When and How to Apply Herbicides on your Lawn

The first and most important part of herbicide applications needs to occur before you step foot in a store to buy any. You must identify the target weeds. Why? Because we don’t want to have an unsuccessful application, failureor apply pesticides unnecessarily. Think about the amount of time and effort you put into your lawn, prioritize that time and effort efficiently and effectively. Identify the weeds.

Once that’s done you will know if it’s a grassy weed or a broadleaved weed. You can find out its lifecycle and what is the best method/approach to eliminate it and what types of herbicides to use.

Types of Herbicides

Contact, nonselective, selective and systemic are the 4 classes which can be further broken down into post and preemergent forms.

• Selective
  • Controls a select species without adversely affecting others.
  • This is most used method of control for home lawns.
• Nonselective
  • Kills everything that it contacts.
  • Used for paths, hardscape areas where injury to other plants isnt a concern.
• Contact
  • Only kills what it touches.
  • Doesn’t translocate to other parts of the weed.
• Systemic
  • This takes longer to work as it has to be translocated throughout the plant in order to kill it, sometimes days to weeks. Glyphosate is an example of a systemic weedkiller that is also nonselective.

Now we have to know about pre and post emergent herbicides. Preemergent herbicides are applied to the soil to come into contact with seeds and newly emerging seeds. Timing of seed germination is key here as a preemergent doesnt stay in the soil for too long, weeks. So again, knowledge of weed and lifecycle are crucial here.

Post emergent herbicides are applied to active developed weeds. The majority of post emergent herbicides applied on lawns are selective and are readily available at your local hardware store. They can cause browning or leaf burn of desirable grasses, so caution and accuracy is needed during application. Read the label for accurate rates and make sure to mix to the right quantity. Watch the weather, particularly rain events and high temperatures.

Ok now we understand the different types of herbicides let’s put a couple of hypothetical situations out there.

Scenario 1:

I have a bluegrass/ryegrass lawn that has dandelions, clover, greater plantain and a few thistle.

Course of action is to use a selective post emergent concentrate herbicide. Concentrate because I want to be able to apply it at different rates if necessary and I get more value out of it. I firstly check to see if my weeds are listed on the label, and it is safe to use on my lawn species. Then I check the label and see it is a mix of dicamba, 2,4D and Mecoprop-p and this is classed as a 3-way herbicide, three active ingredients that will have a different impact on the weeds.

Next, I want to buy a 2- or 3-Gallon pump sprayer. Remember 1 gallon of water roughly weighs 10lbs so a 3 gallon can get a feel heavy. My calculation is 1 gallon of water covers 1000sqft when sprayed. When I mix up the chemical it says a range between 1-1.5oz per 1000sqft. Through experience I know that I want to apply at the 1.5oz per 1000sqft rate. So, I’m going to put in 3 oz total into my 2-gallon sprayer.

Now I’m going to check the forecast and make sure it’s not going to rain for about the next 8 hours; pesticide needs time to enter the leaf and I want a dry leaf to apply onto. Therefore, I’m going to wait until about 10 – 11 am when morning dew has gone and when dry I’m going to go out and spray weeds. Wear gloves and try to get a fine spray that covers about 6″ per spray. Don’t apply a direct stream of mix as this increases the herbicide quantity and will result in damage to desirable grasses. A fines spray, enough to wet the weed leaves without runoff is necessary to ensure a quality application. Now I’m going to make sure no one enters the area until dry and let the pesticide activate inside the weeds.

Scenario 2:

I have a 15000sqft Common Bermuda dormant non overseeded lawn, and last year I had a massive crabgrass infestation.

In this situation I want to go after all the seeds the crabgrass has produced over the course of last season that sit in the soil waiting to emerge. This is the best management practice after I have identified and understood the lifecycle of crabgrass.

Crabgrass seeds start to germinate when the soil temperature rises above 50F and stays above that average. So, I will monitor my soil, a simple meat thermometer can be used if you have one and measure about 4″ into the soil profile.

Now I want to give me lawn a feed in the spring too, so I am going to buy a granular fertilizer wrapped with a crabgrass preemergent. My time is valuable, and I want to be efficient and actually enjoy my lawn as well as work on it. Consequently, I’ll give my lawn a boost coming out of spring and kill crabgrass seeds at the same time.

In the hardware store I see that the product I am looking at covers 5000sqft per bag (usually says on front of bag), therfore I need 3 bags. Over applying products like this will not benefit your application in anyway and will probably lead to leaf burn and scolding.

Stick to application rate and spread on your dry lawn within a time window where you know there will be rain within the next day. When applying this type of product, set your fertilizer spreader to the correct setting after reading label, do a perimeter lap first and then do the inside section in straight lines making sure you throw the product to the previous tire track and no further. This will ensure appropriate coverage as spreaders fan out products when spreading and edges of the fan has less coverage. Throwing tire track to tire track compensates for this and will provide accurate coverage. Now leave it to either rain or irrigation to water it in and create a barrier to the surface for emerging crabgrass seeds.