs and what shrubs look good together. For a broader design framework, see landscaping with ornamental grasses.