- Dithiopyr 2L, is capable of providing limited post-emergent control of crabgrass until the 2-3 leaf stage -
So, if pre-emergents do not prevent weed seeds from germinating, how to they prevent grasses and some broadleaf weeds from invading our turf? To be correct, you want the weed seeds to germinate. Once a seed germinates, the initial or primary root emerges from the seed. At this point, if a pre-emergent has been applied, when the young root absorbs moisture from the soil, the pre-emergent does its job. Depending on the specific active that is contained in the pre-emergent, either the root or initial leaf is prevents from developing and that plant dies. All of this activity occurs below the turf canopy and is never seen. This is probably the reason that people state that the seeds are prevented from germinating. One pre-emergent, Dithiopyr 2L, is capable of providing limited post-emergent control of crabgrass until the 2-3 leaf stage. This still mostly occurs unseen by the turf manager. Pre-emergents are chemicals that are highly un-soluble. This is important because you want the active to remain at or very near the soil surface because this is where the majority of the weeds are and germinate. To make a proper application, you should have the pre-emergent applied 1-2 weeks before the weed seed germinates. This allows time for the pre-emergent to form a barrier across the soil surface. It is better to go early vs. late when applying your pre-emergent. Pre-emergents are mainly broken down by soil microbes. These micros are also controlled by soil temperature. If the soil temperature is cold, no microbe activity occurs and the chemical sits in the soil unaffected. With that said, going earlier not later is a good policy to follow.