The Pre emergent is the most misunderstood part of lawn care. On the surface, it’s fairly straightforward; It prevents weeds before they appear, but does nothing to stop actively growing weeds that have already poked through the surface. Simple, right? But beyond this basic explanation, you can often get conflicting and confusing information. Let's dig deeper into pre-emergents to understand the most common mistakes we make.
Mistake #1: Applying pre-emergent when seeding
Pre-emergent doesn’t only prevent weed seeds from punching through the surface. It’s non-selective, which means it will prevent any type of seeds from developing, including your premium grass seed. This is why most lawn care pros will recommend choosing either seeding or applying pre-emergent, and never both at once.
We recommend applying a pre-emergent in the spring and seeding in the fall. Weed pressure in spring is very high, and the grass seeds planted in the spring will be quickly outrun and crowded out by weeds later in the year.
Mistake #2 April/May is is too late to apply pre-emergent.
Most assume that by April and May it’s too late to apply any. In some states like Florida, Texas, and parts of California that’s certainly true, but for most of the country April and May are still well in range for pre-emergents to work effectively.
To explain, seeds germinate primarily based on soil temperature. When soil temps are consistently around 50-55 degrees consistently, it sends the signal for seeds to start growing. However, some seeds are slower to get that signal than others, and don’t start growing until soil temps are climbing. The point is, new weed growth occurs across a range of soil temperatures, anywhere from 55 to 75 degrees. When the soil temp hits 75 degrees it becomes too hot and weed seeds can no longer grow. When the soil is that hot, it’s usually well into the summer months in most states. Until then, pre-emergents will be effective.
So think of it as working on a spectrum. It’s most effective in the early/mid spring, but can still be effective at stopping weeds well into May and even June in some areas.
Mistake #3: Pre-emergents prevent germination
Pre-emergents do not actually prevent seed germination. Any seed in the right conditions will germinate. Pre-emergents work by creating an area near the surface which prevents new seeds from developing a root system, making it nearly impossible to break through the surface and reaching sunlight. And of course without sunlight, the weed cannot thrive. Ultimately, it achieves the same results as preventing germination, but it’s still important to understand the difference.
Mistake #4: One Pre-emergent application will prevent every weed
No Pre-emergent is 100% effective. Some weeds will always get through no matter how perfectly timed the application. Because weeds sometimes germinate later in the season, it’s always possible for weeds to pop up after the pre emergent to wear off (usually a 2-3 months). For this reason, it’s usually a good idea to put down two applications of pre-emergent; one application in the early spring, and one in the late spring. And even with multiple apps, it is important to understand that pre-emergent effectiveness is measured in degrees. If you do everything right, you can reduce the weeds in your lawn by 80-90%, but it will never be 100% effective.