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Take Your Temperature

Planting vegetables before May

Until this year, a month-by-month portion of Eugene Weekly’s annual vegetable planting guide began in May. In a way, that’s logical — May is about when our heavier valley soils become workable. These days, however, with many people building raised beds and all-season gardening becoming ever more popular, lots of gardeners know that the planting year can start a lot earlier. 

Here’s the classic test of a soil’s “workability” or readiness to cultivate: Gather a handful of soil from 5 or 6 inches down and squeeze it gently, or drop it on a board. Does it crumble and fall apart? Then it’s ready to dig. But what if you have a bed or two you worked over in fall and amended? They may be ready to plant as soon as the soil is warm enough for seeds to germinate. 

Digging around in some old computer files, I came across an article from the OSU Extension Service relating vegetables’ seeding times to soil temperature. “February is prime time to select and plan for planting this year’s ‘cool season’ vegetables,” it says. “And it’s a good time to find or purchase a soil thermometer. The soil temperature is the best indicator of when to plant each type of vegetable, no matter what climate zone you live in.” Some seed catalogs include optimal soil temperatures for planting various crops under cultural guidelines.

I suspect that experienced gardeners can tell by feel and instinct when it’s time to sow. But if you want to get a super early start on your garden and need some reassurance that it’s worth the bother, a thermometer may really help. Many crops germinate at soil temps as low as 40 degrees. The OSU article lists arugula, fava beans, kale, lettuce, pac choi, parsnips, peas, radicchio, radish and spinach seed in that category. One fine afternoon in mid-February, I was thinking of planting some peas and a second crop of fava beans. I grabbed my brand new thermometer and took the temperature 2 inches down in two sunny beds that had been only lightly mulched, and was surprised, given the amount of cool weather we’d had, to find that both were close to 50 F. 

Once it rises above 50 F, I could in theory be planting leeks, onions and chard. At 60 and above, beets, broccoli, carrots and even beans. (Once sprouted, beans won’t tolerate any frost, however, so they’ll need the protection of a cloche or row cover.) You should  expect soil temperature to vary with the site, the soil type and its moisture content, and, of course, the weather. Raised beds may achieve these temperatures long before flat land, especially in a wet season. Protecting raised beds from rain with boards or plastic will help the soil warm up faster, once it is uncovered. So will removing a heavy mulch, which acts as an insulator. 

So if you like direct seeding, don’t let decent weather in February, March and April go to waste. To get an accurate temperature reading in late winter or early spring, you’ll need to insert the thermometer 2 inches deep into the soil, the Extension Service advises. “For most reliable results, take the temperature at the same time each day for several days in a row and average them out,” preferably at midday. 

If you use transplants, make sure they are ‘hardened off’ before they go in the ground. (That means exposing them gradually to outside temperatures and sun over several days.) Wait until the soil warms above 70 degrees to plant out warm season vegetables such as tomatoes, peppers, cucumbers, corn and squash. At that point, the Extension Service recommends pushing the thermometer about 4 inches into the soil to get an accurate reading. Be prepared to protect things if a hard freeze is forecast. Small row covers or cloches are ideal for spot protection.