A Guide To Using Weed Preventer Products

A PreEmergent lawn care product eliminates weeds at the earliest stage of growth — before you even see them. Several key factors are important to consider if you want to use this type of weed killer effectively. 

Have you hesitated to use a preemergent herbicide because you didn’t understand what it does or how to apply it? Did you try one and have poor results? 

This article will explain the concept of weed prevention: 

  • the benefits,
  • the drawbacks,
  • when and how to apply it.

Learn to avoid the mistakes that lead to disappointment and more work!

A separate concluding article will assist you in selecting the best type of PreEmergent for any situation, including lawns, landscaped garden areas, vegetable gardens and non-vegetation areas. 

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Weed control & herbicides

Weed Free! (Almost)  

Have you spent countless hours pulling weeds by hand, or repetitively spraying weed killers? We all hope for a better way to improve the appearance of our lawns. Welcome the PreEmergents: a group of garden products that offer a respite from that aggravating problem. No, they do not offer automatic salvation as in ‘use once and forget about it’. But they are one of the most important weapons in a gardener’s arsenal.

Typical applications occur in early spring before warm season weeds appear and in autumn for cool season weeds. These preventative herbicides actually can be helpful throughout the majority of the growing season. Various weeds germinate at different times, so repeated applications of the correct chemical can eliminate the majority of the weeds in any lawn or yard. Note these benefits:

  1. The weed is eliminated permanently, because the seed is allowed to germinate, and the young tender sprout is killed. A PreEmergent is activated in the top layer of soil, where the weed seeds sprout, and by design it stops the plant from growing.
    This is one key advantage over just putting down new soil or mulch to bury the seeds.
    Weed seeds remain viable many years, waiting for a chance to pop up. Pre-emergents break the cycle of constant regrowth of new weeds.

  2. A minimal amount of labor is involved in applying either a granular or a liquid version. The chemical is activated by applying water, usually within a couple of days, sufficient to soak it into the ground. It can remain effective in the soil for several weeks up to several months, depending on the ingredient used and the concentration of it.
    This benefit comes right at the time when there are so many other gardening tasks to which you want or need to give your attention.
  3. Deal more easily with any remaining weeds that manage to escape this treatment. (Wiping out those stragglers deliberately and personally gives you a sense of satisfaction, since you finally have the upper hand!) Most importantly, you are able to knock down any remaining population of weeds before they go to seed.
    Without relying on a preemergent to reduce the infestation, it always seems to be a losing battle trying to kill every weed before it reproduces.
  4. Each successive season the need to spray a post- emergent weed killer is lessened and perhaps gone for good. The ideal status of completely eliminating weeds would also require that you take other measures to get your lawn healthy, thick and strong, and able to out-compete any challengers.
    In many yards, due to outside infestations, the need to continue with this herbicide will continue. Yet the advantage will be yours.

What if...? or Oops!

  1. Some weed seeds are not affected by certain pre-emergents, so it is critical to check the label for a "List of Weeds" that will be susceptible.
    (If your undesirable weeds are not mentioned, don't hope for the best -- check another product!)
    The active ingredient of each herbicide must be evaluated for your specific situation. Do NOT grab the cheapest or prettiest bag and expect to get lucky.
  2. Do NOT plant new grass seed when you take this approach (with one possible exception). Most products are non-selective, meaning they could be called Seed Preventers instead of Weed Preventers.
    (A potential negative impact also exists if you desire to lay down grass sod immediately in the treated areas.)
    Most typical lawn grass seed will not grow, so you must calculate the timing of any application to allow a window for starting a new lawn or reseeding a meager lawn. The label should advise you.

  3. A pre-emergent is NOT effective once the weeds are showing. (Again, I’ll mention one exception later.) The chemical must be situated in place prior to the weed seed starting to germinate. Being tardy is inexcusable. You wait, you lose. If you miss the opportunity, these products will do nothing.
    This is one of the biggest frustrations for the gardener who just didn’t get to the chore soon enough, and the cause of many complaints that ‘the stupid stuff didn’t work’.

  4. Spreading out the herbicide so that it is evenly distributed is critical. As explained later, you don't have to be meticulous, but leaving any untreated sections will guarantee unfavorable results.

  5. It must also be watered into the soil within the appointed time period, which varies from 1 to 3 days. The proper amount of water is essential. Failure to do so, or taking a shortcut, (or a long one) can/will result in failure. You don’t have to be perfect, but a careless approach is prone to disappointment.
  6. The soil must remain undisturbed in order to maintain the barrier which is established by applying a weed pre emergent. If the ground is cultivated by any means, just in the top couple inches of soil, the chemical may not contact the seeds, and they will be free to grow.
    Digging done by gophers, moles, dogs, chickens or other critters can interrupt that barrier.
    Pulling out existing weeds, especially if they have a large taproot like dandelions or a dense root system, can leave behind a channel or safe zone for the weeds to survive.

  7. Yes, it is a chemical. Many people must or prefer to avoid another source of chemical exposure. Although the presence of this active ingredient is minimal compared to spraying a weed killer and leaving it on the foliage to have its effect, it is what it is.
    A single organic variety provides an alternative, but one that will be more expensive and shorter-lived than the chemical type.
  8. A pre-emergent has to be replaced, or replenished. In some situations it requires two or more applications per year. Next year will be the same. However it is possible that continued elimination of weeds, not letting them reseed (at least from your own yard), can bring you to the point where you may be able to skip the repeat part of the process.
    (This ideal status would also require that you take other measures to get your lawn healthy, thick, strong and able to out-compete any challengers.)
    In most cases, the need to continue with this herbicide will remain, but hopefully the need to spray a post-emergent weed killer is lessened and finally eliminated.
  9. Some brands of chemicals may cause staining on concrete or stone patios and walkways.
    We’ll identify those later on the product page, but be aware that the potential exists to make a mess. Hopefully, the manufacturer will post a visible warning on the package, but do search for it.

Do you Procrastinate?

Many folks just want to get the product down and get on with life. Do it when you think about it, is that o.k.? As long as you are early enough, as mentioned before, there is no harm in getting the pre emergent down way ahead of time. It just might wear out too soon to get the later batch of weeds if you start too early. 

So if you realize that your yard conditions will likely require a second application anyways, just get it down and breathe easier. But what if you are late? Investigate the concerns about timing at this article When To Apply Crabgrass Preventer. Much of the information will apply to broadleaf weeds as well as the crabgrass concern that is emphasized. (It’s never over, if you don’t give up!)

The gardener who wants to know specifically when to apply a preemergent is going to have to do a bit of research. The temperature of the soil as it starts to warm up in the spring is going to be the primary indicator of the pending arrival of various weed species.

You can be prepared for that by checking with the resources available in most states through your State University Extension program or through your County Farm Agent. (Yes, even if you live in the city, like San Diego for example, there is a wealth of helpful information from the farmer’s friend!) They will typically have charts or articles written up that offer the expected emergence date for common weeds in your area.

You can be fairly accurate by using an inexpensive soil thermometer. Testing should be done in moist but not soggy soil. First, poke a hole with a stiff rod to avoid damage to the probe, but not too big, as the soil must be tight against the probe. Notice the differences in various locations that are sunnier, dryer, or more exposed.

A fall or winter application is necessary in some geographical areas. Locations that deal with annual bluegrass or annual ryegrass will want to get their lawn covered prior to the initial seasonal autumn/winter rains.

In some southern climates, a winter application is necessary to protect against crabgrass as early as January or February. 

A summer application can be helpful, even necessary, in those areas where certain weeds wait for the hot weather. Many spots need to be ready for a second onslaught by some early weeds that can reseed and grow the same season, like the ever-present dandelion. 


GRANULAR products are the most common type found in everyone’s local garden department. (In my opinion, it is unfortunate that they are usually combined with fertilizer in local retail stores. It is better to specialize when working on an issue of this nature. Get the pure reemergent if you can find it.) 

These can be spread by hand just as you would throw out chicken feed. (Well, maybe you’ve seen them do it in a movie!) However, this technique can result in erratic coverage.

Pre-Emergents can be applied through the same devices used for granular fertilizers. SPREADERS COME IN MANY STYLES AND OPTIONS. 

hand-held spreader works great to cover a medium sized area, as it throws out the product quite evenly as you turn the crank.
Be aware that the cranking speed and walking speed will greatly affect the rate of dispersal and the distance the granules are thrown by a "hand-held"  spreader.

For larger lawns, a push-type lawn spreader is more efficient, the same type that many people have for applying fertilizer. These are either drop-type or broadcast type.

Most drop-type spreaders will specify on the label or have on-line instructions for what the setting should be on different units. Only a drop-type spreader confines the preemergent to a well-defined area. This can keep the product out of areas to be protected like flower beds, fish ponds, and vegetable gardens. 

Broadcast type spreaders cover a large area quickly. They can be push type or tow-behind a garden tractor. They can produce uneven coverage unless the operator maintains a steady pace while applying. They fling out an herbicide in a broad manner, potentially getting into areas where you don’t want/need the protection. Yet they remain the most popular for  big area coverage.

Safe Application Tip:
If you find yourself dealing with this as a likely issue, and need to keep it out of a flower bed where you want to plant seeds or annuals, have someone walk along with you and hold a large sheet of cardboard, a blanket or tarp that can deflect the granules so they spread into the desired area only.

If you have a relatively small area that needs to be treated, especially just a few bare spots here and there, some manufacturers of lawn and garden products offer a shaker can or shaker bag that is quite handy. You may be able to create one from a coffee can, by drilling properly sized holes in the top. 

In all cases, the intent is NOT to cover the ground completely. You just want an even, consistent dispersal. In a square inch, you might only need a couple of granules, since typical application rates allow 1 pound to cover from 200 − 500 square feet. (Before you start: Read the Label Again! Calculate how large is your area! Plan your approach!) 

Some brands offer a LIQUID Preemergent in either a ready-to-use-spray bottle that attaches to your hose, or a concentrate to mix in your own tank sprayer. These are very convenient —  a very quick way to apply the product to a large area. They are often more economical as well. 

Be especially cautious, however, if the lawn area that you spray abuts any location where you will be planting soon. And even though it is liquid, it will not work if it remains on the grass blades. 

Whichever method you select, it is imperative that you follow up and water the product adequately to get it into the top soil layer.

This does not have to be done immediately, but within the time indicated on the label, usually 1-3 days. Waiting too long exposes the chemical to light and/or heat that will diminish its potency.

You can irrigate the lawn area, or allow rain to water it in, as long as it will be a sufficient amount. In most cases, you need not worry about using too much water. The exception is with the organic corn gluten pre-emergent which can get over-diluted, unlike the chemical products.

Options and Choices.

A lot to consider, right? Already this is a long article. Let’s evaluate this important concern on another page. I’ll get that posted as quickly as possible.  

Top of Page

With Pre Emergent Down, Can I Plant Grass Seed or Sod? Questions & Answers to explain 4 different situations affected by this.

Learn When To Apply Crabgrass Preventer for the exceptions everyone loves to hear.

Can you apply Preemergent for Crabgrass in Bermuda when the lawn is dormant?

My Garden Needs: Crabgrass Preventer See a variety of products listed in the Garden Counselor Store.

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