Herbicides

Herbicide Group 5 Target

Photosystem II (PSII) Inhibitors — block electron transport; common in pre/post grass & broadleaf control.

Herbicide Group 6 Target

Photosystem II Inhibitors (different binding site than Group 5); used on broadleaf weeds (e.g., Bromoxynil).

Herbicide Group 7 Target

Photosystem II Inhibitors — mainly soil-active; inhibit photosynthesis by blocking electron flow.

Herbicide Group 10 Target

Glutamine Synthetase Inhibitor — causes ammonia buildup; fast-acting burn-down (e.g., Glufosinate).

Herbicide Group 14 Target

PPO Inhibitors — cause membrane destruction via reactive oxygen; good for small broadleaf weeds (e.g., Flumioxazin).

Herbicide Group 15 Target

Long-chain Fatty Acid Inhibitors — root & shoot inhibitors; strong pre-emergent grass control (e.g., Metolachlor).

Herbicide Group 22 Target

Photosystem I Electron Diverters — produce free radicals that destroy cell membranes (e.g., Paraquat).

Herbicide Group 27 Target

HPPD Inhibitors — block carotenoid synthesis causing bleaching/whitening (e.g., Mesotrione).

Glyphosate MOA / Group

Group 9 — Inhibits EPSPS (Aromatic Amino Acid Synthesis).

Imazapyr MOA / Group

Group 2 — Inhibits ALS/AHAS Enzyme (Amino Acid Synthesis).

Triclopyr MOA / Group

Group 4 — Synthetic Auxin / Plant Growth Regulator.

What is a pre-emergent herbicide?

Applied before germination; prevents root/seedling growth (e.g., Pendimethalin).

What is a post-emergent herbicide?

Applied to actively growing plants (e.g., Glyphosate).

What does a non-selective herbicide do?

Kills nearly all plants it contacts (broad-spectrum).

Herbicide Group 1 Targets

ACCase inhibitors — lipid synthesis blockers; grass-selective (e.g., Clethodim).

Herbicide Group 3 Targets

Microtubule/root inhibitors — block cell division and prevent root extension (pre-emergent).

Why is Triclopyr selective toward broadleaves?

Monocots (grasses) can metabolize or tolerate it; dicots experience uncontrolled growth and die.

How does Glyphosate behave in soil?

Binds tightly to soil; non-mobile; minimal leaching.

Ideal timing for Imazapyr in woody plant control

Late summer — during root energy transfer so herbicide moves into roots.

What is the "hack-and-squirt" method?

Applying herbicide directly into cuts exposing the cambium to stop resprouting in woody plants.