Seed Germination and Soil Temperatures

Okiedawn OK Zone 7February 1, 2013

Since Reed mentioned planting radish seed this weekend, I thought I'd link Tom Clothier's Vegetable Seed Germination Data Base -for anyone who might find the information useful. We refer to it a lot on this forum in the January-May planting season.

The information is useful because if you sow your seeds when the soil is way too cold for them, they will germinate very slowly and there's also a risk that they may rot if they sit in the soil for too long waiting for it to warm up. Conversely, with cool-season seeds, if you wait until the soil temps are too high for optimal germination, that can be a problem.

Sometimes we find ourselves between a rock and a hard place here in Oklahoma because the best soil temps for a given vegetable to germinate may occur much later than that crop needs to be planted in order for it to beat the heat and produce well before the temperatures are too hot for good production. Still, between the OSU Garden Planning Guide and the guidance on soil temperatures, most gardeners here manage to get a good stand of crops in late winter through late spring.

I plant based more on my soil and air temps than on a calendar date, and also based on my experience with the specific temperatures and freeze and frost patterns observed in our microclimate here where we live. The planting calendar is a helpful guide, but it is based on averages, and real life rarely mimics the statistical average.


Here is a link that might be useful: Seed Germination Data Base-Vegetables

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Dawn, what is that website that gives us our current soil temps? I recall it being posted last year in the spring, and that was something I relied heavily on. I remember putting in my Favorites, but when my older computer crashed, I lost that info.

Appreciate it and thanks!


    Bookmark   February 1, 2013 at 3:22PM
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Dawn, great info! I've had black fabric over mine for about three weeks so, hopefully, soil temps will be okay.

    Bookmark   February 1, 2013 at 3:32PM
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Susan, Dawn may know of a better one, but the Mesonet has one. Click on 'More Maps', then 'Soil Moisture/Temperature'.

    Bookmark   February 1, 2013 at 3:48PM
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Thanks, Carol, you're the bom diggity!


    Bookmark   February 1, 2013 at 5:21PM
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Okiedawn OK Zone 7

Reed, I love Tom Clothier's data base. It helps take a lot of the guesswork out of knowing when to plant seeds. At his website there's also a similar set of data for other plants like annual and perennial flowers.

I first really came to understand the relationship between actual soil temperatures and seed germination/plant growth and performance from Dr. Sam Cotner's incredible book "The Vegetable Book: A Texan's Guide to Gardening". I also learned so much from him about the way air temperatures impact growth and performance of our vegetable plants. While there are many gardening books that I love, love, love and find useful, Dr. Cotner's book is the only book I consider absolutely essential and responsible for whatever success I have had in raising vegetables.

I grew up in a gardening family so understood all about when we planted each vegetable, but reading Dr. Cotner's book in the mid- to late-1980s helped me understand it all in terms of, not dates, but rather soil temps and air temps. After that, vegetable gardening became so much more easy to understand and manage. I've completely worn out my first copy of Dr. Cotner's book, and my second copy is starting to look pathetically well-used.

And for anyone who hasn't seen the soil temperature part of the OK Mesonet, I'll link the three-day average map below. You can increase your own soil temps by mulching the soil heavily in the fall to help hold in the heat, or by covering it with black plastic several weeks before you want to plant. You also can use clear plastic placed over PVC hoops to form low tunnels over beds you want to heat up. The same greenhouse effect that keeps plants warm in our greenhouses will heat up soil in a low tunnel in no time at all.

I usually won't plant anything until my soil temps have been in the right range for good germination for 3-5 days and the forecast looks favorable for the next 10 days too.

I take my own soil temps using a thermometer with a metal probe, but also check the Mesonet maps to follow general trends. If you have raised beds or sandy soil, you may find that they warm up more quickly than grade-level soil and more quickly than dense clay soil.


Here is a link that might be useful: 3-Day Ave Temp Bare Soil at 2

    Bookmark   February 1, 2013 at 9:55PM
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Bumpity Bump! Happy Spring everyone! Thanks again, DAWN. Muah!

    Bookmark   March 7, 2013 at 8:39PM
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Okiedawn OK Zone 7

Hi Bon!

Thanks for bumping it back up so we don't have to go looking for it.

It is that time of the year.

Hope y'all are doing well up there. We're burning up down here, but I think that the rain (if it falls as predicted) will give us a few days of relief. The higher humidity today felt wonderful compared to the days this week when our RH bottomed out in the 20s or upper teens and my skin felt like it was going to crack and fall off.


    Bookmark   March 8, 2013 at 11:21PM
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Who knew?

There is a direct relationship between soil temp and germination of seeds, plant growth, and performance. Ignore this relationship and risk losing your crop.

I was dimly aware of this - I don't plant seeds of warm season crops when the soil and air temps are cold. But I didn't know that there is such a strong correlation between temperatures and germination rate/ successful germination. This may explain why some seeds have lower than expected germination rates when I start them inside.

I sowed pepper seed on Feb 9, most germinated in 8-9 days. I took the flats off the heat mat, and expected the rest to germinate in a day or two. Didn't happen. The soil temp in the flats dropped to our house temperature - 72-68. A few seedlings came up recently, not many.

According to the table, at 77 degrees, germination rate for peppers is 98% in 8 days. At 68 degrees, germination rate is 96% in 13 days .

I concluded that I must have some bad seed (although most was purchased this year).

Thanks Dawn!

    Bookmark   March 9, 2013 at 10:56AM
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Okiedawn OK Zone 7


You're welcome. All I did was link Tom Clothier's page, he did all the work!!!

The genius of Tom Clothier and the years of work he must have done to collect the info in his database just amazes me. Can you imagine the time it took to conduct all these germination tests for years to get enough data to compile his database? Not only does he have the info for veggies, but if you look at his webpage, there is similar data for other types of plants.

When I was a kid, we planted by dates. You know....just like all the standard state university recommendations. I know why state universities and extension services recommend dates---because it is user-friendly. But plants and seeds don't use dates. They never look at a calendar and say "Okay, it is March 10th, time for me to sprout". They respond to many natural processes, including soil temperature and air temperature. Once I learned that (from Dr. Cotner's book), I became a much better gardener because I understood that I should ignore planting calendar dates and focus on soil temps and air temps. I just cannot describe what a difference this has made. Then I found Tom Clothier's database which were much more detailed and specific than Dr. Cotner's recommendations. Between the two of them, they made me into a much better gardener.

When I planted by date, whether direct-sowing or transplanting, I did notice that some years the seeds popped right out of the ground and the transplants took off growing like crazy, but other years they did not. Now i do understand those different behaviors likely were linked to soil and air temps, and available soil moisture. It really has revolutionized how I garden.

I had a good garden before I learned about soil and air temps and learned how to use those to my advantage. Since learning this info and applying it, I have a great garden. I've gotten so much smarter in how I garden over the last 30 years or so.....just imagine how much more there is to learn though!


    Bookmark   March 9, 2013 at 11:38AM
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Yeah.. best gardening group on the net. I'm becoming more versed in the terminology and my searches more effective. Thusly, you need not repeat yourself with me and save that for the newbies. *thumbs up

I'm SO grateful you teach this and, so, I learned it from the 2nd season onward. True head-ache and time-saver. for this seasons it tells me I'm not too late to get started on some of these. While others have been growing inside, I can plant mine and they're likely to catch up depending on the variety. I just don't have the room in the house any more!

love you guys

    Bookmark   March 9, 2013 at 9:17PM
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