On Soil Temperature and Seed Germination

The last time the soil was as warm as it is right now, this early in the season, was 2012. That year we didn’t have a proper winter at all, and I had been harvesting lettuce from the garden until well into the middle of January. Last year at this time the ground was still frozen. This year the frost laws have already come and gone, and the soil temperature is climbing — both at two and four inch depths — into the low fifties. Today, as if by clockwork (it’s wet and the weather is nasty to boot) the county came by and graded our road into a soft, red-dirt mess.


A couple of years ago while talking to Maasai elders they assured me and the other journalists I was traveling with that they no longer used their traditional Maasai calendar, but had instead transitioned entirely to a western method of time and season-keeping. Later, the woman who ran the NGO nearby said that she was almost certain they do still use the Maasai calendar, but that they also always deny it in the presence of westerners. Western methods are viewed as more legitimate, more scientific. The modern western world doesn’t suffer famine and the traditional African tribal men do not want to be seen as lesser; as being at fault for the hunger and poverty that wracks their communities. So they tell people they have abandoned the folk wisdom passed down through their generations full stop for a “proper” calendar with twelve months, three hundred and sixty-five days, four seasons. By the time the woman at the NGO had confided in us we were a couple beers and a whole-roasted-goat-over-an-open-fire away from the days meetings. The elders were asleep in their bomas; a short hike over a dusty hill slung low out into the black sky west of Kilimanjaro.

There was no going back. To this day I wish that hadn’t been so; that I’d had the opportunity to tell them about our folk wisdom. About how the western calendar that adorns my wall tracks the phases of the moon, as well as the days of the month. About the racks of Farmer’s Almanacs in every Tractor Supply Company in the country. Maybe even the common saying in my neck of the woods that you shouldn’t plant until the leaves on the oak are as big as a squirrel’s ear, if only I thought I could figure out how to translate “oak” and “squirrel”. Not to make the point that scientific methods of production shouldn’t be embraced — this is a post about soil temperature and seed germination, after all — but that they don’t have to be mutually exclusive; that often the old folk wisdom has a little science behind it, too. The leaves on the oaks around here tend to be about the size of a squirrels ear just about the same time the soil temperature is just right for planting our major summer crops — corn and soybeans. The old farmers might not have known that connection, or maybe they did, but also understood that, “don’t plant until the soil is fifty or sixty or seventy degrees (depending on the crop)” wasn’t as memorable. A lot can happen to a seed in the ground.

Left too long and too wet it can rot and decompose. Left too long and too dry it can shrivel and die. Left too long under almost any circumstances and it can become dinner for the local pest population from varmints to birds to insects. I once lost three plantings of green beans to a very hungry family of shrews, before I turned the dogs loose in the garden and the Schnauzer brought me their heads (and bodies, she’s not a total barbarian.) The sooner after being planted a seed germinates the less risk there is of the crop failing before it even begins. Most seeds need two things to germinate well: moisture and warmth; water and heat, the two fundamental elements of life as we know it. For seeds started indoors or in greenhouses both are easy enough to come by, but outdoors is a little trickier. You can go by the size of the leaves on the oak, or you can use a soil thermometer. A little of both probably never hurt. Many universities — especially in agricultural states — also have weather stations scattered throughout the country with the data they record daily available online.

I usually watch the data from a local weather station online (which is what I have charted above) until it consistently records a reasonable planting temperature. Then I use a combination of a soil thermometer, my own judgement and folk wisdom about the weather we’re seeing, the forecasts of a couple local meteorologists whose work has proven most accurate over the years, plus a glance at my own schedule to see when I will have time to plant. I guess you could say while some regard an approach to planting as an absolute science, I consider it more the art of applying science to real life. Either way, this chart of optimum soil temperatures for germination, by crop, is (one of) my bible(s). The shortest length of time from planting to germination can be expected when planting into soil at the optimum temperature, and deviations from that temperature in either direction will delay seedling emergence until, at some point on the continuum the seeds won’t germinate at all — those are your maximums and minimums. Whatever your approach to planting I hope it helps you, too.

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