The class of weed control products known as pre-emergents are highly effective herbicides if used properly.   They are relatively simple to apply and can save a lot of headache later in the season if care is taken to apply them properly.  Sometimes though, weeds will grow even after you've applied pre-emergents. Let’s look at a how they work, and how some common issues can make it seem like they "didn't work".

Pre-emergents can be applied as liquid solutions, or more commonly, in granular form, which consists of a coating of the pre-emergent chemical on an inert carrier.  In the presence of water the chemical is washed off the carrier and onto the soil.  The active ingredient forms a thin barrier on the top layer of soil, chemically inhibiting the germination process of seeds.  Think of it as a chemical weed mat.  By preventing seeds from germinating in spring, desirable plants get a chance to grow in and shade the soil, minimizing or preventing weed germination later in the season.  Pre-emergents generally have little or no toxicity to already-germinated seeds-that’s why you can apply them over your turf or garden beds without damage.

So why do they sometimes “not work”?

Timing of application

Most weed seeds germinate at a certain soil temperature, about 50F.  In the case of an early spring warm-up or delayed application, the weed seeds may have already germinated unseen before the per-emergent application.  Any seeds trying to germinate after application will be controlled, but the ones already growing (and too tiny yet to see) may make you think the application was ineffective.  Areas where the soil warms up faster due to location, like next to a concrete walkway or a south facing slope that absorbs the sun’s heat, will cause weeds seeds to germinate faster than surrounding areas, resulting in decreased control.  Get your first application down in late February or early March. 

Quantity or Coverage

You must use exactly the coverage amounts called for on the application instructions.  If you have to cover 10 thousand square feet, using the amount to cover 8 thousand won’t work “almost as well”.  It may not create enough of a barrier to work at all.  Be sure to have accurate measurements of your treatment area, and purchase enough to cover as specified.  It’s OK to have leftover product; closed tightly and stored in a cool, dry place they can be reused in your next application or the following year.

Accurate application is also critical.  If your spreader or the path you take results in “lumpy” application, your control will be reduced.  Any area that did not receive sufficient chemical will act as a hole in your virtual weed mat.  That’s where weeds will germinate.  Invest in a good spreader, calibrate it correctly, and apply in a grid pattern: one half in one direction and one half in the perpendicular direction.

Mechanical interference

Your lawn or garden beds are full of weed seeds just waiting to grow.  Any activity that causes a “hole” in your pre-emergent barrier, even when applied at the correct time and in the right way, will give weeds an opportunity to germinate.  Pets or children digging in the lawn, mole activity or your weekend garden projects can poke holes in your chemical weed barrier.  Of course some of this is inevitable, but it’s not a failure of the pre-emergent.  Control any breakthrough weeds with a spot post-emergent herbicide application.

Your weeds already germinated

Some weeds prefer cool temperatures in fall and winter to germinate, growing while it’s cool and going dormant in summer. These can be controlled with a fall application of pre-emergent around September 15 and spot treatment with post-emergent herbicide.  If you need to reseed your lawn in fall, however, you can’t apply pre-emergent within 8-10 weeks of seeding as it works on grass seeds also.

Your weeds are Perennials

Not every weed that you see in the lawn in summer came from a seed the previous spring.  Some weeds are perennial, and the live plant lurks below the surface or as a nearly invisible cluster of leaves.  Since these weeds are already germinated, a pre-emergent has no effect on them.  (But it will prevent any seeds they dropped the previous year from germinating. Call it a partial win.)  Treat them with spot applications of post-emergent herbicide that targets roots, like Roundup.

Your weeds aren’t coming from seeds

Some weeds are particularly nasty in that they can grow from tiny pieces of root, bulb or stem that remain after being hand pulled, cultivated, or tilled out. Canada thistle, bindweed and wild onion are just a few. Identify the weed and use an appropriate herbicide.

They need to be reapplied

Pre-emergents are generally effective for about three months, but environmental factors can shorten effective time.  Sun, heat, and rain degrade most chemicals, and herbicides are no exception.  While some rain immediately after application is beneficial, heavy, flooding rains can wash away the chemical before the barrier is created.  For best results, avoid applying before heavy rain is predicted, and plan a reapplication well within the three-month window.