'optimum' soil temps?

dave_f1 SC, USDA Zone 8a(7b)March 10, 2009

I'm interested to get some feedback on what many consider 'optimum' soil temps for germinating various veggie seed. Beginners realize that different types of veggies prefer to be grown under certain temperatures (air), but not realize that soil temperature affects germination and early growth. But what does 'optimal' soil temperature actually refer to? I assume all those numbers in various tables take it to mean the temp at which you'll get the highest percentage and/or fastest germination. But is it always best to sow seed in that 'optimal' range? I think it could be a mistake to just look up your crop in one of these germination tables and then start sowing when your soil reaches that 'optimum' temp. Especially for spring veggies. I've seen many tables cite that the optimal temp for pea germination is 70F. Ok, maybe that's true. But certainly it could be a mistake in my climate to wait until the soil reached 70F to plant a spring crop of peas. They would be burned up in the summer heat before cropping much. It's best to sow when soil temp is around 50F maybe. I guess I'm saying there's a disconnect between these data tables and the real world sometimes. What do you think?

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In your example of peas it is true they germinate faster/more reliably in warm soil, but grow best in cool air.

This is why I don't sow spring peas outside, but inside and transplant as soon as they germinate.

A lot of what we consider spring plants are more accurately thought of as fall plants. Planted in soil that is still warm, but with air temps cooling down from the summer's heat.

I have attempted to sow seeds for cool season crops 'as soon as the soil can be worked' and it almost always produces very poor results. Germination takes forever and sometimes the seeds seem to disappear/rot.

    Bookmark   March 10, 2009 at 4:24PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

I guess I'm saying there's a disconnect between these data tables and the real world sometimes. What do you think?

I don't think it is so much a the "charts vs. the real world" problem as it is the wide discrepancy in some of the charts and getting folks to use and interpret them properly.

Just getting folks to think about soil temps instead of just air temps and the date on the calendar is a BIG step, real progress. ;) Then comes the learning how to measure soil temps and recognizing what weather changes affect it and which ones do not. But you are correct that, just as with most info sources, there are good charts and bad ones.

IMO the best charts make a distinction between planting seed temps and planting plant temps. Unfortunately, many folks don't pick up on that distinction.

And the best charts will provide a MINIMUM soil temp for germination (for those who just can't wait) and also give info on how to artificially raise that minimal temp closer to the OPTIMUM planting temp. OPTIMUM defined as that temp which will get the best stand of vigorous seedlings but not necessarily the fastest rate of germination.

Good article below - shows for example that the germination number of peas begins to level and then fall off at soil temps much above 50 but that the fastest germination occurs at 77 degrees. Clearly, as you point out, waiting for soil temps of 77 to plant peas is out, but planting at 50-55 degree soil temp will get you the optimal stand of peas.


Here is a link that might be useful: The effect of soil temperature on sown seeds.

    Bookmark   March 10, 2009 at 7:34PM
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dave_f1 SC, USDA Zone 8a(7b)

Thanks digdirt, that is a useful chart since it shows both germ time and germ percentage for various soil temps. I have seen this before, but was concerned that the author cites the reference as being unknown. Which is a bit weird. But I have found this to be pretty darn accurate for the crops I grow. I used to sow spinach indoors or outside at the end of the summer, but never had even a decent stand. This tables shows why.

As far as soil temps go, what do you think is the best depth to measure? For transplants, I guess where most of the roots will be. But what about for sowing seed? I know soil temp very near the surface can fluctuate widely from day to day.

    Bookmark   March 10, 2009 at 7:59PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

Here (link below) is an example of what I would consider a far from ideal chart. Note the pea info. ;)

As to where to measure? Yeah, maybe it's just me but for transplants I usually measure at 6" or just below what would be root level depending on the type of plant. ie: tomatoes and peppers go much deeper than beets. It is one of those "getting closer to optimal tricks" - if the temp at 6" is 50 then the soil above that level is warmer and so closer to optimal, follow me?

Seeds I measure an inch or 2 below planting depth for the same reason. Granted surface temp is going to fluctuate more but it is also a very temporary fluctuation.

One thing to keep in mind too is raised beds vs. in ground beds (I have some of both). Raised beds will warm faster so seeds can be planted sooner in them than in-ground beds. Black plastic mulch can be a good friend in the early spring garden. ;)


Here is a link that might be useful: Germination Tables

    Bookmark   March 10, 2009 at 9:16PM
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Check if your state extension office has a web page like the one listed below. It shows the soil temperatures across the state here in IA. I use this instead of getting my own soil thermometer. The state takes temps at 4 inches.

Here is a link that might be useful: IA State Soil Temps

    Bookmark   March 11, 2009 at 1:19PM
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