The Soil Computer

by Carl Strang

The recent moderation in temperature at last makes it seem like spring is nearly here. How do you decide that winter is done and spring has arrived? I don’t know about you, but for me the astronomical or calendar spring doesn’t cut it. In the Chicago area it seldom feels like spring, yet, at the equinox. That leaves us with subjective measures, which can be different for every person. There are all kinds of signs and indicators we might use. I am outside during my lunch break at Mayslake Forest Preserve most days of the week, so I know that I will find my measure of spring there. At first I considered the date when the ice is off the lakes, but that varies wildly and inconsistently among years.

Mays’ Lake on March 19. Mays’ Lake was clear soon after, but a shelf of ice persisted on Trinity Lake several days more.

Mays’ Lake on March 19. Mays’ Lake was clear soon after, but a shelf of ice persisted on Trinity Lake several days more.

This year Trinity Lake was not clear until March 29, 11 days later than the previous record in my 5 years at that preserve. My next thought was to look at the phenology data, and here I found something more satisfactory. There are 3 flowering plants that consistently bloom earlier than the others (not counting horticultural imports). Looking back over the records, whichever of those three I find blooming first could well mark the start of spring. Here they are:

Common blue violet

Common blue violet

Spring beauty

Spring beauty

White trout lily

White trout lily

This measure takes advantage of the soil computer. The soil adds up the degree-days of heat, influenced by the warmth of the days, the cold of the nights, and the depth of the snow. Eventually that sum produces an output, expressed by the plants when the soil warms enough for them to grow and to bloom. By this date last year, all three of the indicator species were blooming for more than a week and a half at this point. This year, they still are still some days away. That fits for me, subjectively, because I’m not feeling spring, yet.



  1. tamara said,

    April 3, 2013 at 2:50 am

    Thank you for the lily-in our forests we do have only the dark pink variety!

  2. April 17, 2013 at 5:52 am

    […] decided that, for me, the arrival of spring will be signaled by the first bloom of one of three species of plants, I have been watching for new growth. A week ago there was […]

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