May 06, 2012

Soil Temperatures and Gardening

The question that many amateur gardeners like myself often ask is:

Why are my seeds not germinating and rotting in the soil?
When will the soil be warm enough to sow my seeds or transplant my plants?

Well, I don't pretend to be an expert in this, but I suspect it has something to do so with the temperature of the soil.

You can purchase or adapt a thermometer for taking soil temperature measurements
Lets talk about soil temperature and how this compare to air temperature, a subject that is interesting beyond gardening.

The first thing to keep in mind is that the temperatures you hear about on TV or read about in the newspaper are for the air temperature, in shade, at a height of roughly 2 meters (6 feet).   So when meteorologists talk about surface air temperatures...that is what we mean.  And such temperatures SHOULD be taken above native vegetation, which is increasingly rare.

Now the temperature of the ground surface is often quite different than the air temperature.  During sunny days, ground temperatures can be much warmer than the air temperature (10-40F is not unusual), and on a cold, clear winter nights, when the ground radiates heat to space, the ground temperature can be 1-8F cooler than air temperature.  After relatively warm periods, heat conduction from the warmed soil below can keep the surface temperature warmer than the air temperature at night.

Several of these characteristics are evident in this plot of surface and ground temperatures at the WSDOT road weather site at Silica Road (near Quincy in eastern Washington, along I90) for Sunday.  Time advances to the left in this figure.   The road temperature zoomed up to roughly 105F, while the air temperature was in the low to mid 60s!  Wow.  And last night the air temperature was cooler than ground temperature as the former dropped in the upper 30s.

 Such super-large temperatures changes just above road surfaces lead to the famous water on the road mirage (see my book for details on this).

Now what about the soil temperatures?  The deeper you go into the soil the weaker the daily temperature variation becomes, with the soil temperature increasingly  reflecting the average temperatures of the weeks and months before as one descends.  Here an example of the soil temperatures at roughly 1 inch (2.4 cm), 6 inches (15 cm), and 12 inches (30 cm).  Lots of daily variation in the top inch (in this case a range of 30F), but only a 2F range at 12 inches.  And you can see it takes a while for the warming to propagate down into the soil.   Go down 5 feet or so and the temperature hardly varies throughout the year.

So what are the current soil temperatures around the State?   Lets look at soil temps at 8 inches down from the highly useful Washington State University AgweatherNet .  Here are the values today...mid 50s over western Washington and the mid to upper 60s over the western portion of eastern Washington (60 and above are in yellow).

Now lets examine  the 8-inch deep soil temperatures in Seattle since January 1 (see graph).  A steady rise until the cold spell this week, and now it is rising again with the warmer days. Roughly upper 50s F.

 Ok, so what does this have to do with the germination of vegetable seeds....and particularly why my bean seeds are just rotting in the soil?

Take a look at the typical soil temperature required for germination of various seeds (see below).  Big variations.   Here in western Washington we are now good to go for corn, spinach, carrots, and peas (not shown, but ok at 50F).  But beans need soil temps in the 60s.  Boy did I make a mistake planting those too early!   Next time I will check the soil temperatures before I sow. 

Vegetables or not, soil and ground temperatures are fascinating, and during the winter knowledge about them can save your life when roadway icing is threatening.


  1. If you make a pile of kitchen and leaf and grass compost, it can automatically raise the soil temperature to start your seeds growing when the rest of the soil is too cold! Sometimes in the Northwest, you find yourself waiting a long time for those soil temperatures to rise!!! Thanks for those soil links, love the agri-graphs!

  2. Wonderful article, Cliff. I always look at the ten day forecast to determine when to plant corn and beans. As long as the days are relatively sunny with highs in the 60s, or above, I feel I am good to go. The soil warms quickly with the sun high in the Seattle sky. I planted my corn on Sunday, the fifth. I will keep my fingers crossed that the ten day forecast remains accurate!

  3. If you know your weeds and/or volunteer veggies and have a soil thermometer you can keep track one year at which temperature they germinate. Next year no need for thermometer - just use weed germination to determine soil temps.

  4. If you average soil temperatures all year, you will get a number close to the mean temperature for the location.

    For most soils, at a depth of 20-25 feet, the delay for the summer "heat wave" to arrive is about 6 months. Where the mean temperature is comfortable (say, 60 degrees), some folks bury air ducts at the proper depth and get free heating and air conditioning! In the middle of winter, last summer's heat wave is going by. In the middle of summer, last winter's cold wave is going by. The 60 degree mean annual line runs across the deep south, then turns north along the west coast due to ocean warming. The most common use of this "free" heat/AC is tempering large commercial chicken coops.

  5. Thanks, Cliff, for addressing one of the ways the weather affects our gardens. As a full-time gardener, I am always watching the forecasts--not just for the day, but several days out--so I can plan my activities.

    You wrote: "Mulching my soil might have helped keep in the daytime heating." Having mulch on the ground during the winter and spring affects how the soil heats up. If you want your soil to warm up ASAP so you can seed, at a certain point in late winter you need to pull back the mulch so the sun directly contacts where you're going to sow.

    And it's not just the thickness of the mulch; it's also the reflectivity of the surface. For example, take a look at your photo of the thermometer in the ground. Note how much reflection there is from the dead leaves, especially the leaf in the upper right corner. If the photons that hit that leaf were instead allowed to hit dark moist soil, the latter would warm up much sooner.

  6. Here's a silly thought I've wondered about on and off...

    The earth is, very roughly speaking, a ball of molten fire surrounded by the thin, temperate crust which we inhabit. This inner core is constantly radiating heat away, spending some time in our atmosphere before heading off into space.

    What effect, if any, does this heat have on our climate? Conversely, if we fast forward a few billion years to the point where the core has cooled down, what will the earth's climate look like (assuming all else stays equal)?

  7. The second graph scaling seems wrong - >35C is definitely too warm at 30cm down; <40F year 'round probably too cold.

  8. Great post on soil temps and how they affect germination and plant growth!
    Soil scientists and agronomists, however, usually measure soil temps at 5cm deep (about 2 inches), not 8 inches.
    Good news for Seattle gardeners is that the top 5cm warm up much faster than deep down at 8 inches/20 cm.
    You're right that it's still too cold for those tomatoes and squash though!

  9. Just of more incidental note really - Albeit. .. Of course, actually digging, and loosening up whatever ground / soil, would work to add a degree or two. — Tilthing in whatever amendment, most likely a few more, as has been suggested more initially above.

  10. Thank you! I was wondering why my beans were not emerging, while the peas were coming up.

    I need to get my tomatoes and peppers out soon since I'll be out of town the last week of May. I don't think I can rely on anyone else in the family to actually water them when they are frying in the window.

  11. Cliff, when is the probcast coming back?

  12. Thanks for the soil temperature chart. We try to get a jump on the season by starting seeds indoors and planting out plants. Unfortunately is is still freezing overnight in Sammamish, just east of Seattle. The National Weather forecast showed overnight temperatures were going to be above 42°, not the 32° we had on May 11th. Is there a source for a more accurate local weather forecast? We don't want to put our tender veggies out and have them freeze.

  13. Selecting a high soil is definitely likely to help the cultivation in long term so various factors such as productivity,cost as well as quality matters probably the most.Dark top soil may be the needed component from the any farm but help the maqui berry farmers in most aspects.


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