The answer to this question is not as straightforward as you might think. There are many types of weeds out there, and the way they react to different herbicides varies dramatically. Knowing what type of weed you have can help determine how long it will take for that weed to die after spraying with an herbicide.

Reasons for weeds in the lawn are:

  • Prolonged drought
  • Lack of nutrients
  • Lawn mowed too short
  • Poor soil
  • The lawn has not been sacrificed for a long time
  • Gaps in the lawn carpet where weed seeds could spread

Mode of Action of Weed Killers

Most weed killers act as contact herbicides on or through the green leaves. With so-called corrosives, you can almost watch the foliage wither within a short time; leaves that are not wetted remain intact. The result is similar to a chemical weed hoe, which removes all above-ground plant parts like a hoe. Annual weeds usually die completely, but root weeds resprout after some time. Since the spray broth must dry on the foliage and not be washed off, dry weather accelerates and improves the effect. The active ingredient remains outside the plant or, at most is absorbed locally by the leaf. Foliar corrosives are effective even at low temperatures below ten degrees.

Systemic agents are taken up by the plant after wetting the green leaves but are then transported with the sap flow to all parts of the plant – including unaffected leaf sections and the root tips. The agents interfere with metabolism and are therefore also called growth herbicides. The result is complete destruction of the plant, whose metabolism is revved up until it dies – it grows to death. However, success is only seen after days. Since the plants have to actively cooperate to transport the active ingredient, the agents only work reliably at higher temperatures and work best in sultry growing weather. Glyphosate, which has come under discussion, is one of these agents.

So-called soil herbicides combat dormant seeds. These agents are of little importance in the garden but are often a component of lawn herbicides, which also act to a small extent through the roots.

Most weed killers do not distinguish between good and evil and go after anything green indiscriminately. These total herbicides should only be used when there is adequate distance between weeds and crops, when you can cover the crops in the bed, or when spraying when there is no wind at all. Selectively acting herbicides are few and far between.

Lawn Herbicides

If the green carpet is riddled with weeds, lawn herbicides can help. They leave grasses alone and only go after the dicotyledonous weeds. That means, however, that couch grass and chicken grass are also spared. Lawn herbicides are sprayed or watered, with spraying being faster and using less product. The weeds turn brown after a few days and seem to literally disappear into thin air.

Prerequisites for a good effect are (night) temperatures above ten degrees. If it is colder, the plants are much too sluggish and do not transport the active ingredients to the tips of the roots. Mow the lawn beforehand and then wait another four days until treatment so that the cut wounds on the blades close.

Tip: High-quality lawn mixtures can prevent weeds. They are more expensive but save trouble and follow-up costs. Brand-name seed forms a much denser lawn in which weed seeds hardly find any gaps to germinate. The cheap mixtures grow well in the first year, perhaps in the second. But then their forage grasses show their true colors: no dense scar, but gaps. Then the formerly price-conscious hobby gardeners start spending a lot of money on care products and equipment, which then, however, only temporarily eliminate the symptoms – the weed growth.

Apply Lawn Weed Killer Correctly

For lawn weed killer to work well, the herbicide solution must be applied evenly to the leaves of the lawn weeds and remain there for an exposure time of about eight hours. These tips will help ensure that you apply lawn weed killer successfully:

  • To ensure pesticide exposure time, it should not rain on the day (and night) of weed control.
  • Since 90% of the active ingredients are absorbed through the leaves, the lawn should not be mowed before treatment with weed killer.
  • Warm, moist soil and mild (night) temperatures increase the effectiveness of the growth enhancers!
  • Proper dosing of weed killers is critical: too much water quickly dilutes the active ingredient solution to the point of ineffectiveness. The equation of adding more water = more herbicide = larger area treatable with lawn weed killer is a fallacy!
  • Fine watering sprayers are best suited for liquid lawn weed killers.
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