Applying pre-emergent weed control or a fertilizer with pre-emergent herbicide in the spring to combat annual weeds such as crabgrass can be a great, low-effort way to improve your lawn. But how long does pre emergent last, and will you need to re-apply it for a full season of annual weed control?
This article was last updated on by Lawn Chick Owner Sarah Jameson
Article content reviewed for accuracy by Certified Horticulturist Nicole Forsyth, M.S.
In today’s article I’ll share the answer to these questions, and provide my tips for success when applying pre-emergent herbicide to your lawn.
Realistically, it depends on your lawn. If you have a thick and healthy lawn, you probably won’t have to do a second application. The thickness of your turfgrass will help create shade from the sun. Weeds need sunlight to germinate and thrive, so if you have a dense canopy of established grass, delaying germination of annual weed seeds is probably enough to block their presence for the year.
But if you have a thin lawn or are struggling with patchy grass, a second application of pre-emergent is usually advisable.
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Will Pre-Emergent Last the Whole Season?
So, how long does pre emergent last? Well, every pre-emergent herbicide product is a bit different when it comes to how long it’s effective.
In general, one pre emergent treatment can last between three and five months. This means that most people can apply this once annually and get good results. If you have a lawn that’s a work in progress, you may want to apply pre-emergent twice per year.
Many professionals say you should do a split application. This means applying one month apart. This can help make your pre-emergent more effective.
Nicole Forsyth, M.S., a certified horticulturist and member of our expert panel explains that “a split application of pre-emergent herbicide is more effective and you are putting less chemical down at once. Any weed seed that is not killed by the initial application (either it was missed, or perhaps germinates at a different time) has a higher chance of being terminated.”
Make sure you do your research when choosing the product to use on your lawn. Read the label very carefully and ensure that you follow all the instructions.
What’s the Best Time of Year to Apply Pre-Emergent Weed Control?
It’s very important to apply your pre-emergent weed control product at the proper time of year. Most of the negative reviews you’ll see online for these products are from people who applied the product too late, after annual weeds had already started to germinate.
Remember that pre-emergent herbicide must be applied before weed germination. This type of product doesn’t kill weeds, it blocks their germination, so if you wait too long you’ll render it completely ineffective.
Many weeds, such as crabgrass, are annual summer weeds.
Pre-emergent will never be successful if you apply it too late. Many homeowners try to get the timing with their yard’s soil temperature just right, but in my experience it’s better to apply it at a time that seems too early than to wait too long and miss your window.
Is Pre-Emergent Weed Control Potentially Harmful to my Lawn?
As long as you do your research and choose the right pre-emergent product, it will not be harmful to your lawn (unless your lawn is nothing but weeds).
It’s also good to keep in mind that you should not apply treatment immediately before overseeding, or when laying seeds for ornamental plants. If you do, it will interfere with their growth and germination as well.
There are some pre-emergent herbicide products which work in tandem with overseeding projects (like this product from Scotts), and if you plan to overseed in the springtime I always recommend using this product so you can get your new turf off to a great start and delay competition from annual weeds.
Should I Use Pre-Emergent Even if I’ve Not Had Prior Issues with Weeds?
If you have never had weed issues in your lawn, that doesn’t mean that you never will. In my view, annual application of pre-emergent herbicide is one of the foundations of building and maintaining a healthy lawn.
For that reason, I feel it’s still a good idea to use pre-emergent weed control even if you’ve never had a problem with weeds before. Remember that seeds from weeds, such as crabgrass, can be transported by the breeze onto your lawn, and it’s much easier to stop crabgrass with pre-emergent than it is to kill it later with a post-emergent herbicide.
Even if you already have a thick and healthy lawn, you should still treat your lawn with some kind of pre-emergent herbicide early in the spring, before ground temperatures warm enough for seeds to germinate.
Do Not Spot Treat with Pre-Emergent
If you have a few problem areas where you always struggle with crabgrass, poa annua or other annual weeds, it can be tempting to save some cash and apply pre-emergent only to those areas.
I don’t recommend that.
Rather, I encourage you to apply pre-emergent weed control to your entire lawn all at once rather than trying to do spot treatments.
This is called broadcast treatment and it is always the most effective approach. These annual weeds produce thousands of seeds each year, and those seeds travel easily on the wind, on pets, on shoes, and even on the blades of your lawn mower.
Treat your whole lawn each spring for best results.
Other Tips and Facts
Below I’ll share some other facts about pre-emergent herbicide to help you use these products successfully on your lawn:
Don’t Plant New Seed After Application
You shouldn’t plant new grass seed after applying a pre-emergent. There are some exceptions to this, but most pre-emergent products are non-selective. This means that they act as general seed preventers. If you are planning to overseed in the spring, again – use this product for best results.
Get the Right Weed Control
Make sure you get the right pre-emergent product for your needs. There are different products, and each blocks different types of weeds.
Take a look at the list of weeds that is provided on the product label. If the weeds you want to prevent aren’t included, you know that the product isn’t for you.
Apply Carefully!
Any time you work with herbicides it’s important to wear PPE to keep yourself protected, and to read the manufacturer’s recommendations about when it is safe for children and pets to enter the lawn again.
Make sure to evenly distribute the product, and keep an eye on the weather.
It’s important to always read the product instructions to find out what you should do for watering.
With most products, you will need to water the product into the soil within the specific time period. This is often within one and three days of application. Certified horticulturist Nicole Forsyth, M.S. says that “within 3-5 days is fine” in most cases.
You must also ensure that you use the correct amount of water.
With most pre-emergent herbicides you will need to apply once annually, but if you have a thin lawn with bare patches, you’ll probably want to double-up on your application to block weeds for the entire season as your turf fills in.
Certain kinds of garden chemicals can cause stains on concrete, as well as walkways and stone patios. Be careful when applying the product, and quickly clean up any granules that remain on your hardscaped surfaces with a shop broom after you finish.
Types of Pre-Emergent
When it comes to applying pre-emergent, you have options.
Granular Pre-Emergent
The granular form is most common with pre-emergent products. You should be able to find this kind of product very easily at a local hardware or box store, but my favorite product (this one from Andersons) you’ll probably have to order online.
To spread the granular product, you should use a spreader. While it can be tempting to use a small handheld spreader if you have a small lawn, I do recommend using a walk-behind broadcast or drop spreader with herbicides so you get less of the product on you and into the air.
The spreader you use to fertilize your lawn will work fine.
The Spreader I Use & Recommend
I’ve owned and used a number of different broadcast spreaders, and if you want the best one available, I recommend The Andersons Yard Star spreader. It is American made, can hold 50 pounds of material, rolls smoothly and easily, and is the most accurate spreader I’ve ever used.
Liquid Pre-Emergent
It’s also possible to find liquid pre-emergent products. Most of these are concentrated products that you’ll dilute in water and spray with a pump or backpack sprayer.
These are very effective, and can offer a cost savings if you have a larger property, but for some homeowners mixing and spraying herbicide can be intimidating.
Pre-Emergent Can Help Maintain a Beautiful Lawn
How long does pre emergent last? It depends on the product, but most will effectively block seed germination for 3-5 months. This means that if you have an established, thick lawn, you’ll probably only have to do a single application in the spring. If you have a thin, patchy lawn with significant areas of bare soil, you’ll probably want to do two applications per year.
Either way, pre-emergent weed control is a wonderful way to prevent weeds from growing on your lawn. That old saying: “An ounce of prevention is worth a pound of cure” is certainly true when it comes to lawn weeds.
Your effort and investment will be well worth it when you enjoy a green, weed-free lawn.
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-Sarah
