The Homeowners Column

The Homeowners Column

A Few Herbicide Basics

Photo of Sandra Mason

Sandra Mason
State Master Gardener Coordinator
slmason@illinois.edu

As gardeners we make choices all the time about what plant lives and what plant ends up on the compost pile. I've ripped out many plants that a few years before in my naiveté I had purchased and planted with the dream of beauty and brevity in maintenance. But they quickly became thugs and I cursed their very existence. Whether it's crabgrass or ribbon grass, sometimes we will need to edit in our gardens. Weeds are just our personal definition of a plant out-of-place.

"Know thy enemy" In last week's column I discussed the importance of identifying the weedy culprit. Identification provides information about the plant's life span (does it come back every year from seed or does the mother plant return each year with all her baby seedlings?) and reveals the time of year it is most vulnerable to management. I also discussed all the many ways we can lessen unwanted plant issues: physical removal by hoeing, tilling or pulling; applying mulch to beds and paths; and planting cover crops or ground covers. Whatever you do, do not let weeds produce seeds.

Remember in your lawn, garden or driveway, bare soil is the battle ground. Like patrons to an outdoor concert, weedy plants look for spots not covered with a picnic blanket.

Herbicides are another tool in the tool box of weed management. Implement all the other tools first before reaching for an herbicide.

First let's define a few terms. Herbicides are pesticides. A pesticide is the general term describing any chemical used to destroy, prevent or control any form of life declared to be a pest. Insecticides, rodenticides and fungicides are all pesticides. Pesticides may be organic or inorganic (synthetic). If it's meant to kill something, it's a pesticide.

Herbicides may be selective or non-selective in what plants they kill. Selective herbicides are formulated to kill one group of plants with little or no harm to another group. For example 2, 4-D lawn weed killers selectively control broadleaved plants such as dandelions while not harming lawn grass. Selective herbicides are useful in areas where we have desirable and undesirable plants growing together. Non-selective herbicides such as glyphosate or herbicidal soap control or will cause damage to most any plant. These can be particularly useful in non-garden areas such as driveways, paths or brick patios. Non-selective herbicides require great care in usage to prevent damage to desirable plants in garden areas.

Herbicides work by contact or as a systemic. Contact herbicides kill plants through direct contact to the unwanted plant; therefore, good spray coverage of the entire plant is crucial. Contact herbicides are typically non-selective in what plants they kill. Systemic herbicides such as glyphosate move within the plant to other parts of the plant; therefore, covering the entire plant with spray is not as crucial. Systemic herbicides are particularly useful with perennial weeds and hard-to-kill weeds such as poison ivy because the herbicide can move into and kill underground roots.

The timing of herbicide applications depends on the intended weeds and on how the herbicide works. In general garden herbicides are either preemergent or postemergent. Preemergent herbicides such as corn gluten for crabgrass control must be applied before seedlings emerge and can be effective against annual weeds that return each year by seed. Postemergent herbicides control existing weeds. Generally postemergents are much more effective on young plants and actively growing plants.

Herbicides may be inorganic (synthetically produced in a lab) or organic (originate from naturally occurring chemicals).

My apologies to people who hate sequels. I had not originally intended to make this herbicide discussion a two-parter. Next week I discuss more specifics of organic and inorganic herbicides and homemade herbicide mixtures. On that note, do you have a home remedy you use to kill weeds? Let me know.

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