Ornamental grasses are perennial plants that bring structure, texture, and movement to the garden. The timing of their new growth is not uniform across all varieties, but depends entirely on the specific physiological type of grass planted. Knowing whether you have a cool-season or warm-season variety is necessary for determining the proper schedule for spring maintenance and care. Attempting to prune or divide a grass at the wrong time can significantly stress the plant and delay its seasonal awakening.
Understanding the Core Difference: Cool Season vs. Warm Season Grasses
Ornamental grasses are divided into two categories based on their preferred growing temperatures. Cool-season grasses utilize the C3 photosynthetic pathway, which is more efficient in cooler temperatures and lower light conditions. These varieties put on the majority of their growth during the mild weather of spring and fall.
Warm-season grasses employ the C4 photosynthetic pathway, which is better adapted to high heat and intense sunlight. They remain dormant throughout the winter and early spring, waiting for consistent warmth to begin their active growth cycle. Cool-season types slow down in summer heat, while warm-season types require it to thrive.
Timing the Spring Awakening for Cool Season Grasses
Cool-season ornamental grasses are the “early birds” of the garden and begin their growth cycle quickly as winter fades. New shoots often emerge in late winter or very early spring, typically around March or April, depending on the local climate. These grasses are triggered by the first sustained rise in temperatures above freezing and do not require the soil to be fully warmed.
Examples of these early growers include Fescues, such as Blue Fescue (Festuca glauca), and Blue Oat Grass (Helictotrichon sempervirens). Since many cool-season grasses are semi-evergreen, they may retain some color through the winter, and their new growth appears rapidly from the center of the clump. Their vigorous growth period ends as summer temperatures consistently exceed 75 degrees Fahrenheit, at which point they often slow down.
Timing the Spring Awakening for Warm Season Grasses
Warm-season grasses delay their spring awakening until conditions are reliably warm. This group requires consistently warm soil temperatures, generally waiting until late May or early June to show new growth. The soil must warm up to at least 65 degrees Fahrenheit for their roots to become active, a much later trigger than for their cool-season counterparts.
This delayed start often causes concern for gardeners who see their cool-season varieties already greening up while the warm-season clumps remain brown and dormant. Common examples of these grasses are Miscanthus (Maiden Grass), Fountain Grass (Pennisetum), and Switchgrass (Panicum virgatum). Cutting these grasses back prematurely, before the danger of frost has passed, can expose the tender new shoots to damaging cold.
The key to successful growth is patience, as they will not respond to early spring care. Once the summer heat arrives, warm-season grasses enter their most productive phase, quickly developing their full height and producing their characteristic plumes later in the season.
Essential Spring Maintenance: Cutting Back Old Growth
The dead foliage from the previous season must be removed to ensure the health of the new growth. This old growth can otherwise mat down, blocking sunlight and air circulation needed by the emerging shoots, which can lead to crown rot or fungal issues. The timing of this cut-back must align with the grass’s specific growth schedule.
For cool-season grasses, the old foliage should be cut back in late winter or very early spring, just before the first signs of new green growth appear. Waiting too long risks cutting into the tender new leaves, which can stunt the plant’s early development. These varieties should be cut back only by about one-third of their height, or to about 4 to 6 inches above the crown.
Warm-season grasses are usually cut back later, often in early to mid-spring before their new shoots emerge. These deciduous varieties can be cut back much harder, typically to a height of 4 to 6 inches above the ground. Tying the old foliage into a bundle before cutting can make the cleanup process much easier.