Maximum soil temperatures for germination
I realized that soil
temperature was the limiting
factor keeping spring crops from sprouting early, I felt like I'd
discovered the scientific way to time my spring plantings. I
probably should have realized that there was a flip side to the soil
temperature coin --- many fall crops have trouble germinating in hot
This summer has been considerably warmer than average, but I assumed that as long as I kept my fall plantings well watered, they would grow. Not so. I managed to sprout a single spinach seedling out of three beds, only a third of my peas came up, and I wasn't able to germinate Asian greens and lettuce until I planted new areas at the shady end of the mule garden.
I finally thought to pull out my soil thermometer one morning at the end of August, at which time I discovered that the earth's temperature two inches down ranged from 65 degrees Fahrenheit in the more shaded front garden to 68 degrees Fahrenheit in the sunny mule garden. Checking back at 6 PM when the sun had pounded down all day, the mule garden soil temperature had risen to 81 degrees. Yes, mulched spots were considerably cooler --- clocking in at the low to mid seventies --- but my fall plantings have all gone into bare soil since the seedlings are too small to push aside mulch. A few weeks earlier when I was vainly planting spinach seeds, I suspect the daytime soil highs easily exceeded the 85 degree Fahrenheit absolute upper limit for spinach germination.
So what's the solution for getting fall vegetables to germinate in the middle of a hot summer? Lowering soil temperature is probably the reason that some old gardening books recommend placing mulch or old boards over your fall plantings --- you just have to remember to uncover your seedlings as soon as they sprout. Another option is to plant fall vegetables in a cool, shady location, but that idea will backfire when winter cold nips those spots early. Perhaps shady beds could be used as nursery plots for larger fall vegetables, and the seedlings transplanted out into the main garden later? Or maybe there's a way to rig shade cloths to keep the soil temperature in range? I'd be interested to hear how those of you who live with this sort of summer weather every year get your fall garden to sprout.
Our chicken waterer kept the flock healthy despite four months with highs in the 90s.
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