Found a tool online that provides average soil temps to help know when to apply your pre-emergent herbicide / fertilizer (ie just before the 5 day average reaches 55 degrees)
https://www.greencastonline.com/tools/soil-temperature
(I have no affiliation with this site nor do I know how accurate it is but seems legit based on my sample size of one location)
For those that could use a Pre-emergent herbicide 101 (courtesy of Google Gemini):
Pre-emergent herbicides, which are often found in pre-emergent fertilizers, work by preventing weed seeds from germinating. Here's a breakdown of how they achieve this:
Creating a Chemical Barrier:
* When applied, pre-emergent herbicides form a chemical barrier in the top layer of the soil.
* As weed seeds begin to sprout and their initial roots emerge, they come into contact with this barrier.
Inhibiting Root Development:
* The active ingredients in pre-emergent herbicides interfere with the weed seedling's ability to develop its root system.
* This disruption prevents the seedling from establishing itself and absorbing the necessary water and nutrients.
Preventing Emergence:
* Essentially, pre-emergents stop the weed seedling from successfully breaking through the soil surface.
* The seedling either dies or remains dormant beneath the surface.
Key points to understand:
* They target germinating seeds, not established weeds: Pre-emergents are designed to prevent new weeds from growing, not to kill existing ones.
* Timing is crucial: They must be applied before weed seeds begin to germinate to be effective.
* They don't prevent seed germination itself in all cases, but they prevent the seedling from developing.
* They create a barrier in the top layer of soil.
Therefore, pre-emergent herbicides provide a preventative measure against weeds by disrupting the early stages of their growth.
Applying pre-emergent fertilizer effectively hinges on timing, and that timing is largely dictated by soil temperature. Here's a breakdown of the key factors:
Key Timing Factors:
-
Soil Temperature: The most critical factor is the soil temperature. Many weed seeds, like crabgrass, begin to germinate when soil temperatures consistently reach around 55 degrees Fahrenheit.
Spring Application:
- This is the most common time for pre-emergent application, targeting weeds that emerge in late spring and summer.
- Aim to apply it before the soil consistently reaches that 55-degree mark.
- A rule of thumb is to apply before or in conjunction with the early stages of the "spring green up" of the lawn.
Fall Application:
* Fall applications are important for preventing winter weeds, such as poa annua.
* Apply when soil temperatures begin to decline, typically before they drop below 55 degrees Fahrenheit.
Practical Tips:
* Monitor local weather forecasts and soil temperature trends.
* A soil thermometer can provide accurate readings.
* It's generally better to apply pre-emergent slightly early than too late.
In summary, the key is to apply pre-emergent fertilizer before weed seeds germinate, with soil temperature being the primary indicator.
by cmullins77
5 Comments
The common lawn pre-emergents (prodiamine, pendimethalin, and dithiopyr) work to help reduce the germination of certain seeds… Mostly grasses and only a handful of broadleaf weeds. The labels will list which weeds are targeted. To prevent more broadleaf weeds, a specialty broadleaf pre emergent like isoxaben is required.
Pre-emergents work by preventing the germination of seeds of the target species. So in order to be effective, a pre emergent needs to be applied BEFORE those seeds germinate. For winter annual weeds (annual weeds that are present in the fall, winter, and spring, like poa annua), a pre emergent needs to be applied in the fall before soil temps fall below 70F. In order to prevent summer annual weeds (like crabgrass), a pre emergent needs to be applied in the spring before soil temps reach 55F. (In very southern areas, timing can be more closely tied with periods of higher moisture AND climbing soil temps. Consult your state extension service for more specific guidance)
Pre emergents will not kill existing weeds. Pre-emergents alone will rarely control a weed problem. Pre-emergents are tools to **reduce** the need for post-emergents. They rarely eliminate the need for post emergents.
The labels of pre emergents have many important instructions and use restrictions. ALWAYS READ THE ENTIRE LABEL. For example, you are limited to 2 applications of each active ingredient per year.
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It is WILD that Boulder’s soil temps are 20 degrees warmer than ours here in Cleveland Ohio right now.
That’s the site that’s linked to by automod whenever someone says “soil temps”
[this tracker](https://gddtracker.msu.edu/) is an even more accurate way to time pre emergents. It has only been “ground truthed” in the great lakes region, but in theory it should be accurate everywhere in the contiguous 48.
Mesonet app
SO I’ve been doing hard scaping and have a lot of areas now that as are just soil…some with patchy grass…and then grass.
Should I overseed the bare soil spots..the patchy grass spots..and lay down a pre-emergent in the grassy areas?
Cause if I lay down a preE everywhere now..I don’t think I’ll be able to overseed till June