Japanese stiltgrass (Microstegium vimineum) is an aggressive, non-native annual grass that has rapidly invaded turfgrass and natural areas across the eastern United States. This fast-growing weed forms dense mats that suppress desirable lawn grasses, making its control a common landscape challenge. Managing it requires selectively killing the weed while leaving the surrounding turfgrass unharmed. Successfully achieving this selective control depends on understanding the plant’s life cycle and applying the correct chemical or cultural strategy at the precise time. This requires a targeted approach leveraging the biological differences between the invasive annual and the established perennial turf.
Understanding the Target: Japanese Stiltgrass Lifecycle
Japanese stiltgrass is a summer annual, meaning it completes its life cycle within a single growing season and relies entirely on seed for the following year’s growth. Germination begins unusually early, often in late winter or early spring, even before common weeds like crabgrass sprout. This early start gives it a competitive advantage over many desirable turf species.
The plant develops a shallow, fibrous root system, but it also develops aerial rootlets, or “stilts,” from nodes along its sprawling stems. Later in the season, typically from mid-September through October, each plant produces numerous seeds. These seeds fall to the soil, creating a persistent seed bank that can remain viable for several years. Preventing this annual seed production is the most important long-term control objective.
Selective Pre-Emergent Control Methods
Pre-emergent herbicides are a foundational component of long-term control, as they prevent stiltgrass seeds from sprouting. These chemicals establish a barrier in the upper soil layer that kills the seedling as its root tip emerges from the seed. For selective control in turf, the active ingredients Prodiamine, Pendimethalin, and Dithiopyr are highly effective against Japanese stiltgrass.
The timing of application is important because stiltgrass germinates earlier than other summer annuals. Applicators should target an application window in late winter or early spring, weeks before the typical crabgrass prevention timing. Rainfall or irrigation is required after application to incorporate the herbicide into the soil surface, activating the chemical barrier. Consulting product labels for region-specific application rates and timing is necessary to maintain the health of the established lawn while maximizing weed control.
Selective Post-Emergent Control Methods
When Japanese stiltgrass has already emerged, post-emergent herbicides are necessary to kill the growing plant without damaging the turf. The most reliable selective option for cool-season turfgrasses, such as fescue and bluegrass, is Fenoxaprop-p-ethyl. This active ingredient is a grass-selective herbicide designed to control annual grasses, like stiltgrass, while being safe for established perennial turfgrasses.
Fenoxaprop-p-ethyl works by inhibiting lipid synthesis in susceptible plants, causing growth cessation and eventual death. Applying this chemical early in the plant’s life, typically in mid-summer when the stiltgrass is still young, provides the best results and requires lower application rates. Higher rates or repeat applications may be necessary for dense, mature infestations, especially if the plants have started to tiller and spread.
Non-Chemical and Cultural Management Strategies
Mechanical and cultural practices should complement chemical control, particularly to reduce the size of the seed bank. Hand-pulling is a viable option for small, scattered infestations because the plant’s shallow root system allows for easy removal. This manual removal must be completed before the plants set seed in late summer to prevent new seeds from entering the soil.
Maintaining a dense, healthy lawn is one of the most effective long-term cultural controls. A thick turf canopy shades the soil and physically inhibits stiltgrass germination. Raising the mower blade to the highest setting encourages the desirable grass to grow taller and cast more shade. Mowing or trimming the plants in late summer, just before they flower, can prevent seed head formation.