Longer answer: Research by Bill Gordon showed that even if you keep everything else the same, night spraying can put at least three times more product in the air than daytime application, even if weather conditions are similar and there is no temperature inversion in place.
In both the burndown and in-crop studies, the most effective timing was usually midday, followed by midnight. Coles says, “Since night spraying was usually more effective than dawn, night spraying could be a good option when daytime opportunities for spraying are limited.”
The herbicides in pre-seed burndown and in-crop trials performed most effectively when applied in the day time (12-2 p.m.). Night time (12 p.m. – 1 a.m.) gave better results than the least effective dawn time (4-5 a.m.).
Avoid spraying weeds if the wind has picked up ahead of a storm, or in any particularly breezy conditions. Wind can cause the herbicide to drift and cause damage to plants you don't plan on killing.
Temperatures in the low 90s or above can hinder herbicide uptake. Wait a few days for things to cool off to protect your corn from weeds and added stress. Avoid spraying herbicides, if possible, when temperatures surpass 90 degrees. During a heat wave, weeds shut down and can't uptake herbicides.
Always apply herbicides during the middle of the day when the sun is shining strongly to disrupt photosynthesis. Active spraying during the winter can also ensure that the weeds are not allowed to germinate properly before the arrival of spring.
You usually don't want to mow your lawn within 24 hours before any such treatment. This is because if you mow right before an herbicide treatment, there isn't much leaf area left to absorb the weed control. If a broadleaf weed control doesn't come in contact with enough of the plant's foliage, it won't be effective.
One of the questions we often get asked is - "do you have to pull up dead weeds after spraying them?" The short answer is yes, but it's important that you pull the dead weeds up in a very specific way to ensure that they don't come back to haunt you the following year...
Although spraying Roundup® For Lawns on your weeds is quite satisfying, resist the urge to do it too much. It works so well; one application is usually all you need to kill a weed. And some weeds can take as long as three weeks to die. If it's still hanging in there in two weeks, give it another spray.
Effect of Temperature on Herbicide Efficacy
The ideal temperature for applying most post-emergence herbicides is between 65°F and 85°F; however, that window is not always practical with other fall practices. Herbicides can be applied at temperatures of 40°F to 60°F, but weeds may be killed slowly.
The most effective homemade option is a mixture of white vinegar, salt, and liquid dish soap. Each of these ingredients has special properties that combine to kill weeds. Both the salt and the vinegar contain acetic acid, which serves to dry out and kill the plants.
This is why most weed killers are designed to evaporate within 24 to 78 hours. This means that for the most part, it is safe to plant anything, edible or non-edible, in a place where you have sprayed weed killer after three days. If you want to be extra sure, you can wait a week or two before planting.
Roundup works faster when weeds are actively growing. If weeds are not growing vigourously (e.g. in cold weather, or if the plant is stressed by drought), the process may take up to 4 weeks.
With Roundup® Weed & Grass Killer products, one application is usually all it takes to get rid of most pesky weeds.
There are pros and cons to both spraying and pulling weeds, and we have a good rule of thumb you can use when choosing a method. Hand-pulling is easier when you are focusing on a small area. Spraying weeds is ideal when you're dealing with a vast area or a loftier infestation of weeds.
Flame weeding is considered an organic method of weed removal. However, if the weather is hot and the fire danger is moderate to extreme, Roundup may be the better option to avoid accidentally starting a fire or risking fines from the local fire district or other government agency.
I know that Roundup used to kill all your weeds when they were 2 to 4 feet tall, but like other herbicides, Roundup is best when your weeds are 2 to 4 INCHES tall. Use REAL ammonium sulfate (AMS).
Most Roundup® Weed & Grass Killer products deliver visible results in a few hours, though some take a little bit longer. For Roundup® Max Control 365, it takes 12 hours to see visible results, but the trade-off is year-long hassle-free weed control.
You are not required to use any protective clothing when using Roundup, although we do recommend that when using any lawn & garden products that you wear closed shoes, protective glasses, dust mask, and gloves, where appropriate. Don't forget a hat and sunscreen!
It can persist in soil for up to 6 months depending on the climate and the type of soil it is in. Glyphosate is broken down by bacteria in the soil. Glyphosate is not likely to get into groundwater because it binds tightly to soil. In one study, half the glyphosate in dead leaves broke down in 8 or 9 days.
Roundup is also believed to poison the helpful microorganisms in the soil. Kremer said the glyphosate leaches into the plants through the roots. How long does it take for Roundup to get to the roots? Just one to two weeks.
When is the best time to apply herbicides? In Australia our most common weeds like Bindii and dandelion are Winter annuals, this means they germinate in the soil from around April-June and start setting seeds in mid-Spring. These dates leave us a window between July and September for best results.
Don't spot spray
I would recommend spraying your whole lawn rather than spot spraying. Spot spraying can lead to applying too much chemical on each weed (which can burn the lawn), and it can lead to only spraying part of the weed.