Calamagrostis x acutiflora ‘Karl Foerster’ is a celebrated ornamental grass, valued for its striking, strictly vertical form and early emergence. Proper planting distance for this feather reed grass is a deliberate choice that determines the final appearance and health of the landscape design. Correct spacing is the primary factor in achieving either a dense, uniform screen or showcasing the distinct shape of a single specimen plant. This initial decision is paramount because the grass’s growth habit makes correcting crowding issues difficult later on.
Understanding Mature Size and Growth Habit
This particular cultivar is a cool-season, clump-forming grass, meaning it maintains a tight base and expands slowly outward from the center, unlike rhizomatous grasses that spread aggressively. The mature height of the foliage blades typically reaches between 3 to 4 feet, but the slender, wheat-like flower plumes extend the total height to 5 to 6 feet in optimal conditions. The basal foliage clump itself is relatively narrow, generally achieving a mature spread of 2 to 2.5 feet wide.
The clumping nature ensures the plant will not quickly fill large gaps between individual plants. This predictable, contained growth allows for precise planning, but initial under-spacing will result in a sparse, disconnected look for several years. Since the seeds are sterile, the plant only expands slowly by its root crown. The upright, narrow habit allows this tall grass to be used effectively in relatively small spaces.
Spacing for Specific Landscape Goals
The distance you select, measured from the center of one plant to the next, must directly align with your intended visual outcome. For creating a solid, uninterrupted visual barrier or privacy screen, tight spacing is necessary. Placing the plants 18 to 24 inches apart on center is the recommended minimum distance. This tight arrangement encourages the mature clumps to merge quickly, forming a seamless, dense wall of vertical foliage and plumes within two to three growing seasons.
When the goal is a mass planting or a drift that looks full but still allows individual plants to be appreciated, a slightly wider spacing is more appropriate. A distance of 2.5 to 3 feet on center will allow the plants to touch upon reaching full maturity while maintaining adequate air circulation between them. This spacing promotes healthier growth by reducing moisture buildup that can lead to fungal issues. Using a staggered, triangular pattern rather than a straight line in mass plantings creates a more naturalistic and visually appealing effect.
To showcase Karl Foerster as a standalone feature, or specimen planting, the grass must reach its full, distinct shape without interference. The plant should be spaced a minimum of 3 to 4 feet away from any other large plant or structure. This generous distance ensures the plant has ample room to express its full verticality and highlight its architectural form.
Preparing the Site and Installation
Preparing the Soil
Once the center-to-center measurements are marked, proper installation is necessary for successful establishment. Begin by digging a planting hole that is no deeper than the root ball but is at least twice as wide. This wider dimension allows the roots to easily grow into the surrounding loosened soil, reducing transplant stress. Before placing the grass, incorporate a 2 to 4-inch layer of organic material, such as compost, into the prepared soil to enhance drainage and nutrient retention.
Planting Depth and Watering
When setting the plant into the hole, ensure the crown—the point where the roots meet the stems—is exactly level with the surrounding soil surface. Planting the crown too deeply can lead to crown rot, especially in heavier soils. After backfilling the hole and gently firming the soil to eliminate air pockets, water the plant deeply to fully settle the soil around the roots.