How Does Weed Killer Work?

How Does Weed Killer Work?

Learn to recognize the types of weeds and choose your herbicide accordingly

** This post has been updated for 2018 **

A lot of homeowners are satisfied with a decent yard if it isn’t the worst on the block.

Little do they know that upgrading your landscaping can increase the value of your home by 5 – 15%*. Chances are you’ve peered out your window to look at your neighbors’ lush, bright-green yard and asked, “What’s the best weed killer for lawns?” Relax, you’re in the right place.

It’s safe to assume you’re researching pre-emergent and post-emergent herbicides because there’s an outbreak or annoyance of weeds in your lawn. Correctly identifying the target weed prior to purchasing or applying the herbicide is critical. You’re most likely dealing with:

  1. Annuals
    • Grow, produce seeds & die within a single year. Easiest to kill, generally.
    • Annuals focus on one thing: reproduction. Soon after reaching their goal of flowing and seed production, annual weeds die. Not to worry, though, the mother plant leaves hundreds of seeds behind to continue another lifecycle the following year.
  2. Biennials
    • Live for two years; the first year is vegetative development followed by flowering and seed development.
    • Biennials grow only foliage their first season, followed by the formation of flowers and production of seeds.
  3. Perennials
    • Live from season to season producing seeds each year.
    • Perennials are persistent, typically completing a lifecycle in three to five years. Foliage can be seen for the first few years with noticeable flowering and seed production coming along later on.

In addition to the three types of common lawn weeds listed above, they can be further distributed into two groups: broadleaf and grassy weeds. Broadleaf weeds typically have wider leaf blades and do not resemble grass (ex. Dandelion). Broadleaf weeds will often bear colorful flowers of different shapes and sizes. On the other hand, grassy weeds have long, narrow leaves void of any color or unique leaf shape. Many of you are probably familiar with the bright green grassy nightmare: nutsedge. The ability to identify grassy weeds typically depends on recognizing growth habits, vegetative features, and seed heads.  Start studying!

Types of Weeds

We can divide herbicides into pre-emergent and post-emergent categories. A pre-emergent herbicide is applied prior to germination and before the weed has emerged. Oftentimes pre-emergent herbicides are used to control annual weeds. A post-emergent herbicide is used when weeds are already established (visible), or to spot-treat lone weeds.

When you take a closer look at the herbicides on the market, they can be further separated into two groups: selective and non-selective. Selective herbicides control the target weeds without damaging desirable turfgrass species. Non-selective herbicides kill all vegetation (including turfgrasses) and should be used on weeds not controlled by selective herbicides.

When is the most effective time to apply herbicide?

Application timing is vital in broadleaf weed control because knowing the target weed lifecycle can result in a successful application.

Completely preventing weeds can be a difficult challenge as weed seeds are spread in a variety of ways: wind, water, animals, soil amendments and even lawn equipment. Ensuring proper watering, fertilization, drainage and other techniques can help keep your lawn as healthy as possible.

Remember, don’t mow your grass too short as it will decrease the grass’ ability to shade the soil from sunlight, increasing the potential for weed germination.  On that same note, don’t mow immediately before or several days after application as more available leaf surface on the weed is better for absorption. Make sure to inspect your lawn frequently to look for new weeds that have germinated and require treatment.

With a pre-emergent herbicide, it is best to apply the product prior to the weed seeds germinating. You\’ll want to pay close attention to the soil temperatures as this will determine the start of the germination period. When dealing with post-emergent herbicides, it is safe to apply the chemical at any point in the growth cycle, however, maximum success is achieved when the herbicide is applied to a young, actively growing weed.

Weeds Graphic


  • Regrow over a few to many seasons
  • Appropriate air temperatures, warm soil temperatures and adequate fall moisture allow for better herbicide uptake

Pro Tip: The best time for control is fall

Summer Annual

  • Germinate in late spring & through summer months
  • Mature at different times making it difficult to control with one application

Pro Tip: Best time for control is after they emerge but before they flower

Winter Annual

  • Germinate in late summer to early fall, over winter, and then flower and produce seed in mid to late spring
  • Pose huge problem in sports turf and home lawns

Pro Tip: The best time for control is fall

Annual Grasses

  • Complete their life cycle within one calendar year
  • Most common annual grassy weed is crabgrass
  • Usually germinates mid to late June

Pro Tip: Apply pre-emergent when soil temperatures approach 55°F

To understand some of the more complex nuances of the products and their chemistries, we connected with an expert, Jeff Schmidt, Midwest Sales Representative for PBI Gordon, and he was able to provide some applicable advice on using herbicides on your home lawns.

Jeff Quote

What factors specific to a home lawn would determine the right product or brand of pre- or post-emergent to use?

One of the main factors for post-emergent broadleaf weed control is air temperature. Using the correct formulation of post-emergent herbicide will aid in control of the target species.

  • Amine herbicides tend to be better suited for warm weather.
    • An example of an amine is Surge® Broadleaf Herbicide. Proving an economical option for late spring, early summer and fall weed control, Surge Herbicide is known for tackling knotweed, plantain, oxalis and lots of other broadleaf weeds.
  • Esters are generally more suited for cooler temperatures and penetrate the leaf surface better than water-based chemistries.
    • An example of an ester is SpeedZone® Herbicide. SpeedZone is known for its quick, exceptional control of broadleaf weeds such as plantain, ground ivy, clover, and spurge. Taking effect in as little as a few hours, SpeedZone Herbicide is a great option for residential sites, golf courses, athletic fields, and commercial areas.

spraying herbicide

How important is it to water in the herbicide vs. the drying period?

When applying post-emergent broadleaf herbicides, drying time is a critical factor. Active ingredients won’t work as well if they’re not allowed to be taken up by the targeted plant. And, if the product is still wet when it rains, the product may be washed off and not taken up by the plant.

The herbicide label will often state the rainfast period. For example, Surge® Broadleaf Herbicide for turf is rainfast in as little as six hours, whereas TZone™ SE is rainfast in as little as three hours. Esters typically have a shorter rainfast period compared to amines, as esters move into the plant surface much faster than amines.

If using a granular weed-and-feed, application should be made when the dew is on the turf in the morning. The product should not be watered-in for 1-2 days. This allows the herbicide active ingredients to stick to the target weeds. Refer to the label when using granular weed-and-feed products.

Why might someone see different results when seemingly applying a herbicide in the same manner? (Ex. external factors that may affect chemicals effectiveness)

As mentioned above, temperature comes into play when selecting the right formulation of broadleaf herbicide (amine vs. ester). There are other factors that can also affect a successful application as it relates to environmental conditions, including wind, dry conditions, and excessive temperatures (hot and cold).

Another less-publicized factor is accurately calibrated spray equipment. It is critical to know the spray volume as it relates to square footage a sprayer is covering. This ensures the right amount of product is being applied to each site. Knowing the spray coverage per 1,000 sq. ft. is critically important for effective weed control, especially tough to control weeds such as white clover, ground ivy and others.

Choosing the right herbicide

Finally, deciding on the correct herbicide to treat your (or your client’s) lawn is not as simple as one would hope. With lots of different choices on the market and in the stores, pay close attention to the product label, description and which weeds you need to control.

Some herbicides on the market will target both broadleaf and grassy weeds. For instance, PBI Gordon offers Q4® Plus which is a post-emergent for both broadleaf and grassy weeds. A couple of other post-emergent options include Defendor from PROKoZ which specifically controls broadleaf weeds and Drive XLR8 from BASF. Drive XLR8 effectively controls clover, crabgrass, dandelion, foxtail, kikuyugrass and more.

You should be able to get a handful of meaningful information from the herbicide label such as ingredients, restricted uses, specific uses, personal protective equipment, application directions and weeds controlled. If you ever have any questions or confusion regarding which herbicide to use and how to use it, please reach out to a local Reinders professional for help. When in doubt, remember: the label is the law.


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