What You Need to Know About Pre-emergents

Crabgrass prevention.

You hear your lawn care provider talk about it, you see they applied it on the invoice, or maybe you see it on the shelves at your local garden center or hardware store. But what exactly does crabgrass prevention mean for your lawn?

Crabgrass prevention, better known as a pre-emergent, is a herbicide used  for managing certain annual weeds such as crabgrass, foxtail, and spurge. It is usually combined with the first application of fertilizer of the year. Pre-emergents tackle weed seeds by chemically pruning the radical root and blocking the enzyme that causes the plant to create more roots.

There are several types of pre-emergents on the market, I have found that in our area the brand name Barricade (scientific name: Prodiamine) works best. Some lawn companies use a different product called Dimension (scientific name: dithiopyr.) I choose Barricade because I see less weeds break through during the summer months. This product usually lasts until the first of August which is great if you want to do fall seeding.

Most pre-emergents aren’t selective in what types of roots they inhibit from active growth which is why seeding is best in the fall when the presence of pre-emergence is minimal. Fall is also the best time for seeding because the soil temperature is warm during the day, cooler at night, and usually free of heavy rainfall which might wash the seed off its target.

What do I need to know before using pre-emergents?

Before using pre-emergents, it is best to consider the life of the turf. If the turf is considered healthy, a pre-emergent is great because the roots have been established. If the turf is weak, pre-emergents may cause more harm than help because it inhibits the turf’s ability to promote root growth. If the turf is also filled with young seedlings, it is not recommended to use a pre-emergent. Pre-emergents block roots from growing therefore killing the new grass. Remember: healthy roots are needed in order for pre-emergents to benefit the turf.

Another situation in which I would choose not to use a pre-emergent is in an area with a lot of shade. Using pre-emergents in shady areas where the turf is struggling causes turf decline. Shaded areas are also less likely to produce foxtail, crabgrass, spurge, purslane, and knotweed because they are sun-loving weeds. Without these your lawn won’t need pre-emergents anyway.

What do I need to know after using pre-emergents?

If a pre-emergent is used on a lawn, make sure not to overseed for about 5-6 months depending on weather conditions, rate of which the product was applied, and the type of pre-emergent used. If you must overseed, break the barrier of the pre-emergent by aerating or heavily raking the soil in the area you want to re-seed. Although this will help, the percentage of seeds that germinate might be reduced because there will still be pre-emergents in the soil.

What to look for when selecting a pre-emergent?

If you’re purchasing a pre-emergent from the store, you’ll want to read the label on the bag. It will either say that the product includes pre-emergent or crabgrass preventer. The chemicals to look for are Prodiamine or Dithiopyr. If you’re using a lawn care provider, you’ll want to ask them what they are using. Be sure to also let your provider know if you are planning to re-seed in the spring so they use a starter fertilizer instead. (Note: I recommend seeding in the fall in order to yield higher germination rates.)

Knowing how pre-emergents work and choosing what products are best in certain situations is one of the keys to having a healthy lawn. Every lawn is different; know what’s best for yours!