 Pre-emergent? Post-emergent?: Types of chemical weed killers
Herbicides are used to kill weeds but can have unintended consequences:
- Some are toxic, and some mimic human and wildlife hormones.
- Some are mobile and can get into aquatic environments.
- Some contain acids or oils that can cause damage to eyes and injury to aquatic organisms.
All pesticides, even those labeled as "safer" should be used carefully and never sprayed in or on water.
Pre-emergent herbicides prevent weed seeds from growing and have little or no effect on existing weeds.
- Using a pre-emergent chemical herbicide anticipates a problem that may not exist, so application may not be necessary but will still impact the environment.
- Mulches, fabric barriers, and corn gluten are safer pre-emergent weed controls.
Post-emergent herbicides kill existing weeds. They are classified as selective and nonselective.
- Selective post-emergent herbicides kill either broadleaf weeds or grasses, but generally not both.
- These often contain endocrine disrupting chemicals and may impact human as well as environmental health.
- They are usually used to control weeds in lawns. Weed and feed products contain selective herbicide products.
- Nonselective post-emergent herbicides kill or damage all types of plants.
- "Contact" herbicides destroy only the plant tissues they touch.
- They are typically acid- or oil-based.
- Contact herbicides work well in controlling smaller annual weeds but may not work as well on larger perennial weeds.
- They tend to be the least-toxic herbicides.
- "Systemic" herbicides are absorbed, and circulate through the plant to kill the roots and the top growth.
- Systemic herbicides work well in controlling perennial weeds that re-sprout from roots.
- Roundup® (containing glyphosate) is the most common example.
Some people use household vinegar to kill weeds. New products on the market based on acetic acid, citric acid (found in lemons) or other acids likely work better than plain vinegar. They are stronger and contain a detergent or soap to make the product stick to plant leaves. Acid-based herbicides will kill or damage any plants they touch, including grass, and may be most useful for managing weeds in gravel and on patios and sidewalks. They break down quickly, causing less environmental harm than many other chemical herbicides. Caution: These products contain a higher percentage of acid than plain vinegar and – if splashed in the eyes – may cause severe irritation or even damage.
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