When it comes to weed control, it can be tricky to find a product that selectively kills weeds but not other desired plants. Weeds are often defined as unwanted plants that grow where they are not wanted. They compete with crops and garden plants for water, nutrients and sunlight. While hand pulling weeds works for small infestations, most gardeners and farmers rely on herbicides to efficiently remove large weed populations. The key is finding an herbicide that targets the weeds, without damaging nearby ornamental plants, vegetables, flowers, trees or turfgrass. This largely depends on the herbicide’s mode of action and method of application.
Most herbicides are designed to disrupt biochemical processes that are vital for plant growth and survival. However, weeds and crops vary in their susceptibility to different herbicides. Herbicide selectivity relies on these differences to kill weeds, while leaving desirable plants unharmed. There are several types of selective herbicides:
- Contact herbicides destroy plant tissue on contact. They can selectively remove weeds if applied directly to the weeds, without contacting crop plants.
- Systemic herbicides move within the plant. Selective systemic herbicides are absorbed by the leaves or roots of weeds, transferring to their vascular system which crops do not absorb.
- Hormone herbicides mimic plant hormones like auxins or gibberellins. Low doses stimulate crop growth, while high doses overstimulate weed growth, killing weeds but not crops.
- Mode of action herbicides target biochemical pathways or enzymes that weeds rely on more than crops. For example, ALS inhibitor herbicides block a key enzyme that exists in weeds but not in crops.
Additionally, the timing, rate and placement of the herbicide application affects which plants are impacted. Herbicides may be applied before planting, pre-emergence, or post-emergence of weeds and crops. Lower herbicide rates are often used for selective control, while higher rates are non-selective. Spot treatments directly onto weeds can avoid contact with desirable vegetation. No herbicide is 100% selective, but using the right product at the right time reduces unnecessary plant damage.
Pre-emergent herbicides control weeds before they sprout and are a go-to for selective weed control. They create a chemical barrier that kills emerging weed seedlings, while established plants remain generally unaffected. Timing pre-emergent herbicide application prior to weed seed germination is critical. Consult your local Cooperative Extension for optimal pre-emergent herbicide application timing in your region.
Common selective pre-emergent herbicides include:
- Pendimethalin – controls annual grasses and small-seeded broadleaf weeds in lawns, turf, ornamentals.
- Dithiopyr – selective pre-emergent for crabgrass and other grassy weeds in lawns.
- Oryzalin – prevents germination of annual grasses and small-seeded broadleaf weeds around established trees, shrubs.
- Trifluralin – controls grasses and small-seeded broadleaf weeds in warm season turfgrass, landscape beds.
These residual herbicides break down over time, so reapplication is needed every 1-3 months during the active weed growing season. Furthermore, irrigation or rain after application improves their activation and performance. Use caution when applying pre-emergent herbicides over or near the root zones of desirable plants. Root uptake can sometimes damage non-targeted vegetation.
Post-emergent herbicides are applied after weeds have already germinated and emerged above ground. Systemic post-emergent products translocate throughout the weed’s vascular system after foliar application. Contact options destroy plant tissues and cells directly where applied. Post-emergent herbicides are absorbed quickly through weed foliage and roots to kill existing unwanted plants, rather than preventing new weeds. Their ability to selectively target weeds depends on the product, formulation and timing.
Selective post-emergent herbicides for lawns and ornamental areas include:
- 2,4-D – systemic that controls broadleaf weeds like dandelions, clover, plantain in established turfgrass.
- MCPA – similar systemic activity on broadleaves as 2,4-D but safer for some ornamentals.
- Dicamba – effective on annual and perennial broadleaf weeds, often mixed with 2,4-D or MCPA.
- Quinclorac – controls crabgrass and other grassy weeds in turfgrass and landscape beds.
- Glyphosate – non-selective systemic, but selective when spot sprayed directly onto weeds.
Timing is key with post-emergent weed control. Young, actively growing weeds are most susceptible. Mature weeds that have already flowered and set seed are harder to control. Make applications when weeds are small for best selectivity and performance.
Organic weed control relies on plant-based herbicides derived from natural materials like corn gluten, acids, oils and plant extracts. Organic herbicides are non-toxic, safe for pets and wildlife, and break down quickly in the environment. However, most organic products provide only contact weed control. Corn gluten meal provides pre-emergent control by inhibiting root formation in emerging weeds. Acetic acid (vinegar) and citric acid herbicides create holes in weed cell membranes on contact. Essential oil herbicides like clove, cinnamon and lemongrass oil burn through weed cuticles and foliage. These organic herbicides kill emerged weeds well, but offer very little residual activity to prevent new weeds.
Improving Herbicide Selectivity
Certain management practices can help improve herbicide selectivity when weed and crop plants are susceptible:
- Use low herbicide rates for selective control of younger weeds.
- Make separate applications on different dates to avoid crop damage.
- Spot treat weeds or use a shielded sprayer to avoid crop contact.
- Apply post-emergent herbicides when crops are past the susceptible growth stage.
- Allow sufficient time between pre-emergent herbicide application and planting vegetation.
- Utilize herbicide-resistant crop varieties if available.
Most Common Non-Selective Herbicides
While selective herbicides target specific weeds, non-selective options indiscriminately kill all plant material. Non-selective herbicides are often used for total vegetation control in areas like fence lines, driveways and under gravel. Some common non-selective herbicide active ingredients include:
- Glyphosate – systemic herbicide found in Roundup and other products.
- Glufosinate-ammonium – also systemic, used for vegetation control.
- Diquat dibromide – fast-acting contact herbicide for aquatic and terrestrial weeds.
- Pelargonic acid – organic contact herbicide derived from plants.
Keep non-selective herbicides away from any desired vegetation, as even minor contact can cause irreparable damage. They are useful tools for pavement cracks, gravel areas, fence lines and other zones where plant growth is prohibited.
Weed Control Without Herbicides
For gardeners wishing to avoid herbicides altogether, there are several non-chemical weed management options:
- Hand removal – Hand pull or hoe weeds, roots and all.
- Smothering – Use mulch, plastic sheets or landscape fabric to block light.
- Soil solarization – Solar heating kills weed seeds and seedlings.
- Corn gluten meal – Organic pre-emergent for blocking weed seed germination.
- Vinegar – High concentrations provide non-selective contact weed control.
- Flaming weeds – Heat treatment damages cellular structure.
- Cover crops – Outcompete and smother weeds through rapid growth and biomass.
For severe weed infestations, combinations of these methods may be required. Hand weeding works great for small patches. Vinegar spot sprays remove emerged weeds. Solarization prepares sites prior to planting. And cover crops crowd out weeds throughout the growing season. Though labor intensive, consistency with multiple non-herbicide strategies can provide effective weed control.
Key Tips for Selective Weed Control
Here are some key tips to remember for selectively removing weeds without harming other plants:
- Know the weaknesses of your weed species and target them when most vulnerable.
- Understand herbicide modes of action to choose products that affect weeds but not crops.
- Always read and follow herbicide label instructions for selective use in your situation.
- Apply at the right time in the weed’s life cycle for optimal selectivity.
- Use a shielded sprayer, sponge applicator or spot treatment when needed.
- Combine manual, cultural, and chemical weed control methods.
Controlling weeds without damaging nearby ornamental plants is possible with the right herbicide selection, application method, timing and management. Pre-emergent herbicides selectively form a chemical barrier that prevents vulnerable weed seeds from sprouting. Post-emergent products applied directly to actively growing weeds can eliminate emerged weeds alone when spot treated. Non-selective herbicides kill all plant material and must be kept away from desired vegetation. Contact organics like vinegar offer non-chemical weed control. And hand weeding continues to be the most targeted way to remove weeds without harming other plants. Using multiple selective control strategies is key to effectively managing weeds in lawns, gardens and crop fields.