You go, Steve! :)

Try as we might to convince ourselves, outdoor gardening is far from a purely scientific process. There are just way too many variables...

To me, soil temperature is a joke. Was when I was a wheat farmer, still is as a gardener. When? and if cloudy or sun shining, wet or dry?, which spot?
I can get temps from 40 to 80 on the same day in my dry soil.
Might work at 2 in the morning by the dark of the moon.
Yes! I have tried to measure it, many times.
They all so tell me that tomatoes are a no at over 90 degrees in the South. Sure glad that is not the case here in Colorado or we would never pick a tomato.
Now! I feel better, Kenny

The difference between soil temp in the morning and at the end of the day can amount to something - especially at a shallow depth and in dry soil. Of course, as the depth increases the difference becomes minimal.

Throughout the Northwest, AgriMet (Bureau of Reclamation) keeps track of soil temperatures. Morning to Afternoon: at 8" into the soil on the 1st day of summer last year, AgriMet tells us that temperature varies just over 2°F in Aberdeen, ID. At a 4" depth, it varied nearly 5°. At a 2" depth, it varied over 6°.

The bureau is setting up one of these stations just a few miles from my garden. I assume that other agencies are doing this kind of thing in other farming regions. In Canada, it is the job of the Ministry of Agriculture in the various provinces, as I understand it.

Season of the year makes a difference with the greater intensity of sunlight. But, we are usually just talking about springtime temperatures for germinating seed.

Linked below, you can see the difference between average soil temperatures (morning and afternoon) during the months of April and May. It is quite a difference as they move into May but very little difference during April.

Soil moisture also changes things. Morning to Afternoon: In April, the average change is less than 8° in Lethbridge, and that is at a 2" depth.

All in the interest of providing some guidance for growing . . .

Steve

Now I am on the other side of the fence on this one. I like to watch the soil temp. I use it as one more tool in my tool box to decide when to plant. It works best for me when I combine the soil temp with other events in the garden.

Like: the lilac leaf when the soil is 45°, my lilac bloom at 50° soil and the grape leaves are when the soil reaches 60°. Weeds are also good indicators of when safe to plant. When the goat heads sprout the soil is 70°.

If I take the temp at 8am in the same central place in the garden, then I don't confuse myself with data overload because it is just an indicator, not a cast in stone rule.

Thanks Steve for the CSU link. It is useful information, especially for people like me that did not grow up gardening, have never been farmers and don't have mentors (other then our internet friends).

Bonnie, the activities of the perennials in our outdoors are probably the very best of indicators of what we should be doing!

They are out there everyday, all year and reacting to the totality of the environment.

We can really get crowded down into single variables and . . . I realize that this sounds like a contradiction to what I've said above but it is just part of my efforts to understand what is going on "out there."

Steve
The human understanding is like a false mirror, which, receiving rays irregularly, distorts and discolors the nature of things. ~ Francis Bacon

Well now, Im gonna tell everybody how to do it! I have a VERY scientific system for deciding when Im gonna plant my veggiesand pretty much everything else!

Are you holding your breath???

I do it whenever I have enough time!

Problem solved! Now everybody knows "when" to plant their veggies! Heh heh heh!

There is, of course, one caveat to that! Theres the cold season ones, and the warm season ones. But then, I havent even gotten the "warm season" side of my garden turned over yet, so the chance of putting them in "too soon" is unlikely! The problem there, however, is that the root crops go in the warm season "side," so theyre not in yetand need to be gettin in soon! Altho if I HAD gotten them in with the REALLY cool season stuff, theyd probably just be sittin there with all the cold weather weve been having anyway, so maybe its a good thing I "didnt have time!" At least, thats how Ill rationalize it to myself! And the other rationalization is: the root crops have ALL summer, and even into fall, to grow anyway, so they have LOTS of time!

What has REALLY amazed me this year is that I got my lettuce and spinach and peas in on the 18th, weve had mostly COLD weather since thenwith lots of wind, except for a few warm days, and on the 27th I discovered that the first lettuce (Winter Density Romaine) had ALREADY come up! I almost fell overor would that be, got blown over! Of course I had been going out there EVERY day to lookin the rain, in the snowbut I never REALLY expected to see anything up yet! The next day, on the 28th, three more lettuces and one spinach were up, and on the 29th, the first pea was up! I really was shocked! When the cold weather came inand stayedafter I planted them, I had started worrying that they were all unhappily ROTTINGtho I do think it was probably too cold for that to happen! So now I only have one lettuce and two spinaches that havent put in an appearance yetand four kinds of peas. I expect them all very quickly as soon as it warms up again.

Nothing to do with temps, but Im running a "grand experiment" this year! I dug out a "trough" about 3" deep all the way down the rows for each lettuce and spinach, and filled them with mostly finished homegrown compost before planting the seed. Then I covered the seed (lightly) with more compost. Im gonna see how much that helps with the IMMEDIATE bolting problem Ive always had when they were planted directly in the BAD clay! Its gonna be interesting, and, of course, Ill be reporting back later!

Nowhaving given you all that wonderfully scientific method for deciding when to plant, I DO have another chart to link here! This is the one Ive found most helpful in the past when I was getting "a feel" for what I should be putting in "when I had time!" I like this one because it gives you a range for how well the seeds germinate at different temps, so instead of looking for a certain specific tempyeah! Like thats gonna work at altitude!you can get a "feel" for the general conditions they need! (Seems to me I may have gotten this link from somebody here on RMG! You, Digit?)

Ive only ever actually measured the soil temps a couple times, and that was more out of curiosity than anything. Once you get down which seeds need cool/cold, and which need warm/hot, I think it really has more to do with GUT (and good luck!) than with a thermometer! Wing it! It usually works out! (Gotta get a "bird" reference in here somewhere!)

So for anybody whos new to veggie gardening, I recommend the linked chart below to help you get your "feel" tuned up, and then just go for it!

Skybird

I agree with all that has been said above. And use a combination of everything I have and then like Skybird said I usually go by my gut. And old gardener where I grew up always would take a few seeds of what he wanted to plant and plant them. Then when they sprouted if the forecast looked good he would plant the full amount of what he was going to plant. I do take soil temps. But as stated they can be misleading. Mine at 5 inches has been around 64 degrees for around 3 weeks. I know some corn farmers who go by the soil temp who planted 3 weeks ago like a did 2 short 10 foot rows. My seeds and theirs just started coming up last weekend. Most of the corn farmers just started planting this week. The thing as I stated in another thread that has hurt here in the cool night temps we've had over the last 3 weeks. This tends to slow germination. I watch the tree leaves, the lilacs, the apple blooms and the weeds. Even if they germinate I wait till they start growing. I've had Kochia weeds and others up for over 3 weeks. Just the last week have they started growing. Warm season grasses like bermuda are another good indicator. Overall I tend to go with what has worked in the past. Unless I plan on providing protection of some sort. I usually plan on putting the majority of my 80 plus tomato plants out May 10th or later and don't transplant peppers or plant okra till around Memorial Day weekend. This has been the dates that has worked consistently over many years of gardening. But like everyone else I always have to push the envelope a little with a few. Like I tell those who ask me if they can plant early. Just don't plant something you don't want to lose when you plant early. Otherwise it never hurts to gamble. Jay

I solve all these problems by being constantly behind on my gardening chores. So there's never a chance of getting stuff planted too soon!