Pre-Emergent and Post-Emergent Herbicides

What are Pre-Emergent and Post-Emergent Herbicides? (Full Guide)

In Lawn & Garden, Lawn Care by Jamie

Are you having trouble with weeds constantly inviting your lawn and garden? There are two forms of weed control you might want to know about.

Pre-emergent herbicides are designed to kill weeds before they have a chance to grow, post-emergent herbicides require application when the weeds are fully grown. Pre-emergent and post-emergent herbicides are chemically designed to either prevent weeds or to kill them if they already exist. 

Learn how they work and how to apply them, in the article below.

What Are Pre-Emergent Herbicides?

If you want to abolish weeds before they have a chance to germinate, you need pre-emergent herbicides.

Pre-emergent herbicides are used before the appearance of weeds in the garden or on the lawn.  This is not to say that the chemicals prevent germination but rather that they prevent the production of new root cells in young weed plants.

Without weeds, seedlings are unable to feed and grow and eventually die. This entire process occurs beneath the grass blades, so you never have to see emerging weeds. The weed type, weather, and timing will all influence the precise formula and administration of pre-emergent herbicides.

When to Apply Pre-Emergent Herbicides?

As is the case with most plant pesticides, the application method is determined by the type of weeds at hand and the weather. Pre-emergent for winter annuals should be used in the fall, as this is when the seeds germinate.

Summer annuals germinate in the spring, which is the optimal time for pre-emergent application. If you’re unsure which sort of weed is the most troublesome, a spring application will likely manage the bulk of pests.

Types of Pre-Emergent Herbicides?

There are two main types of pre-emergent herbicides, and they both function similarly. That means that which one you choose is mostly a matter of personal preference.

Liquid herbicides come with a type of garden sprayer, which makes it easy to apply the product. Some liquid herbicides are concentrated, meaning you have to dilute them in water before application.

Granular herbicides literally have small grains and are best applied using a cup, drop spreader, or broadcast. A drop spreader is the best choice if you’re looking to cover a larger area at once.

What Are the Best Pre-Emergent Herbicides?

Some of the best pre-emergent herbicides that you can apply during the fall season include:

  • Prodiamine – the most long-lasting and cost-effective because of its low application rates.
  • Oxadiazon – an excellent granular choice that is safe to use on all turf and ornamentals.
  • Isoxaben – effective against a large number of broadleaf weeds.

Quali-Pro Prodiamine, Pre-Emergent Herbicide, 5 lbs, Yellow Granules

How Are Pre-Emergent Herbicides Applied?

To properly administer your weed killer, you’ll need the appropriate instruments, the appropriate sort of pre-emergent herbicide, and the ability to systematically distribute or spray your herbicide.

Additionally, consult your weather app before applying your pre-emergent to ensure that your herbicide does not wash away during a rain shower. Wait until the ground is totally dry before spraying the herbicide, particularly if you have just experienced a period of regular rain.

To apply liquid herbicides, you’re best off using a sprayer. Some products already have once included, but you can also purchase one if you’re making your own concentrate. Read the label on how to dilute the product with water. Apply it to your lawn systematically.

You can either use a cup to sprinkle the granules on the soil or a spreader for granular products. You want to go across the entire lawn and spread the product evenly across the surface of the soil.

Important: Make sure you water the lawn three to five days after applying the pre-emergent herbicide of your choice. This helps saturate the ground.

What are Post-Emergent Herbicides?

If pre-emergent herbicides were used before weeds could germinate, post-emergent chemicals are to be used when the weeds are fully-grown. It is basically a product that you use to kill existing weed with the possibility to help prevent new ones.

As you’re going to learn later in this article, there are multiple types of post-emergent herbicides. Make sure to carefully read a product’s label before you buy it.

When to Apply Post-Emergent Herbicides?

Herbicides applied post-emergently perform best in late spring when emerging weeds are tiny and actively growing. It is considerably easier to eradicate little weeds because actively growing weeds are more likely to absorb the herbicide and spread it throughout the weed, all the way down to its roots.

You can also use post-emergent herbicides in the summer, but it may take many treatments to completely control the weeds you’re targeting. They will also be effective in the fall when weeds prepare for winter.

What Are the Best Post-Emergent Herbicides?

Here are a few suggestions:

  • Speedzone/Speedzone Southern – effective broadleaf weed management that is safe for turf grass. 
  • Roundup QuikPro – provides strength and speed, even in cold weather, with visible effects in as little as 24 hours.
  • Roundup ProMax – a trusted weed killer that is rain-fast in 30 minutes.

SpeedZone Southern Broadleaf Herbicide for Southern Turf-2.5 Gallon 715387

Systemic vs. Contact Post-Emergent Herbicides

Systemics are particularly effective against perennial weeds since the plant absorbs them directly, causing them to spread throughout it, destroying the weed completely.

Contact herbicides are good for smaller weeds and annuals and can kill the exposed portion of the plant. For most weeds, killing the foliage equals killing the entire plant.

Selective vs. Non-Selective Post-Emergent Herbicides

Selective post-emergent herbicides are designed to kill or injure just the weeds for which they are branded. This is advantageous in the event that you over-spray and gets herbicide on flowers, grass, or other plants that you do not wish to kill

Non-selective herbicides are an excellent alternative if you need to clear a broad area of undesirable vegetation in poorly manicured regions or if you intend to later reseed or transplant desirable turf or plants. Additionally, they are useful along and in the crevices of driveways and sidewalks. They are used for broad weed control.

How Are Post-Emergent Herbicides Applied?

To achieve the optimum effects, post-emergent weed killers must be activated and applied under carefully controlled settings. Once you’ve determined your weed’s requirements and the formula to apply, you must consider the rate of application, residual activity, and contamination or leaching in specific soils.

Apply it on a day where the product has a minimum of 30 minutes and, in some situations, up to 8 hours to dry. The temperature has to be between 55 and 80 degrees Fahrenheit. After the drying period, you have to water the herbicide.

Make sure to never spray the product on windy days. The wind could cause the product to spread to surrounding plants. It may land on the skin and be inhaled, leading to potential respiratory problems.

What’s the Difference Between Pre-Emergent and Post-Emergent Herbicides?

The main differences between these two products reside in how they work and when you should apply them.

As the name implies, pre-emergent herbicides take a proactive approach to weed control by killing weeds before they become visible. Rather than interfering with the growth process, these herbicides aid in preventing germination, the process by which weeds form and sprout from the ground.

Post-emergent weed management chemicals effectively eliminate established weeds and are effective against a wide variety of weed species. They can be employed to treat individual weeds rather than entire turf areas.

Can I Apply Pre-Emergent and Post-Emergent at the Same Time?

But you don’t have to do that. Pre-emergent kill weeds before they bud when watered into the soil. Post-emergent work by being allowed to dry on sprouting weeds.  When one sort of herbicide is appropriate, the other is not.

How Much to Hire Someone for Pre and/or Post Emergent Herbicide Services?

The average national cost to hire professionals for pre-emergence weed control costs between $70 and $90 per application. For post-emergent weed control, it costs between $65 and $85. Note that some costs can have a range depending on where you live.

How Much to DIY?

If you decide to apply the products yourself, you will only pay for the product itself. You can check out our article for some great product suggestions for pre- and post-emergent herbicides.

Final Thoughts

It’s important to understand the main differences between pre-emergent and post-emergent herbicides. If you want to prevent weeds from invading your lawn and garden, then you have to use pre-emergent products. For fully-grown weeds, you want to use a post-emergent herbicide.