Mesotrione (Tenacity / Torocity / MESO 4SC)

Overview

Mesotrione is the only pre-emergent herbicide safe for use at the time of seeding. This makes it essential for any spring overseeding, fall renovation, or new lawn establishment project. Generic versions (Torocity, MESO 4SC) are chemically identical to the Tenacity brand at a lower price.

How It Works

Mesotrione inhibits a specific enzyme (HPPD) involved in photosynthesis, causing treated weeds to turn white (bleaching) before dying. It provides approximately 28 days of pre-emergent protection — shorter than prodiamine but safe for emerging grass seedlings.

Dual Action

Mesotrione works as both pre-emergent and post-emergent. When sprayed on established weeds, it causes visible white bleaching within days — this means it is working. Effective post-emergent targets include bentgrass, nimblewill, and various broadleaf weeds.

Critical Application Rules

• At seeding: Do NOT add surfactant. The mesotrione alone is sufficient and surfactant can damage emerging seedlings.
• On established grass: Add surfactant for better leaf adhesion and post-emergent activity.
• Maximum 3 applications per year per label.
• Fine fescue caution: Do not use on seed mixes containing more than 20% fine fescue by weight — fine fescue is sensitive to mesotrione.