So those leafy spawns of Satan are back in your backyard, aren’t they?
Sure they have and you’re losing your sleep over it, being a caring landscaper you are. Post-emergent weeds killers are the surefire way to get rid of those pesky weeds that have already sprouted their annoying little heads out of the soil. The market is flooded with options but not every pre-emergent herbicide will do the trick for your lawn.
Before picking up a product from the catalog and dousing the soil with it, take a step back, grab a cup of coffee, and do a little homework. That’s what I’m here for.
I have done my best to curate the most authentic information, some from my own experience, some from reliable sources, only to make you exit this page with zero ounce of doubt left in your mind.
Before I provide you with the list and in-depth reviews of the best post emergent weed killers that do the trick without hurting your pocket or beloved saplings, here are a few key pointers for you, my keen learner:
- Best Post Emergent Weed Killers – Comparison
- Reviews of the 7 Best Post Emergent Herbicides You Can Trust
- Compare-N-Save 016869 Concentrate Grass and Weed Killer
- RM43 76502
- Green Gobbler Vinegar Weed & Grass Killer
- RoundUp 5200210 Ready-to-Use Weed & Grass Killer
- Roundup 5705010 Extended Control Weed and Grass Killer
- Southern Ag Amine 24-D Weed Killer
- Ortho 0436304 GroundClear Vegetation Killer
- Types of Post Emergent Weed Killers You Need to Know about
- Things to Consider While Choosing the Best Post Emergent Weed Killer for Your Target Plant
- The Bottom Line
Best Post Emergent Weed Killers – Comparison
|Name & Brand||Visible result||Active Ingredient||Size||Price|
|Compare-N-Save 016869||2-4 days||Glyphosate||1-Gallon|
|RM43 76502 32Oz Total||5 days||Glyphosate + Imazapyr||32-Ounce|
|Green Gobbler Vinegar||6 hours||Vinegar||1-Gallon|
|RoundUp 5200210||3 hours||Glyphosate + Pelargonic||1.33 gallon|
|Roundup 5705010 Extended||6 hours||Glyphosate + Pelargonic||32-Ounce|
|Southern Ag Amine 24-D||3 days||2,4-D Dimethylamine salt||32oz|
|Ortho 0436304 GroundClear||6 hours||Isopropylamine||1.33 gallon|
Post Emergent Weed Killers 101: Why Timing Is Everything in Weed Management?
You got to figure out what kind of weed you’re dealing with and how badly they have spread to find a fail-safe remedy for it. One little caveat here: many post-emergent herbicides are non-selective and systemic in nature.
I am going to tell you what those geeky terms mean later in the article but for now, all you need to know is that they knock out every plant that comes their way. Therefore, post-emergent weed killers should be only used for spot-treatment when the timing is perfect.
Now, what’s the perfect timing for treating weed with post-emergent herbicide?
It depends on the kind of herbicide you’re using and the kind of weed you’re dealing with. Having said that, the rule of thumb is to make sure that the weed growth is at its peak when you apply the treatment.
That’s when your target is most vulnerable. Peak growth timing varies from plant to plant but in general, the window between spring to fall is regarded as the ideal time for nuking those unwanted weeds with a post-emergent weed killer.
Low heat slows down the germination process, preventing the herbicide solution from penetrating a waxy cuticle layer of the leaf. Extremely high temp (above 85 degrees F) affects the herbicide’s efficiency in two ways:
a) It either puts the plants under extreme stress which increases the risk of crop injury or b) It makes the plants more tolerant to herbicides due to increased photosynthetic activity which consequently reduces the herbicide’s absorption/translocation/metabolism rate.
So the limited window of time between spring to fall, when it’s neither too hot nor too cold, and the soil is neither too dry nor too moist, would be ideal time to target most perennials and hard-to-control species like spurge, veronica, violet, and wild violet.
Reviews of the 7 Best Post Emergent Herbicides You Can Trust
If everything I wrote so far is making total sense to you right now and gave you the much-needed perspective, we can move on to the next section in which I have rounded up 7 best post emergent herbicides to help you take control of the weeds before they take control of your land.
This one up here is a contact systemic post emergent herbicide that lets you spot treat those pesky grass weeds and tough-to-beat perennials. The core ingredient of the strong concentrate in Glyphosate which is a non-selective herbicide, meaning it will wipe out anything it touches. So you might want to keep it away from your young tree saplings and ornamentals.
The good part is, it’s a foliar applied weed killer which doesn’t cause any long-term damage to the soil. You might have to wait anywhere between one to two weeks to reseed, fertilize, and aerate the treated area.
It’s not just the “getting a gallon for a peanut” factor that gives Compare-N-Save such a massive popularity. This post emergent is a lethal chemical weapon for over 200 species of weeds including poison oak, fescue, Canadian thistle, chickweed, clover, ground ivy, and the list goes on.
The product is profusely used by professional agriculturists to spot-treat thick shrub beds, veggie and flower beds, cold and warm-season pastures, crop fields, orchards, and various species of turf grasses such as bluegrass, Bermuda grass, St. Augustine grass, ryegrass, and the likes.
It’s a liquid solution with a whopping 41% of pure glyphosate which is obviously going to wipe out acres of metabolically active annual weeds in 2-5 days.
On top of it, it’s a rain-fast solution that takes merely 2 hours to become rain-proof. I highly recommend it for home garden prep and lawn renovation. Since it will completely terminate any green you spray it on, you can use it for purging acres of weed-infested land and start over in a jiffy.
That’s phenomenally fast! It will also work great on non-crop sites like sidewalks, rocky landscapes, driveways, fences, corners of your patio where those nasty weeds crop up every year.
Some perennials and creeping perennials like Canadian thistle, Veronica, and wild violet might require 2-3 shots if you don’t notice any signs of major injury in 10-14 days. If you are dealing with old weeds that have been in your yard forever, two sprays on two consecutive days, just to be on the safe side.
One thing to take note of here is that 41% glyphosate won’t speed up the weed removal process because the plant will eventually hit a saturation point which would render the use of higher doses useless.
So what’s the hype about 41% glyphosate concentration, you ask? Well, it will let you cover thousands of square feet (the 2.5 gallons can cover up to 63000 sq.ft, beat that!) with smaller volumes, thus drastically cutting the yearly cost of weed management.
So stick to the recommended water to herbicide mixing ratio, use a garden hose or battery-powered lawn sprayer to treat the plants on a wind-free, moderately warm day early in the morning for best effect. Also, systemic herbicides need to translocate from the leaf cuticle to the xylem and phloem tissues of the plant to inhibit the enzymes that are essential for the plant’s life.
Depending on how well-developed and widespread the root system of your target plant is, the weed killer can take longer than you expect to completely wipe out certain species. The key to satisfaction is keeping your expectations reasonable because creating a successful weed management routine could take a couple of trials and errors.
It doesn’t work on newly germinated weed seeds, and some hard-to-kill species e.g crabgrass, Johnson grass, and quack grass. Also, you will have to be careful while misting the spray in your well-vegetated areas in spring as it can seriously injure some of your beloved plants in the process.
For the same reason, I wouldn’t recommend this product on newly seeded lawns and turfgrasses. Also, you will have to let the herbicide dry completely before your pets or/and kids can come near the treated spot.
This post emergent weed control formula is enhanced with the lethal weed-killing power of two potent herbicides, namely glyphosate and Imazapyr. The formula is so potent that two to three days after the treatment, it kind of looks like you have torched all the weeds with a huge gas torch. Fast absorption rate allows the solution to be rainproof within roughly 2 hours of application.
To aid smooth and quick translocation to the root and widespread underground storage tissues, this ultra-potent weed killer is combined with a surfactant as well. It’s highly recommended that you limit its application to non-crop sites like fences, cracks in an old building, farm buildings, cabins in the woods, campsites, and driveway areas as it’s a total vegetation remover.
If you accidentally spray it in your yard or grass lawn, it will not only knock out every green it touches but also prevent any seed germination for an entire year.
Although the product labels itself as “safe for pets and kids”, I would advise not letting your kids near anything that contains even a low concentration of glyphosate until the solution is completely diffused. It might not do anything severe to them but can surely plague your little ones with eye irritation and your furry friends with mild gastrointestinal problems.
First of all, what a dark product name! Coming to the point, Green Gobbler is a ready-to-go biopesticide containing 20% white vinegar. The high concentration of acetic acid in 20% vinegar can sucker-punch annual broad leaves like dandelion, clover along with some tough guys like musk thistle, crabgrass, monkey grass, quackgrass, pigweed, oxalis, and creeping charlie.
The scientific process of drying and deforming plants using acetic acid is called desiccation. Vinegar, unlike glyphosate, doesn’t possess any toxicity. Therefore, kids, pets, and people suffering from some kind of chronic disease can safely take a walk in the treated location immediately after application.
To aggravate the absorption and movement of the herbicide inside the plant body, you can mix a strong surfactant or a few drops of your good old dish soap in the solution. The herbicide is highly effective, even in small volumes. The only warning here is that the Green Gobbler doesn’t discriminate against its victims.
It will kill anything green it touches, so use a quality sprayer to spot treat weeds on a wind-free, warm day early in the morning for optimal results. It also requires a rain-free period of up to 24 hours, so it’s better to avoid the application in the rainy season.
If your target area is a landscaping bed covered with mulch or have newly seeded plants, you need to properly cover those areas before applying the treatment.
However, if your fully grown plants accidentally come in contact with the weed killer, no need to lose your sleep over it. It’s a biodegradable herbicide which can be neutralized with adequate watering. You can make up for the lost plants by reseeding the land within a 5-7 days after application.
Unable to fully remove certain hard-to-kill species like Japanese Knotweed. If you notice some weeds coming back in two days post-treatment, you’ll have to re-spray the area after 7-10 days.
Everyone is a gangsta until RoundUp walks into your lawn. It’s incredibly potent formula contains the double punch of 18% Glyphosate and 0.73% Diquat. The 5200210 upgraded formula can deliver the ultimate knockout punch to many persistent weeds like Japanese Knotweed, crabgrass, wild violet, yellow nutsedge, and 249 other species.
If used in ideal condition, RoundUp can show signs of crop injury in 3-5 hours. On top of that, the herbicide becomes rainproof in merely 10 minutes and is suitable for round the year application.
However, I’ve noticed that tough-to-kill perennials and grassy weeds respond best to the herbicide in hot and dry summer, and fall when they are in full bloom. Don’t be surprised if some old weeds need 2-3 more shots in the same week for total termination.
Unlike most cheap off-brand products, it doesn’t stop at just preventing the weed seed germination for a few weeks. It kills those pesky little buggers from shoot to root. It’s a pre-mixed spray, so no need to fuss about the mixing ratios.
Use the included wand to spot-treat the target areas and poof! RIP weeds. What’s even better is that you can plant non-edible seed plants on the treated soil a day after and edible plant saplings 3 days after application.
Like all great post emergent weed killers, RoundUp also murders weeds non-selectively. If your meticulously grown flowers, vegetables or ornamentals come in the way, you will have to watch them painfully withering away. Make sure the droplets of the herbicide don’t dash off to your desired plants. Also, it’s essential to wear protective gears to keep yourself protected from the moderately toxic effects of Glyphosate.
Listing the best post emergent weed killers of all time and not featuring Roundup 5705010 in it would have been an act of blasphemy. The weed and grass killer can is a mighty weed murdering machine gun that works effectively on over 250 weed species.
Its patented RoundUp FastAction formula is a deadly combination of 18% Glyphosate, 0.30% Imazapic ammonium salt, and 0.73% Diquat Dibromide. A little goes a long way and if you manage to knock out the weeds with this herbicide when they are at their peak growth phase, you win! Depending on how much your target weeds have developed all this while, and weather conditions, the herbicide can take effect in 6-7 hours.
Some species might need a second application a day apart. But once the herbicide is on the site of action, you can rest assured that those ghastly weeds aren’t coming back for several weeks.
Mind you that it’s not a ready-to-spray solution like the previous product. You are supposed to add 6 ounces of concentrate per gallon. Higher concentration won’t speed up the herbicide effect, so don’t waste your valuable weed-killing weapon for nothing.
The manufacturer mentions that the ideal condition for treatment would be a bright sunny day when the thermometer reading above 60 degree F to maximum 85 degrees F. Quick absorption helps the herbicide rainproof in half an hour. Like all RoundUp herbicides, this one’s also a non-selective total vegetation killer.
Keep this highly potent Glyphosate containing weed killer away from pets and children. Also, you should protect yourself with masks, goggles, and gloves while spritizing the solution to avoid the potential harmful effects of Glyphosate.
This is the first selective post emergent weed killer on this list and is arguably one of the best of its kind. It is frequently used by horticulturists to relieve large crop fields and forestries of serious annual, perennial, and biennial weed infestation.
It kills the crap out of crappy weeds without hurting the beautiful turf beneath. Unless you overspray the solution, it’s not going to form a ring of death around the treated area, marking the ground safe for newly germinated and existing full-grown plants, vegetables, flowers, lawn grasses, and ornamentals.
Considering that you’ll only need 1-4 pints of concentration per acre, it’s a highly economical option for someone who doesn’t want to spend extra bucks on a ridiculously potent non-selective weed whacker that spares nothing that comes its way. Broadleaves blessed with a hardy and complex root structure like poison ivy will require a second round of spray in the following week.
Only suitable for fighting broadleaf weeds. Narrow-leaved weeds and grassy weeds found in pastures, gardens, cemeteries, gravel areas, permanent campsites won’t give in to 2,4-D acid.
Ground ivy, poison ivy, monkey grass, wild violet, crabgrass, Japanese knotweed, dandelion, chickweed, Canadian thistle, Bermuda grass, you name it, GroundClear is the ultimate solution for difficult-to-control weeds. I loved the fact that it is available in both premixed and concentrate forms along with a handy spray wand for precise application around your yard or barn perimeter.
Like all the best-in-class herbicides, this one also kills whatever it touches. I would suggest spraying it only on non-crop areas like rocky landscapes, driveways, fence line, barn, patio cracks, driveways, and grounds where you don’t want to grow plants for at least a year. With the help of the provided wand, you can cover up to 6000 sq. ft in an hour and a half, give or take.
In reality, you will have to mix more than the recommended amount of concentrate to kill some creeping perennials. Also, it can’t kill monkey grass. It doesn’t work as fast as RoundUp, so you have to be patient. Once the herbicide is soaked by the roots, rest assured they are not coming back for six good months.
Types of Post Emergent Weed Killers You Need to Know about
Choosing a post-emergent killer that works out best for you becomes even more difficult when the internet keeps growing heavy, alien terms like “systemic”, “post emergence contact herbicide”, “apoplastically translocated herbicides” at you. It doesn’t matter if you are totally new to all these or a serious landscaper who simply wants to expand his knowledge, I am going to simplify post emergence weed management for you all.
Below I have listed and briefly explained 2 major types of post emergent herbicides under the sun:
A. Foliar Applied Post Emergent Herbicides
This group of post emergent herbicides is the way to go to shoot and kill simple perennials as well as some tough guys with one or two applications. Foliar applied herbicides only interact with the plants and don’t affect the soil which keeps the soil integrity intact.
While this somewhat limits the versatility of these herbicides, with proper planning, you can chalk out an effective long-term weed control routine in keeping with constant fluctuations in environmental conditions.
The entry point of foliar applied weed killers is the leaf of the plant. The goal is to ensure proper absorption of the herbicide so that they can move from the plant to the site of action (which is the cytoplasm of the cell in this case). The cytoplasm is the warehouse of organelles like mitochondria, nucleus, and chloroplasts which are essential for the survival of all living organisms.
When you spray the herbicide to the exposed parts of the weeds, the droplets of the herbicides have to penetrate three barriers in order to take effect. These three barriers are namely cuticle or the outer waxy surface, cell wall, and finally, the cell membrane. This might sound simple on paper but some left structures are more complex than others, thus presenting multiple obstacles in the way of herbicide absorption.
Choosing the right post emergent herbicide for your target weed species would require you to understand what kind of leaf solubility characteristics. Are the target leaf parts hydrophilic/water-loving or lipophilic/oil-loving?
To adapt to different leaf solubility characteristics, most foliar-applied post emergent weed killers contain some kind of weak acid, Glyphosate being the most preferred choice of most manufacturers. They are convenient to mix with different acidic solutions to overcome different obstacles.
For example, if you mix a weak acid like Glyphosate in an alkaline solution, the chemical reaction caused by it lends hydrogen ions to the solution, turning it into a hydrophilic solution suitable for treating hydrophilic leaves.
Similarly, if you pair a weak acid with an acidic solution, it will neutralize the charge of the herbicide. This will boost the lipophilic nature of the weed killer, rendering it a lethal weapon for lipophilic weed species.
Depending on environmental conditions, the type of weed you’re dealing with and the kind of herbicide you are using, it can take anywhere between several days to weeks to show the signs of injury. Auxin growth regulators like benzoic acid and phenoxyaliphatic acid are fast-acting herbicides that show injury symptoms in a day or two.
On the contrary, branched-chain amino acid inhibitors like imidazolinones, sulfonanilides, sulfonylureas can take 3-4 weeks or even longer to show effect but they are an infallible remedy for annual broadleaves.
B. Soli Applied Post Emergent Herbicides
This group of herbicides interact with the soil colloids and cease the growth of unwanted seedlings through photodegradation and sometimes volatilization of the soil. Most soil-applied herbicides have little to no foliar activity.
High-degree soil applied herbicides are mainly used as pre-emergence. However, some herbicides with low soil activity are prominently used as early post emergence to weed out annual broadleaves from established turfs, orchards, vineyards, soybeans, tobacco, lettuce, cotton etc.
These are mainly root and shoot inhibitors that slow down the plant cell division process which subsequently ceases cell wall formation. Dedicated shoot inhibitors are primarily used for controlling perennials that sprout in large and small seeded legumes, tomatoes, corn, potatoes, and ornamental plants.
The crops you want to protect from these herbicides need to be covered or treated with protectant sprays during the herbicide application.
These two main categories can be further divided into the following categories. Note that most post emergent weed killers are foliar applied with no to very little soil activity. I have specified the application type and mode of action (the method if injuring the target plants) of each variant to help you select the most effective weed killer for your target plant.
1. Contact Herbicides/ Non-translocated Weed Killer: For Foliar Application
Contact herbicides are point and shoot weapons. They don’t get into the vascular system of the weeds. Instead, they simply burn the foliage of your target spots. This group of weed killers has zero soil activity, so you don’t have to worry about the collateral damage of your surrounding grass and plant seedlings.
Even if some sections accidentally come in contact with the herbicide, it will only cause minor foliage injury to the exposed sections. Nursery crops usually recover such minor damage in a week or so.
This particular mode of action is an ideal remedy for most annual weeds and some recently mushroomed perennial weed species. In order to injure the foliage, the herbicide has to pass through the stomata layer of the leaves. The stomata opens up only when the plants are at their peak growth period. That’s why experts suggest spraying the solution earlier in the day, which allows the herbicide the rest of the day to diffuse into the stomata.
Perennial weeds having established stems, root systems, and underground storage tissues e,g taproots, tubers, and rhizomes cannot be suppressed with contact weed killers. Some newly germinated species, densely planted shrub and ornamental beds, however, might respond well to 2-3 lethal doses.
2. Systemic Herbicide/ Translocated Weed Killer: For Foliar Application
Systemic herbicides work their way into the root system of the plants, thereby causing a severe internal injury that finishes the whole plant, stem to root. This group of herbicides become an essential part of weed management for uprooting perennials that have already developed a strong and complex root and underground storage system. Burning back the foliage with a contact herbicide won’t do the trick in this case.
One spray of systemic herbicides can only wipe out 60-70% of the plants due their hardy structure of invasive perennial weeds like violet, wild violet, and Lesser Celandine. The best time to attack them would be right before fall and spring when the seedling heads are yet to form. Once the seed heads show up, you will need multiple sprays to wipe them out completely, as simple as that.
The mode of action of systemic or translocated herbicides might seem complex in theory but are actually pretty straightforward in practice. You spray the herbicide to the spots you want to clean up. The herbicide translocates into the root and shoot tips.
The graveness of the weed injury depends on the intensity of the dose. A sub-lethal dose generally weakens the roots, resulting in stunted growth, twisted twigs, malformed foliage, and increased susceptibility to pest attack.
Plant injury symptoms due to lethal doses of systemic herbicides include:
- Bending, rosetting of leaves, shortened secondary roots.
- Pigment loss,
- Yellowing of fresh growth.
- White, pink, purple spots on the leaves, malformed flowers, leaves, and stems.
- And finally, rapid death of the target weeds in just a few days or max two weeks after application.
Lethal doses are mostly reserved for total vegetation control in non-crop fields as well as broadleaves and creeping perennial control in soybean, cornfields, turf grasses, and cool-season grasses. The effectiveness and time taken for showing full symptoms can take anywhere between a few days to 4 weeks or even longer, depending on the environmental conditions and dosage.
The only major downside of systemic herbicides is severe collateral damage it can bring to the adjacent plant saplings, grass, or crops. Even a small dose of translocated herbicides can gravely injure nursery crops, resulting in stunted growth, malformed foliage, and weakened roots.
You might want to completely cover the plants you want to protect or spray them with a protectant spray before misting systemic herbicide to the surrounding weed growth.
3. Photosynthetic Inhibitors or Residual Killers: For Both Foliar and Soil Application
Photosynthetic inhibitors are the serial assassins of the herbicide clan. They are mostly used in pre emergent solutions but often found in industrial-grade weed killers as well.
Their primary applications include but are not limited to hard-to-control weed species in some perennial, and annual crops such as various woody species, mint, celery, corn, asparagus, parsnips, cotton, etc. Pigment inhibitors are a go-to choice for poison ivy suppression as well.
These total vegetation controllers are an excellent choice for uprooting weeds from non-crop areas i.e where you don’t want any vegetation at all such as fence line, pole barn, driveways, cracks of a sidewalk, and patios.
Residual herbicides are known for their fast and effective soil and foliar activity. They seep into the vascular system of the plant apoplastically, meaning they can only move upward in the plant system via the water uptake from the soil.
When inside the metabolic activity center of the plant, the herbicide causes grave injuries to the leaf veins and tissues, resulting in chlorosis which eventually ceases the plant growth.
4. Selective Herbicides
Selective herbicides have limited efficacy on varied plants but they do a terrific job of wiping out certain species, especially broadleaved lawn weeds like dandelion, clover, buttercup while having no harmful effect on the surrounding grasses and tree saplings. Some selective post emergent products are specially formulated to kill cold season pasture while leaving the warm season grass unharmed or vice versa. This kind of weed control is frequently used in forestry to maintain a fast and uninterrupted growth of the young tree saplings.
5. Non-selective Herbicides
Non-selective herbicides don’t discriminate, they wipe out everything that comes their way. If young plant seedlings come in contact with them, that’ll be the end of them. Larger plants can survive the blow but not without facing some injuries. That’s weed management experts recommend taking great caution while spraying non-selective herbicides.
You should strictly avoid misting them in windy conditions and applying it on places where you want to grow vegetation in the next two years. These herbicides are best suited for non-crop sites such as sidewalks, wall cracks, fence lines, cemeteries, forest cabins, gravel areas, and driveways.
Make sure to protect your skin, cover the desirable plants, and handle the chemical with great care while spraying the herbicide.
Post emergent weed killers are mainly sold in two forms:
1. Granular Herbicides
Granular herbicides can deliver a lethal dose to tough old beasts like wild violet, creeping Charlie, Veronica, and most cool-season perennials. They come in tiny rock-shaped particles and are ideal for weeds that develop a thick layer of canopy as a protective barrier. Sprayed liquid might only be able to burn the outer canopy, leaving the foliage, stems, roots under its shelter unharmed.
Soil applied granular herbicides mix with the soil colloids, rendering the ground festered which severely weakens the root system and underground tissues of the plants. Granular herbicides are an affordable way to treat creeping perennial lawn weeds but they need to be handled with caution. Make sure to water the ground adequately after application, and keep your pets and kids away from the treated area.
2. Liquid Herbicides
Liquid herbicides provide a better foliage coverage and usually take down the whole plant in a week or so if you can deliver the right dose to the right plant at the right moment. You have to mix the liquid concentration with water which allows you greater control over the intensity of the dosage.
The ratio of water to herbicide concentrate will depend on what kind of weed you are dealing with. The more developed the root system, the higher the concentration. As simple as that. You have to wear protective gear and apply the spray on a wind-free, moderately warm and humid day for optimal diffusion.
Things to Consider While Choosing the Best Post Emergent Weed Killer for Your Target Plant
Sounds overwhelming, much? That’s okay. Lawn care is no joke. It takes a combination of multiple trials and errors with a generous helping of patience to perfect your timing. You will get there eventually, don’t worry.
Below I have listed the key factors that affect the herbicide effectiveness. Pay extra attention to what I am about to discuss as the future of your lawn/crop-field depends on the following factors:
Extreme temperature and low humidity are the biggest roadblocks to plant growth, and therefore, the absorption rate of herbicides. As I repeatedly mentioned throughout this article, post emergent weed killers, especially the systemic kind, only work when the weeds are actively growing.
The translocation from the stem to the roots takes place via the sugar flow inside the vascular system of the plant. When the climate is too hot or cold and dry, the plant is put under extreme stress which causes the stomata of the leaves to close themselves which slows down the photosynthesis rate.
The sugar movement from stem to the other parts of the plants slows down with this reduced photosynthesis rate which inevitably minimizes herbicide movement inside the plant.
The ideal climate condition to apply a post emergence is when the local temperature is between 55-85 degrees F. Anything lower than 55 degrees will decompose the herbicide, while temperature higher than 85 will simply evaporate volatile herbicide solutions before the leaves can absorb it.
This is the sole reason why most post emergence works best between spring to fall. However, mind you that this might not apply to all situations.
Some groups of post emergence are extremely sensitive to sunlight. Prolonged exposure to scorching sunlight can cause photodegradation of the herbicide, which mainly applies to soil-applied weed killers. Foliar applied herbicides can take effect only when the plants are metabolically active.
In other words, these weed killers can penetrate the cuticle layer of the leaves when the photosynthesis rate is high. Photosynthesis requires exposure to sunlight. You get the drift, don’t you?
Required Rain-free Period
Just like many herbicides can’t work their way into the system in the presence of high heat and sunlight, many (actually, most) herbicides require at least 8 hours of rain-free period to diffuse into the plant system.
If you really have to treat the weeds in rainy weather, look for a rain-fast herbicide which facilitates rapid absorption, requiring only 10-15 minutes free of rain to get the job done.
The Bottom Line
If I have to sum up this post emergent weed control guide in one line, I would say- kill them when the time is ripe.
There’s no such thing as the best post emergent weed killer ever! It’s all about when you use it, how you use it, and on what kind of weeds. Some weeds become too strong to control by the time it’s winter in your country while some are impossible to get rid of its hot and humid climate.
Some species can be dealt with topical, selective herbicides, some stubborn species only give in to total weed killers/non-selective systemic herbicides. So what’s it going to be? Think about it, understand your enemy, and choose your weapon accordingly. That’s all.