Post Emergent Herbicide Weed Killer | How And When To Use

Keeping a garden looking great takes both experience and knowledge. I’ve spent more than a decade working on projects, and I’ve seen the difference that using the right products makes. Post emergent herbicide weed killer is a potent and helpful tool used for managing gardens, but only if you use it correctly. Here’s what you should know about it.

What Is A Post Emergent Herbicide Weed Killer

Post-emergent weed killer is any herbicide that you apply after weeds germinate and while they are still actively growing. We’ll discuss timing in more detail below, but the best time to use a post-emergent herbicide is usually within a few days of the weed sprouting.

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Pre Emergent vs. Post Emergent

Pre-emergent herbicides need to be applied before a weed germinates and before it starts coming out of the ground. They work by coating the seeds and preventing them from germinating.

Many people use a combination weed killer that combines both pre-emergent and post-emergent herbicides and these can often provide for better weed control. A two-pronged approach will not only destroy visible weeds but will also get to work on any seeds that lie beneath the ground and have not yet sprouted.

While you can try this, be careful to avoid laying too many herbicides too quickly. If you’re not sure whether this is safe, contact the manufacturer and ask for their recommendation.

actively growing weeds

Types Of Herbicide

Here are the four primary types of herbicide available on the market. I don’t broadly recommend any of these over the others because the right type of herbicide to use depends on your garden and what you’re trying to control, but I do have some suggestions for each type.

Systemic Herbicide

Systemic herbicides are intended to be sprayed on the leaves and stems of a plant. Over time, they ‘systematically’ penetrate the entire plant, including the roots. This is a good option for stubborn as a weed killer for Nutsedge. This variety of weed is known to be able to often survive and regrow after the destruction of their upper areas.

Systemic herbicides are relatively slow and may take weeks to circulate through a plant. They don’t have an immediate effect, so it can be hard to tell if they’re working straight after you have applied them. It can take a couple of weeks to notice the yellowing or browning of leaves or stems and much longer for the weed to die in its entirety.

I mainly use systemic herbicides in my garden, but I switch to contact options if I want a quick fix.

Contact Herbicides

Contact herbicides are more aggressive than systemic herbicides, directly attacking any exposed part of the plant they land on. They can kill plant tissue in as little as a day, rapidly eliminating unwanted weeds from your line of sight.

However, while contact herbicides are fast, they don’t travel down into roots and won’t kill weeds that can survive losing leafy areas. Expect weeds to return after 6-8 weeks.

Selective Herbicide

Selective herbicides are more toxic to some types of plants than others and these are widely used to control weeds in lawns and borders. Selective herbicides can be relatively narrow in applications, often controlling as few as one or two types of weeds.

These are a great option if you have only one type of weed in your garden and only want to eliminate that specific weed.

Remember that selective herbicides are rarely non-toxic to other plants, just less toxic. If you use too much, you may still end up harming your lawn or other plants.

Be careful about products that mix too many selective herbicides. All herbicides need to meet a minimum level of application to work, and getting the volume necessary for multiple selective herbicides can reduce the overall effectiveness.

Non-Selective Herbicide

Non-selective herbicides do not target specific plant species, and will effectively kill all weeds, grasses as well as any precious grasses and decorative plant life that it comes into contact with. Avoid using this type of weed killer if you want to preserve lawns, borders, trees, or hedges that it may come into contact with during the application process.

This type of herbicide is a popular product for industrial locations, railway embankments, commercial areas, brush clearing, and any place where you do not want any type of plant life to grow and thrive. Non-selective herbicide products are terrific to use on gravel pathways, driveways, and patios too.

Post Emergent Spray vs. Granules

Post-emergent herbicide comes in two primary forms of application: spray and granules.

Granular Post Emergent

Post-emergent herbicide in granule form is a series of one or more chemicals in solid form, typically small round balls. You apply these chemicals by using a special spreader and rolling over your garden. Granular spreaders work particularly well on flat lawns and gravel paths, but they won’t work as well in bumpy gardens or irregularly shaped areas.

You do not need a spreader from the same manufacturer as your herbicide. Many products will suit different brands of herbicides, making these spreaders versatile for many consumers. One consideration is the application requirements with each model. Be sure to follow the directions carefully on the spreader for proper coverage.

I recommend granule herbicides to most new gardeners despite the extra challenge of working with them. This is because it’s hard to get things too wrong as long as you follow the directions and have the right spreader. Spray herbicides are usually faster and more reliable, but they’re harder to apply correctly without a little experience.

Post Emergent Spray

Post-emergent sprays are liquid forms of herbicide that you can spray as a weed killer for lawns that have individual plants growing, allowing you to target them. Most herbicide sprays require diluting with water before applying. Undiluted liquid herbicide is extremely dangerous, so make sure you follow the directions for mixing exactly.

Dilution ratios depend on the product, but most sprays have no more than a few ounces of herbicide per gallon of water.

Sprays are generally effective, especially in areas that are hard to reach with granules, but I don’t recommend this for anyone applying herbicide for the first time. Applying a spray correctly involves knowing how fast to spray it and how much area you’re covering.

It’s easy to get wrong if you don’t have experience, especially since the spray particles are easily carried by even the lightest of breezes and can wind up in areas that were not intended. My advice is to start small. I also suggest spraying pure water first to get a better feel for how fast you should walk and move when applying liquid herbicide.

Best Post Emergent Herbicides

Here are my recommendations for two of the best post-emergent herbicide weed killers currently available.

Roundup For Lawns Ready to Use All-in-One Spray

Roundup For Lawns Ready to Use All-in-One Spray

Roundup is one of the most well-known names in weed control, and with good reason. This ready-to-use mix doesn’t require any diluting and comes with a special spray wand designed to apply it at the correct ratio. It’s effective on most common lawn weeds including clover, crabgrass, dandelions, as well as nutsedge.

This is a selective herbicide so it is safe to spray on lawns including all Northern grasses as well as some warm-season grasses too. The manufacturer has provided a handy list covering more than 250 weeds that this herbicide can effectively kill but it is always worth identifying the weeds you are looking to destroy and cross-referencing with the label and instructions.

I road-tested it on a clump of persistent dandelions that reappear in my lawn each year. I must admit, the results were impressive. I used the spray wand to spot treat the offending dandelions when I first spotted the weeds poking up. For me, this was early spring just as the temperature started to rise above 45F. After just a few days the dandelion weeds began to wither and wilt and within 14 days they had disappeared.

My top tip is to leave the withering weeds well alone and don’t be tempted to pull them up. Roundup for Lawns needs time to penetrate throughout the weed in order for it to fully reach the roots and kill the weed in its entirety.

I’m pleased to report that there was no sign of them for the entire season. However, if you are concerned that more will appear the following year, it is recommended to reapply in the fall.

Another advantage of this post emergent herbicide weed killer is that it dries within 3 hours (especially if you spray it on a dry day). Once dry it is then safe for people and pets to walk on.

Roundup for Lawns is available in three formats. This particular spray is particularly good for relatively small areas with weeds that can be controlled by spot treating. The entire bottle can cover up to 1330 square feet.

Larger areas would benefit from either the larger-sized bottle or the concentrated formula. Both of which need diluting and transferring into either a handheld or backpack sprayer.


  • Easy, effective, ready-to-use spray
  • Effective pre-emergent herbicide for over 250 common weeds


  • Best used as a spot treatment as coverage is less than 1500 square feet

Natural Armor Pet Safe Weed and Grass Killer Concentrate

Natural Armor Pet Safe Weed and Grass Killer Concentrate

Natural Armor’s weed-and-grass formula is an aggressive spray you can use anywhere you don’t want weeds or grass growing, including around trees, on patios, and near water systems. This non-selective herbicide is safe for both people and pets, thanks to a carefully considered mix of chemicals.

This herbicide’s chemical formula includes citric acid to help burn seeds, clove oil as a natural herbicide and fungicide, and sodium lauryl sulfate as another contact herbicide. All of these ingredients are generally effective, and it avoids more toxic ingredients like glyphosate.

This herbicide is branded as a ‘concentrated’ formula, however, it is ready to apply straight from the container and comes complete with an easy-to-use spray wand. Effects are generally visible within a few hours, and within a day at most, so it’s easy to tell if this is working.


  • An effective herbicide that’s safe for people and pets


  • Better for targeted application than applying in a large area

How To Use Post-Emergent Weed Killer

The correct way to use post-emergent weed control products is exactly as described on the label. Do not deviate from the instructions. If you cannot apply it precisely as directed, get a different herbicide and use it that way. Misusing herbicides could damage your yard or, worse, become toxic to people and pets.

Now that I’ve gotten that disclaimer out of the way, let’s talk about the general use processes.

The first step in using post-emergent weed killer has nothing to do with the herbicide itself. Instead, make sure you’re mowing, watering, and fertilizing your yard. Aeration can also help, especially if your yard hasn’t gotten that for a while. For non-lawn areas, make sure you’re protecting the soil and using barriers like bark.

Basic yard care is a major step in controlling weeds. Herbicide alone is not a magical cure that will result in a great-looking yard, but having a healthy yard and garden will do a lot to limit the growth of weeds. That allows the herbicide to function in its proper role as a supporting product rather than a solution to everything.

Next, determine when to apply the herbicide. We’ll discuss this more below, but for the brief explanation, you need to figure out when your weeds are sprouting and apply it then. Most herbicide manufacturers have charts and guides to help you determine the correct time to apply them.

If you’re still not sure, ask an expert at your local gardening store. They can usually help you figure out when to apply herbicides, down to the day.

With the timing set, apply your herbicide following its directions. This usually involves spraying areas at a steady pace or walking a spreader over the area. 

Ensure that you wear protective clothing that covers exposed skin and eyes. Try to wear tall rubber boots while you’re doing this and wash them off thoroughly before going back inside. Many ingredients in herbicides can be toxic and cause irritation or damage to the skin and eyes when exposed. 

You may need to water granules into your yard. This helps the chemicals spread out and create a barrier.

Finally, remember everything you did because you’ll probably need to do it two or three more times, at least, in the coming years. It can take years to get a truly weed-free garden, and even then you may need to apply more herbicide to maintain it.

If you have any leftover herbicide after your application, store it correctly and safely away from children and pets. These chemicals can be harmful if ingested or spill open where a child or animal can reach it. 

Outside of these steps, try to avoid walking over areas where you’ve already applied herbicide. This means starting in the area furthest away and slowly moving backward until you’re on a sidewalk or an area you’re not treating. 

Aside from helping you avoid getting the herbicide on yourself, this ensures your shoes and spreader won’t disturb the spread as much.

When To Apply Post-Emergent Herbicide

The best time to use post-emergent herbicide is once spring is fully underway in your region and plants are growing throughout your garden. The exact time varies by area, but many weeds start growing around the time the soil reaches 55 degrees. It’s difficult to control soil temperature beyond a small area, so look for charts or guides to help with this.

Syngenta has an excellent online chart for this, allowing you to check overall soil temperature by region anywhere in the continental United States.

Apply herbicides during a time where there is no wind or minimal chance of blowing winds. With high winds, herbicides can spread through the air to unwanted areas of your yard or to surrounding properties. 

Ideally, bright and sunny days are best to apply products to your lawn when the temperature is between 55 and 85 degrees. During this time, vegetation is growing and cycling nutrients from the soil, making the herbicide work more effectively. 

Weeds can continue growing from spring to fall, so you may need to apply your product at least once in each season besides winter to control weeds better. Try to avoid layering different herbicides in one area. The chemicals will affect your lawn and soil, so choose one product and stick with that.

How Often To Use Post Emergent

As with how to use these weed killers, the correct answer is always “as often as the manufacturer recommends.” Anything else may stop it from working correctly or result in damage to your yard.

Many products require several applications throughout the growing season but try to avoid using the herbicide if it’s hotter than 85 degrees outside. That can result in too much harm to your lawn and soil, so wait until things cool off before the next treatment.

Also, try to mow your lawn (if applying herbicide there) about three days before you apply it, and avoid mowing for at least three days after. This gives the plants in your yard a little more time to grow and absorb the herbicide, ultimately helping them work more effectively.

Avoid using post-emergent herbicide if your yard has drought or notable disease. Drought puts stress on weeds as much as it does grass, which can stop the weeds from absorbing as much herbicide. Trying to kill them while they’re weak sounds appealing, but in my experience, it doesn’t work in practice.

Conclusion: Best Post Emergent Herbicide

Post-emergent herbicide is an affordable and effective way to get weeds out of your yard as long as you use them correctly. Both of the products listed above work for different areas, so consider purchasing and using each of them to measure their effect on your area. If you’re planning for next spring, make sure to order herbicide early enough to use it at the right time.

FAQ’s Post Emergent Herbicide

Here are some common questions people have about post-emergent herbicides.