Why aren't my seeds growing?

A few weeks ago, I planted some grass seeds where my lawn had turned to dirt.
Nothing happened.

And then a possible reason occurred to me:  the ground was too cold to germinate the seeds.   After a cool winter and a cloudy/wet spring, the soil temperatures HAD to be cooler than normal.

What my lawn looks like

Let's check.

I looked around at a number of seed company and horticultural websites, and their recommended soil temperatures for cool-season grass seed were generally similar.  Here is an example of their advice:

Cool season grass, such as Fescue, germinate best when the soil temperatures are between 50° and 65° degrees F.  These soil temperatures usually occur when the daytime air temperatures are between 60° and 75° degrees.

And what about other types of seeds?  Most advice follows the charts below, with optimal temperatures indicated by the black dots and "practical" by the green ones.  In western Washington, you can forget the optimal temperatures, but the practical ones are within reach for some vegetables.   Beets to spinach are no problem even here during early spring....but what about beans, squash and other favorites that require 60sF to play the germination game?

One network that has a lot of soil temperature measurements is the WSU AgweatherNet.  Their sensors are placed at a relatively deep 8 inches and I would expect that to be several degrees cooler than observed near the surface on sunny days, but useful numbers on cloudy ones.  Furthermore, since the ground is moist, evaporation will cool near-surface temperatures.  

Here the AgWeatherNet soil temperatures for today.  Low to mid-fifties are the rule in western Washington, with a few even in the upper forties.  During the weekend, I stuck a thermometer into the soil where my germination was poor:  54F.  No wonder.

Soil temperatures in my area of north Seattle are thus marginal for grass seed, which means a low percentage of germination.  I need to wait a month for better luck.

I also wanted to plant some veggie seeds (e.g., beans, zucchini), but really need to wait for most of them if I want to directly sow into the soil.   If the above guidance is crrrect, I should wait until soil temperatures are into the 60s F.  When does that typically happen?

To find out, here is plot of soil temperatures for the last few years in north Seattle.  This year is clearly the coolest of the bunch.  Typically, soil temperatures get to 60F around May 1.    This year, mid-May is a better bet.

Now the next 72h looks quite wet (see WRF model forecast below)...very wet for late April...so soil temperatures aren't going anywhere quickly.  Even California gets substantial precipitation, which IS very unusual for this time of year.

So keep your seeds in storage for a while--I might plant my veggie seeds indoors and transplant the small plants... a good approach in our short-season climate;

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