weed seed killer

How weedkillers work

In order to understand how weedkillers work, we first need to understand what a weed is. There are many types of weed and the type you have will determine the weedkiller you choose to get rid of them.

What is a weed?

The dictionary says that a weed is ‘A wild (not deliberately cultivated) plant growing where it is not wanted.’ By this definition, virtually any plant that’s growing where we don’t want it to grow is a weed.

But it’s those unwanted plants that invade and threaten to take over your paths, patios, flower beds, borders and lawns that are the real problem. If you need any help identifying plants that are commonly thought of as weeds, we have recently written an article showing the characteristics for identifying weeds.

What are weedkillers?

Weeds can be very frustrating and extremely difficult to remove, especially without them reappearing. With traditional methods of weed removal proving to be very arduous work in some circumstances there needed to be a more effective alternative.

Weedkillers are chemical-based liquids that are specifically engineered to target different types of weed in the best way possible. Whatever the weeding job that you are trying to tackle, there is a weedkiller that is matched to that job and will do exactly what you need.

When to use weedkillers

Weeds can generally be killed at any time of the year. But the best time to use weedkillers is when they are growing strongly (usually in the Spring and Autumn) because they absorb weedkiller more quickly. This is particularly the case with Systemic and Selective weedkillers which rely on activity within the plant for their effectiveness. Apply liquid weedkillers in calm conditions to minimise the drift of the solution onto wanted plants. Application to the leaves results in weedkiller moving up to the growing points and down to the root tips.

Location of the weed is also important. For example, if a weed is growing in the middle of the lawn then you should use a selective weedkiller, and if it is growing between patios or pavings then a residual weedkiller would be more appropriate.

In situations when you are looking to clear a patch of land and plant up quickly it is possible to use a glyphosate-based weedkiller which would allow you place new plants after only 24 hours without causing the new plants any harm.

How to use weedkillers

Using weedkillers is often simple providing you read and follow the detailed instructions that are clearly printed on every weedkiller pack. Unless you are using a hand sprayer or a Pull & Spray pack, you will need to:

Use a watering can

With a fine rose or a weedkiller sprinkle bar that will give you effective control over where you apply the weedkiller. Remember a separate watering can is advisable if accidental damage to other plants during normal watering is to be avoided.

Use a pressure sprayer

These are amazing for treating large areas. Always read the label to find out which application method (watering can or sprayer) is suitable for the product you want to use. Do not walk across treated areas until dry, to avoid the transfer of products onto desired plants.

Weed by hand

Grasp the weed as close to the ground as possible and steadily pull it out (roots and all). This is fine for the odd one or two weeds, but back-breaking if the weeds are rampant. It is ineffective with deep-rooted or spreading weeds. Find out more about hand weeding.

Dig or forking out weeds

Even more strenuous than hand weeding and care must be taken to remove every bit of the offending plant and roots without damaging the plants to be kept. Any bits of perennial weed roots left in the ground will grow into new plants.

Hoeing weeds

‘Topping’ the weeds with the hoe blade just below the soil surface avoid some of the backache and the weeds have to be picked up if you want the garden to look tidy. Hoeing the roots of perennial weeds often increases the problem and brings the seeds of annual weeds back to the surface. Less effective in wet weather, hoeing perennial weeds often increases the problem. Hoeing is best done on a warm day or a windy day, so the hoed weeds die quickly.


Applying a thick layer of organic matter to the soil surface – called a mulch – helps to suppress weed growth. Find out more about mulching.

Types of weedkillers

Post-emergence Herbicides

As the name would suggest, these herbicides are used to kill weeds that have already germinated. These weedkillers are most effective when the weed plants are in there early stages and before they’ve begun to produce seed(s).

Pre-emergence Herbicides

You’ve got it. Pre-emergence herbicides are the ones lurking out of sight. Apply weed killer in the fall to kill perennial weeds. Perennial weeds store energy in their roots in the fall before they go dormant in the winter. Spraying in the early fall will help prevent the plant from storing energy and re-emerging in the spring (best pop that on the calendar). Pre-emergence herbicides are mostly used to control annual grassy weeds and broadleaf weeds. It’s important to note that pre-emergence herbicides have no effect on weed seeds that have already germinated. The general rule is that pre-emergence herbicides are most effective about for six to twelve weeks. It’s also important to note that these weed killers can also kill grass seeds, so do not apply to lawns within two to four months prior to seeding and equally wait at least one month after seeding to use.

Selective lawn weedkillers

This is the most common type of weed killer – killing only certain species. They usually target broadleaf weeds and are suitable for large lawn areas and do not usually cause damage to the grass. However, some selective herbicides can be harmful to a certain type of grass (these are usually the products targeted at sedges and rushes), make a point of checking before buying a product. Selective weedkillers work by stimulating growth in the weed that cannot be sustained. In effect, the weed actually ‘grows itself to death’.

They take some time to work – possibly up to 4 to 6 weeks – slower when it is cold, quicker in good growing conditions, warm, moist soil and when weeds are growing actively. One application can last for a whole season.

Non-selective weedkillers

Nicknamed the ‘broad spectrum’ herbicides, these kill most of the plants that they come into contact with. They are most popular for removing weeds that are growing on hardscapes: in sidewalks, driveways and other nooks and crannies.

Contact weedkillers

As the name would suggest, this particular herbicide kills the above-ground subject that it comes in to contact with. Repetitive doses may need to be applied to fully kill the weed population. Usually, contact herbicides are classified as non-selective. Contact weedkillers work on contact with the leaves of the plant. They start to work as soon as they are applied to the plant. After as little as 10 minutes, some treatments are impervious to rain. They are fast-acting – the leaves turn yellow then brown and it’s all over in a few days. New plants can be planted or seeds are sown straight after the initial application has dried on the weed leaves, which can be as soon as 24 hours for some types of plant. Contact weedkillers are the perfect choice for killing annual weeds and ‘burning off’ the foliage of perennial weeds.

Systemic weedkillers

This type of strong weed killer travels down the root to the underground system of the weed (when applied directly to the leafy above-ground part). Systemic herbicides are effective in breaking down the weed, however, they are slower in doing so than others. Systemic weedkillers kill from the inside out. When sprayed onto the leaves, they are absorbed and move all around the plant to kill the whole plant – including the roots. This makes them the perfect answer for perennial and difficult to control weeds. Penetration through the leaves takes some time and effectiveness can be reduced if it rains within 6 hours of application. A job this thorough takes time. The weeds should be left undisturbed for a week. Initial effects are normally evident within 7 to 10 days and complete weed control in 3 to 4 weeks.

Residual weedkillers

Residual weedkillers work by creating a weedkilling barrier in the ground that prevents weed seeds germinating. This prevents the weeds from getting established in the first place. They are held on the surface for several months and so control weed growth for a long time.

The death of the sprayed weed can take up to 28 days, however, just one application can last a whole season. Once treated the area should not be dug or disturbed in any way, as the long-lasting control will be reduced.

Chemical weedkillers take the backache and hard work out of weeding. They are also a very efficient way of getting rid of weeds – especially otherwise difficult to control, deep-rooted

perennial weeds. You need to consider the type of weed & it’s location to select the right weedkiller for your purpose. Take note of how products differ and pick the best option for you and your garden. Also, make sure you time it right for maximum effect. Different weedkillers are specially formulated for specific weeding tasks. When skilfully selected to match the job in hand and used with care, chemical solutions will give you the weed control you need.

What is a Weed? The dictionary says that a weed is ‘A wild (not deliberately cultivated) plant growing where it is not wanted.' Read more at Love The Garden!

Kill Weeds In Lawns: Begin With The Basics

If you have a yard, you have weeds. They may lurk in the lawn, thrive under a shrub or flourish in flowerbeds, making weed control a constant battle. It requires patience, persistence and knowledge – of both types of weeds and the weapons you have to eradicate them.

What Is A Weed?

A weed is a plant that’s growing where it’s not wanted. Weeds aren’t something you plant intentionally; they just appear. Often they grow vigorously, outpacing and overrunning desirable plants. There are several types of weeds:

Annual Weed: Completes its life cycle – from germination to setting seed – in one growing season; some annual weeds complete their life cycles in a matter of weeks, producing several generations in a single year.

Fact: Annual weed seeds can lie dormant in soil from 4-40 years.

Examples: Chickweed, Crabgrass, Lamb’s-Quarters, Annual bluegrass

Perennial Weed: Lives for two or more years; plants grow as long as conditions are favorable and frequently die back to soil level with hard frost; new growth emerges at the start of the growing season, originating from roots or stem remains; in warmer regions, some perennial weeds can be green year-round.

Fact: Perennial weeds spread by various means, including seed, stems that root as they creep along or pieces of root.

Examples: Creeping Charlie, Curly Dock, Dandelion, Plantain

Broadleaf Weed: Leaves are broad and flat (not grassy or needle-like).

Fact: Broadleaf weeds are easiest to kill or remove when they’re young and actively growing. Some mature broadleaf weeds develop a layer that makes it difficult for weed killers to penetrate.

Examples: Chickweed, Clover, Dandelion, Henbit

Grassy Weed: Looks and grows in ways that resemble grass; leaves are produced one at a time and look like grass blades.

Fact: Many perennial grassy weeds form rhizomes, fleshy roots that resprout if left behind in soil during hand-weeding.

Examples: Bermudagrass, Crabgrass, Giant Foxtail, Goosegrass, Quack Grass

All About Lawn Herbicides

An herbicide is a chemical used to kill weeds or inhibit plant growth. Anytime you use a weed killer, you’re using an herbicide. Some herbicides have residual properties, meaning they continue to kill weeds for a specified time period following application. There are several types of herbicides:

Non-Selective Herbicide: Kills any green and growing plant, whether or not it’s a weed.

Selective Herbicide: Kills only specific types of growing plants; for example, a selective herbicide may kill broadleaf weeds and not grassy plants, so that you can spray it on broadleaf weeds in a lawn without harming grass.

Pre-Emergent Herbicide: Prevents seeds from germinating or kills germinating seeds before seedlings emerge from soil; must be applied before weed seeds germinate. A common example of a pre-emergent herbicide is a Crabgrass preventer, which prevents Crabgrass seeds from establishing new plants.

Post-Emergent Herbicide: Kills weeds that are actively-growing and have already emerged from soil. It’s an ideal herbicide for spot-treating lone offenders but is often applied to entire lawns. Post-emergent herbicides come in two basic forms – contact and systemic.

  • Contact herbicide – only kills the plant parts the chemical touches; ideal for treating annuals and perennial weed seedlings
  • Systemic herbicide – absorbed by leaves, stems or roots of a plant and moves throughout the plant, affecting every part; effective on annuals and established perennial weeds; must be applied when weeds are actively growing

Specialized Herbicide: To control some especially challenging weeds, like Nutsedge, Clover, Creeping Charlie or Bermudagrass, you’ll want to choose a specific herbicide that’s been proven to be effective. Ask a local garden center or your local Cooperative Extension System office to learn which herbicides will beat your toughest weeds.

Pre-Emergent Secrets to Success

  • Treat the entire area – pre-emergent herbicides create a weed barrier, so if you miss a spot, weeds can sprout there. It’s vital to treat the entire area.
  • Water in – most pre-emergents require watering in, even liquid forms applied using a hose-end sprayer. With liquid herbicides, the volume of water used to disperse the weed killer is not great enough to wash the material into soil, where weed seeds lie waiting to germinate. That’s why you have to water after application.
  • Use caution – when applying pre-emergent herbicides to newly-seeded lawns – or to areas you plan to seed. Read the label carefully. For most products, the label stipulates how many mowings, after seeding, to wait before application. The label also states how long to wait after application before sowing lawn seed.

Post-Emergent Secrets to Success

  • Treat young, actively-growing weeds – they die most easily and create less of an eyesore than mature weeds, which might require repeat applications to completely kill weeds.
  • Inspect the lawn frequently – while mowing is a good time to look for new weeds that have germinated and require treatment.


For both pre- and post-emergent herbicides, timing is critical. For post-emergent herbicides, you’ll have the best success spraying young, actively-growing weeds. Mature weeds may require repeated applications for total kill. Most post-emergent herbicides should not be applied to dormant lawns. If applying in spring, wait until the lawn is actively growing and has been mowed at least twice.

With pre-emergent herbicides, you’ll want to apply the chemical prior to the time weed seeds start germinating, which can be spring or fall depending on the type of weed. For example: cool-season weeds, such as Annual Bluegrass, are usually best controlled with a late summer to early fall application. If you apply too early, these herbicides will have degraded and are useless when seeds start to germinate. Most pre-emergent Crabgrass killers remain active in soil for 6-8 weeks. If you apply too late, the herbicide will not affect weeds that have already germinated.

Weed seed germination occurs when soil reaches the correct temperature. The best way to determine the ideal time to apply pre-emergents is to contact your local Cooperative Extension System office or master gardeners, who have access to regional soil temperature data.

Other ways to gauge application time include using bioindicators, such as plants whose growth signals the correct time for application. For instance, in northern climates, spring Crabgrass applications are often timed when Forsythia is blooming, which frequently (but not always) occurs when soil temperatures are in the 50°F range. Another option is to time applications based on the calendar. For example, if you typically apply a pre-emergent herbicide in mid-April with success, then continue that routine.

Of course, you can avoid the issue of proper application timing altogether by purchasing a weed control product that combines both pre- and post-emergent herbicides. This type of product kills existing broadleaf weeds and keeps them from returning for as long as six months.

So you want to start to kill lawn weeds. To kill weeds in lawn, it helps to know the different types of weeds and the different types of herbicides that specialize in killing them. This article even covers secrets to success, including tips on when to apply weed killers for best results to kills weeds.