When To Apply Weed Killer To A Lawn?

When spring rolls around in North America, lawn weeds start to become a concern for many people. A lawn can quickly transform from the appearance of a weed-free turf, to a nightmare scenario of many weeds such as dandelions, clover, and crabgrass.

In many cases, using a weed pulling tool is not a practical solutions to a weed problem. Applying a weed killer is often the most effective method at handling a weed problem, particularly when weeds are abundant in the spring.   

Knowing when to apply weed killer is essential to efficiently killing weeds while also helping not to waste money and potentially harm the environment. 

When to apply weed killer depends on the type you purchase. There are two primary weed control products: pre-emergent and post-emergent herbicides. Let’s take a closer look at both of these to determine when they should be applied

Pre-Emergent Weed Killer

This type of weed killer is applied prior to weed germination. It will greatly deter weeds from sprouting and should only be used prior to the appearance of weeds. If weeds have started to grow, it’s too late to apply a pre-emergent. You’ll have to use a post-emergent (see the post-emergent section below). 

A pre-emergent is typically applied in the spring and fall as a preventive measure for weeds.

During the spring, it is put down most commonly from late March to late May. Southern states will apply it earlier whereas northern states later. You’ll commonly hear to apply a pre-emergent when the forsythia blooms, which is indeed a good time to do so. The ground should be thawed out with the winter weather in the rear view mirror. The lawn soil temperature should be under 55°F when applied. If the temperature is above 55°F, weeds will  start to germinate. A test of the soil temperature can be measured using a high-quality soil thermometer such as this Vee Gee found on Amazon. 

Fall pre-emergents are best applied in late summer to early fall to tackle winter annual weeds. Again, the herbicide will need to be applied before weeds are showing up in the yard. The soil should be at the 70°F mark as tested with a soil thermometer. 

Post-Emergent Weed Killer

A post-emergent weed killer is what you grab when weeds have already appeared. They work to kill the existing weeds and prevent the spread of future weeds. Generally, you will use a post-emergent in the spring and into June when temperatures are above 55°F, but not extreme. However, when the heat starts to hit its peak (85°F and above) for the year in July and into August, weed killer should be avoided. The lawn is stressed during this time and herbicides can cause harm. Post-emergent weed control can resume in late summer and fall when temperatures cool. 

Selective vs Non-Selective Herbicides

Weed killers are made as selective and non-selective herbicides.

Selective herbicides, as its name implies, are selective in what they kill. For example, they may kill dandelions, clover and other weeds, but not crabgrass or your beautiful lawn.  

Non-selective herbicides are indiscriminate about what they will kill. In other words, they will generally kill any vegetation is in their path, including grass. Non-selective herbicides are useful for renovation projects where you want to completely clear out an area and work from a clean slate.

Read The Labels 

Whether you are purchasing your weed control products online or in a store, it best to read the label before buying any given product. The product label will almost always provide the weeds it will kill, when to apply, how to apply, and whether it is a pre-emergent or post-emergent.

Of course, if your lawn is solely riddled with dandelions, for example, you will want to be sure the weed killer you plan on purchasing will eliminate this type of weed. Several of the popular weed killers on the market are designed to kill multiple types of weeds. If your lawn has a variety of weeds, try to find one product that will eliminate them all.  

Be aware that not all selective weed killers are designed for every type of lawn. For example, the label of the popular weed killer Ortho Weed B Gon states not to apply it to bentgrass, carpetgrass, dichondra, and floratam variety of St. Augustinegrass. Be sure to careful follow the label instruction to avoid unintended lawn damage.