Week Killer 101: Know Your Herbicides

Keeping lawns weed-free is a full-time task. However, there are some handy tools available that make it a bit easier: Pre-emergent and post-emergent herbicides. The pre- and post- designate at what growth stage the herbicides work.


This type of herbicide kills weeds before they sprout. While they won’t prevent germination, they do keep the pests from breaking through the soil. A good example of a pre-emergent herbicide is crabgrass preventers. Once they’re hit with a pre-emergent, the germinated seeds can’t emerge.


Treat the entire lawn! Weeds can still invade any missed spots.

The majority of pre-emergents need to be watered in, including those applied with a hose-end sprayer. While the volume of water from the hose is enough to spread the herbicide, it’s not sufficient to get it into the soil where it can do its job. Follow package instructions, of course, but typically, water after application.

Be careful when using pre-emergents on newly-seeded lawns; that also goes for areas that will soon be seeded. Most pre-emergent herbicides require a certain number of mowings, after seeding, before applying. Conversely, there’s a certain period of time to wait after application before seeding new areas.


As the name suggests, these herbicides kill actively growing weeds that are already through the soil surface. This includes perennial and annual broadleaf weeds (those not resembling grass). Some products even work on grassy weeds (those that do look like grass). Use post-emergents only for existing weeds. They’re perfect for spot-treating individual pests.


Get ’em while they’re young! It’s much easier to kill young weeds than it is mature weeds. Fully-established weeds may need multiple applications before death occurs. Look frequently for newly sprouted weeds and apply a post-emergent right away. Lawn mowing is the perfect time to do it.

Timing is Everything

While post-emergent herbicides work at any time in the weeds’ growth cycle, treating as soon after sprouting is preferred. This makes it a much easier, more economical chore.

Pre-emergent herbicides eventually degrade in the soil. Applying them too early is likely to be ineffective. Most crabgrass herbicides stay active in the ground for six to eight weeks after application.

Check the calendar. If pre-emergent application day was in mid-April last year and it worked – do it in mid-April this year, too.

Another way of simplifying weed control is applying a blended product. This is one that contains a pre- and a post-emergent herbicide. They kill any existing weeds (as labeled) as well as preventing new ones for up to six months.

As always, use safe handling practices when applying any product and follow the instructions on the label.