What Is An Herbicide And How Do You Choose The Best One?

All you need to know about what an herbicide is and how to use them effectively in your own lawn and garden. No more guessing what works and what doesn't work.

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herbicide for weeds

Table of Contents

An herbicide is, by definition, a chemical that kills plants. They are a major component of any weed control or weed prevention program. The best way to know how to treat the weeds in your area is to ask your local agricultural extension. You can find out how to contact your extension service by checking with the Farmer’s Almanac here.

You need to understand that an herbicide can kill any plant, whether it is a weed or a flower. Fortunately, herbicides come with several active ingredients that can direct them to act as specific weed killers.  There are two main modes of action for an herbicide.

Grassy Weed Herbicides

crabgrass preventer

The first major type of weed killer is one that kills grassy weeds. These weed killers usually include glyphosate, which is selective for grasses only. This means that, if you wanted to kill grassy weeds in your planter beds, these herbicides would not harm plants with broad leaves.

One thing that I want to make very clear is that Roundup, a name we are all familiar with, is a brand name, not a specific type of selective herbicide. There are several herbicides, including Roundup, that work against grassy weeds. A generic branded herbicide works just as well and is a great way to save money.

What Weeds Do Grassy Weed Herbicides Work Against?

Most people know what grassy plants look like. They are plants with long, thin blades that grow in clumps. A good example is crabgrass or lawn grasses.

You can get a grassy weed herbicide that has a crabgrass preventer called quinclorac in it. This herbicide is not only specific for grassy weeds in general but crabgrass in particular.

Annual grasses that grow, flower, and reseed all in a single season are considered undesirable in lawns because they grow taller faster and their seed pods are seen as ugly.

If you have these grasses, including poa annua, annual bluegrass, or annual ryegrass, you can control them as well with a grassy weed herbicide applied during their main growing season.

Broadleaf Weed Herbicides

broadleaf weed herbicide

Broadleaf herbicides are also usually applied in early spring to kill off existing weeds. These herbicides act on plants that have large lobed leaves like a dandelion or a thistle. The most common active ingredients in these chemicals are called 2,4-D and Triclopyr.

What Weeds Do Broadleaf Weed Herbicides Work Against?

These selective herbicides will not harm grass unless they are applied in high doses so homeowners usually apply these herbicides to their lawns. Where I live, we use this type of herbicide to kill the wild blackberries that seem to take over your yard otherwise.

Where Can You Use Herbicides?

Herbicides are used mainly in early spring as a weed control in sidewalk cracks and planter beds. You can combine both broadleaf and grassy herbicides to kill off everything, if that’s what your goal is.

The trick with most herbicides is to make sure that they really stick to the leaf and the plant needs to be actively growing for them to work well.

How Big Can The Plant Be To Apply An Herbicide?

Ultimately, you can treat a plant with an herbicide at any point in its life cycle. Of course, more mature plants may require multiple applications or a higher concentration of herbicide to be effective. There are two major divisions among herbicides however.

Pre-emergent Herbicides


You should apply a pre-emergent herbicide before the plant sprouts. They work by preventing seed germination and keeping seeds from growing roots and shoots. Pre-emergent herbicides can be incredibly effective since they prevent the problem before it even begins. 

They do not tend to be as targeted but a great example of a pre-emergent herbicide is a crabgrass preventer. Another thing to understand is that these herbicides will prevent your flowers and vegetables from growing as well.

Post-emergent Herbicides

Post emergent herbicides are what most people think of as a weed spray. These are sprayed on plants that have already sprouted and are actively growing to kill them off. Post-emergents can be applied to the soil at any point in the growing season.

The key with post-emergent herbicides especially is to make sure they stick to the plant as much as possible. If you are using granular herbicides, dampen the plants before you apply the herbicide. If you are using liquid herbicide, mix in a little dish soap to act as a surfactant and spread the herbicide across the leaf.

Final Thoughts

Make sure that you are getting an herbicide with the right active ingredient to treat the plant that you want to kill. Once you decide on the active ingredient you need and choose between a pre-emergent and post-emergent, you are ready to treat your yard. If you apply that herbicide correctly, you won’t be disappointed with the results, I promise.

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Head Homeowner

Hey everybody! I'm Sydney, the head homeowner here. Let me know if you have any questions you didn't find the answer to. Tell me what projects you're working on. I love to hear from all my readers.