To quote Matshona Dhliwayo: “A flower does not diminish in beauty, no matter how many weeds envy it.” But we can provide a friendly assist to maintain a prized bloom, impeccably groomed lawn, or victory garden.

So how can we accomplish that? Hand pulling might be prudent in some cases, but herbicides can be a gamechanger, if you “use the right tool for the job.” Herbicides are designed to chemically manage weeds: devious, out of place plants that sabotage our fragile green thumbs.

Unfortunately, there is no “Swiss Army Knife” of herbicides. They’re as diverse as a mechanic’s toolkit. Avoid the temptation to pluck anything from the shelf and apply it indiscriminately. Selection must be intentional – a careful alignment of herbicide properties with the weed(s) in question.

Fortunately, herbicides are grouped in straightforward ways, often in very black and white pairings. It’s important to understand how these branch points guide the decision-making process, so an appropriate choice can be made. Here’s a step-by-step outline to assist your selection, from most general to most specific.

Restricted Use vs. General Use

Most of these groupings are functional, but this one is uniquely regulatory. This is similar to prescription vs. over-the-counter meds. Restricted use is like a prescription, requiring a pesticide applicator’s license. This entails a certain number of hours of yearly training in proper handling, application, and disposal. These chemicals have unique properties, and the consequences of misuse are higher. As such, these are typically for commercial, professional enterprises. Only approved outlets will sell them with proof of licensure – homeowners don’t have these options available.

General use herbicides are available at garden centers and the like, no special credentials required.

Organic vs. Conventional

Organic herbicides (and other materials) are approved by the Organic Materials Review Institute. A full listing is available at: https://www.omri.org/about-products-list. Inclusion is generally based on whether a chemical is naturally derived, has a low toxicity, and/or is persistent (sticks around) in the environment. However, there are many exceptions. Organic pesticides are typically very broad spectrum (see below) and require multiple, high-rate applications (to do a comparable job to conventional options) due to their generally poor efficacy. Organics are best considered a philosophical, rather than bona fide safety standard.

Pre-Emergent vs. Post-Emergent

Pre-emergent herbicides are a pre-emptive strike. When applied and watered into the soil, they create an inhibitory chemical barrier around ungerminated weed seeds, preventing sprouting. Established plants are spared because most of their roots lie outside this shallow barricade. Obviously, pre-emergent herbicides must be applied in anticipation of a problem. It’s also a solid option for newly prepared beds. If weeds are already growing, you’ve missed the window of opportunity. As an alternative to pre-emergent herbicides, consider growing cover crops in your garden, yet another valuable tool to keep weeds at bay.

Post-emergent herbicides are reactive, requiring a newly germinated (or established) weed.

Selective (Narrow Spectrum) vs. Non-Selective (Broad Spectrum)

This pairing is a subset of the above. Does the situation call for a delicate surgeon’s scalpel (that only targets certain weeds), or a blunt force clubbing (an indiscriminate herbicide)? If you’re looking to perform a “reboot” in the garden and start from scratch, broad-spectrum herbicides are your best bet. Otherwise, you’re probably in the market for some selectivity based on the weeds present.

Mode of Action vs. Mechanism/Site of Action

Another consideration is the often-misunderstood Mode of Action. This is what the herbicide ultimately does to the weed.

Various modes of action include:

This is a vital consideration due to the threat of resistance. Repeatedly spraying the same herbicide against the same weeds leads to resistant populations. To effectively manage, it’s important to switch things up with herbicides that work with diverse modes of action. This keeps weeds off balance and unable to adapt (or at least stifles them a bit).

For every Mode of Action, there’s an even more specific Mechanism/Site of Action (where and how the herbicide disrupts the weed’s physiology) – but that’s a bit too far in the weeds (pun intended) for this blog.

Other considerations include route of absorption. Some herbicides are designed to be applied to the foliage, while others are placed in the soil for uptake by the roots. Another feature is if the herbicide works systemically (taken up and transported throughout the plant) – like an oral antibiotic, or if it works only at the point of contact – like an antibiotic ointment.

Weather/temperature conditions also impact efficacy, as does application technique. For example, will you use a simple hand pump, backpack pump action, carbon dioxide pressurized backpack, or boom sprayer? When in doubt, refer to the label for best practices. In fact, always read the label before applying!

With these facts in hand, you can wield your herbicidal tools with mastercraft skill. Don’t forget to 1) properly identify your weedy rogue’s gallery (your local Master Gardeners or Cooperative Extension can assist) and 2) remember that an herbicide label isn’t just suggested guidance, it’s the law – a binding contract to use as prescribed!

Answered by Tim Durham, Ferrum College

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.