The Carrot Garden Diva thread lives. lol. They really are divas, ya know!

Heather,

I know that the dates for fall planting seem ridiculously early, but there's a reason for that. For the fall garden, we are facing a day-length (the number of hours of sunlight per day) that shortens every day, so that the more deeply we go into autumn, the less sunlight the plants get. The less sunlight they get, in general, the slower they grow. Thus, we have to plant them during the worst summer heat to ensure they have enough days with enough sunlight to produce a nice harvest in the fall.

With the OSU Planning/Planting Guides for spring and fall, there is a statement in the spring one that says: "Differences in suggested planting dates range from the earliest for southeastern Oklahoma to the latest for the northwestern part of the state." The reason for that is that in the spring, southeastern OK warms up earliest and northwestern OK warms up latest. Unfortunately, they did not put a similar statement in the fall planting guide, but if they did, it would be exactly the opposite from the spring statement because in autumn, the cold weather arrives in northwestern OK first and in southeastern OK last. That's why I always advise folks here that the fall range of dates generally is applied with the earliest planting dates (when a range of dates is given) for NW OK and the latest for SE OK. However, I think that when there's a range, you can pretty much plant anytime within that range safely, based on the knowledge that the first frost/freeze of autumn does not always arrive at the same time every year.

With carrots in general, they actually germinate much more quickly in warm to hot soil and more slowly in cool to cold soil. However, there is a point where it gets too hot for them. Based on research data from Tom Clothier that compared germination rates of various veggies at various soil temperatures, we know that the most perfect soil temperature for carrot seed germination is 77 degrees. At that temperature, 96% of your fresh carrot seeds will germinate within 6 days. If the soil is a bit hotter, you still will get an 95% germination rate within 6 days at a soil temperature of 86 degrees. Above that, the soil temperatures are so hot they begin to impede germination. At a soil temperature of 95 degrees only 74% of the fresh carrot seed will germinate within 9 days. If your soil temperature is hitting 105 degrees, 0% germinates. No matter what your soil temperature is at planting time in summer, there are ways to work around it. The most common one is to sow the seeds and then put a sheet or some other textile over the freshly-planted area to keep it shaded for a few days. Also, lightly mist the soil daily so it doesn't crust over on top and form a hard, crusty layer of soil that the young carrot plants cannot break through as they emerge from the seeds. I'm sure there are other tips in the Garden Diva thread that Amy linked, although it was written some time ago and I no longer remember what I wrote there. : )

You can take your own soil temperature using a soil or compost thermometer, or even a meat thermometer from the kitchen, or you can use the OK Mesonet soil temperature maps. I'll link one of the soil temperature maps below. Note that it is for soil temperatures 4" below bare soil devoid of vegetation or mulch. Since carrot seeds are not sown 4" below the surface, you can expect the soil temperature at the proper carrot seed planting depth to be a bit warmer than the 4" temperature recorded at your county's mesonet station or stations.

Dawn

https://www.mesonet.org/index.php/weather/map/4_in_bare_soil2/soil_temperature