When To Apply Weed Killer | The Right Timing For Better Care of Your Lawn Or Crops

Gardeners know that weeds can be such killjoys. All the work that you put into tilling the soil may be lost because some damn plants won't stop growing where they shouldn't. But using herbicide might mean death to your plants because most of them are not selective, so you need to know exactly when to apply weed killer, if at all.

For some traditional gardeners, the best way to kill weeds remains tilling the soil and manually uprooting the weed plant roots one by one. Painstaking, but still efficient.

However, if you're going the faster, easier way of chemical herbicides, hang on. In this article, we'll look at how to decide on when to apply weed killer, and what type you should be using for the errant plants in your garden. You can go to this post if you want to see the best weed killers.

When is the best time to apply weed killer?

There are a few factors that will work together to help determine the best time to apply weed killer.

1. Weather

weed killers are best applied in dry weather

Most weed killers are best applied in dry weather.

Otherwise, rainwater can wash off the herbicide from the surfaces of the weeds that were sprayed. Subsequently, the residual effect of the herbicide will be diluted to the point that it becomes ineffective.  

However, some herbicides are rainproof  for up to two hours after a rainfall, so if the weather forecast is looking cloudy and you absolutely have to spray a weedkiller, you should look for that option.

Another weather factor to consider is wind, especially if you're using glyphosate or any similar nonselective herbicide that kills all plants, desirable or not. Windy conditions can create a herbicide drift, spreading the chemical to other areas of your garden and killing your precious desirable plants.

Additionally, some weed killers work best when they are applied at certain times of the day, yet timing has little or no effect on others.

2. Gardening timeline

Before applying a herbicide, a gardener should consider general planting timeline. Some herbicides are long-acting and can disrupt any seasonal plans for replanting, for example. Weed killers must be applied early enough so as not to disrupt other gardening plans.

3. After-effects

Some herbicides have a lasting residual effect and remain toxic on plants long after they've been sprayed. This could be as long as a month. Before choosing a weedkiller, and more importantly deciding when to spray it, read its label to see how long it takes to work and how long it will linger on your plants.

You could spray a lingering herbicide if you're going to be leaving your garden unattended for a while, say if you're going on a short trip, for example.

Types of weed killers and when they are used

Some weed killers are designed to prevent the problem plants from germinating at all, while some are specifically made for destroying the weeds that have already grown. How do you know when to use which?

Pre-emergence herbicides

gardeners use pre-emergence herbicides

Gardeners use these in areas like an established lawn to disrupt weed life cycles, especially for annual problems like summer weeds.

Common pre-emergence herbicides include dithiopyr and benefin, all which create a chemical barrier in the soil that prevents weed seeds from germinating.

When to apply pre-emergent weed killer

For best results, pre-emergence herbicides should be applied before weed seeds start to germinate, usually in the early spring. You could reapply the weed killer about seven or eight weeks after the first spray, to tackle exceptionally resilient weeds like crabgrass.

Pre-emergent herbicides are effective when applied at any time of day.

Post-emergence herbicides

These are for damage control, and they can be systemic or non-systemic.

Systemic herbicides work by penetrating plant tissue and destroying their roots, so they're most effective when the plants are actively growing. Glyphosate is a commonly used systemic herbicide.

Systemic post-emergent herbicides are best applied at the peak growth time of the plants during the day. Plant growth speeds up or slows down over the course of a day, depending on light level, temperature, and the local climate.

In warm climates, early morning and late afternoon are the best times for applying systemic herbicides. Midday heat causes plant growth to slow down, so the plant tissues are harder to penetrate and the herbicides quickly dry out on the leaves.

In winter, especially in warm climates, some weeds continue to grow.

best time of day to apply systemic post-emergent herbicides is the middle of the day

At this time of year, the best time of day to apply systemic post-emergent herbicides is the middle of the day, when the temperatures are highest and the plants have shaken off dew and frost.

Non-systemic herbicides damage plant tissues and they can be applied at any time of day.

For optimal absorption, post-emergence herbicides work best on new, young weeds because they're still tender and easily penetrable.

If a gardener is battling an already established weed population, like woody shrubs, he'd get better results if he cuts the weed down then applies the herbicide to the new shoots or new growth.

When to apply weed killer to lawn

‘Oh, my lawn is all weeds! But the last time I applied weed killer, it ended up killing off everything, and I had to start from scratch'.  Classic weed killing-gone-south story. But after reading this article to the end, you'd never have to say those words again.

The best time for lawn weed control is when the weeds are actively growing unless you're trying to prevent weeds from appearing. In that case, apply a pre-emergence weed killer before the weeds begin to grow.

Pre-emergent weed killers control annual lawn weeds while they sprout, so the herbicide must be on the lawn before the seeds begin to germinate.

Weed killers are most effective when applied evenly over the entire lawn, but only if you're using a selective weed killer, or else you could lose your whole lawn. Use a weed killer sprayer to evenly distribute the herbicide after mixing it according to recommended ratio.

when to apply weed killer to lawn

The key is to properly time the herbicide application to the growth span of the weeds. So, to control summer annual weeds, apply a pre-emergent weed killer in early spring, and to control winter annual weeds, apply a pre-emergent herbicide in early fall.

For good control over a broad range of weeds in your lawn, try starting herbicide application in May.

If you need to repeat the treatment you still have plenty of time before cold weather makes it impossible.

Do not apply pre-emergent weed killer to a new lawn. Wait till the grass has an established root system, then you can apply a pre-emergent product that contains dinitroaniline.

However, if you can already see annual weeds in your lawn, it's too late to apply a pre-emergent herbicide. You should use a post-emergence weed killer instead.

Using post-emergence herbicides on lawns

Post-emergent weed killers are most effective when the weeds are actively growing. Selective post-emergent herbicides target certain types of plants, for example perennial broadleaf weeds like dandelion, or annual weeds.

For a post-emergent weed killer to control perennial broadleaf weeds, the best seasons for applying are early fall and spring.

Apply the weed killer on a still day when the temperature is between 60 and 80°F, and there's no rainfall in the weather forecast for another 2 days.

For newly seeded lawns, wait until the lawn has been mowed four times. If the lawn is newly sprigged, do not apply post-emergent herbicide until the sprigs are rooted and growing. Apply the weed killer at half the rate recommended on the label, and reapply the product seven to ten days later to treat weeds that may have survived the first application.

When to apply broadleaf weed killer

Herbicides for broadleaved weeds work best within certain temperature ranges. Herbicides for broadleaf weeds control many lawn weeds but usually leave grass unharmed. If temperatures are too low, these herbicides may not be effective. When temperatures are too high, the herbicides can damage turf grass.

broadleaf weed killer

Broadleaf herbicides work best at temperatures between 50 and 85°F. In cool or cold climates in spring or fall, wait until the warmest part of the day to apply them. In warm climates in summer, avoid midday heat, and apply the herbicides in the morning or late afternoon.

Fall (mid-September to early November) is the best time to exterminate broadleaf weeds.

During this time, perennial broadleaf weeds are transporting food (carbohydrates) from their foliage to their roots in preparation for winter. So the weeds will transport the herbicide alongside the carbohydrates to the roots, resulting in the destruction of the broadleaf weeds.

When to apply weed killer in spring

Timing is everything when it comes to spring weed control because spring brings its own tenacious share of weeds, including the notorious crabgrass. This annual weed tends to appear in weak or bare areas of a lawn, and it can survive in extreme soil conditions.

Crabgrass control should be done in the spring with a pre-emergence herbicide to keep seeds from sprouting. Crabgrass germination actually occurs after the soil temperature has reached 55° F for several consecutive days, so you can check with a reputable garden center to fine-tune timing and soil temp conditions in your region and apply the crabgrass killer before the weed sprouts.

Depending on all the factors we have explained above, knowing when to apply weed killer really just boils down to understanding soil and weather patterns. Hopefully, that doesn't seem so daunting now. Here's to a good soil year!

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