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I've been given the instructions "plant young plants only after the ground temperature reaches 70°F (21°C), about 2 to 3 weeks before the last frost."

I looked up ground temperatures in my area and it never goes above 60°F (15.5°C), measured 1m (3ft) down. So how are you supposed to measure the 70°F (21°C)? On top of the soil, 10cm (4in) below, or above or how?

The way I understand this instruction, I can never plant these. Or at least, it makes no sense to expect these temperatures 2-3 weeks before the last frost.

  • what part of the world are you in and which plants are you talking about, specifically? – Bamboo Feb 10 '18 at 23:19
  • I'm in Denmark, in zone 8a. These instructions were for thyme, but they seem to contradict each other a lot so I just wanted to know the "theory" behind soil temperature vs. air temperature. – Speceus Feb 11 '18 at 12:22
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    I think its over complicating things, having to worry about degrees of temperature - general rule for small plants is to plant in spring (usually late March or early/mid April, depending on locale, as growth is starting), or around September time. When you can see weeds germinating in the soil, its warming up already, so that's a guide - using that, common sense (is the ground still frozen, snow everywhere and so on) and the month of year is how most people do it. – Bamboo Feb 11 '18 at 12:42
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The soil temperature is measured at the depth you're going to plant your seed. I guess seed potatoes could be planted 1 metre below ground but at that depth the soil temperature likely doesn't vary much all year round. And soil thermometers are only 30 cm long at best.

You plant some plants before the last frost so that they get as long in the ground before the ground gets too hot and they bolt. Some plants also need some cold to improve flavor, and tolerate a mild frost. The last frost of the year should be the mildest.

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  • Ok, but it's still contradicting since we don't have temperatures of 70°F (21°C) anywhere at that time (2-3 weeks before the last frost). So should I just ignore the temperature instructions and sow 2-3 weeks before the last frost even if both soil and air is no way near 70°F (21°C)? – Speceus Feb 11 '18 at 12:25
  • Contradicting what? Maybe the seeds you have just aren't suitable for your climate. Anyway soil temperature will determine if your seeds will germinate. – Graham Chiu Feb 11 '18 at 12:37
  • Since you live in Denmark where the temperature might reach 21 degrees only at peak summer you'll need to germinate your seeds inside, and transplant them when they're strong enough to withstand your cool climate. – Graham Chiu Feb 11 '18 at 12:59
  • @Speceus In spring ground temperature is usually higher than air temperature. After winter, the ground warms up earlier than atmosphere. Remember those schematics in geography classes? Rays of light reach the soil and then are reflected back, so practically the soil (not the sun) warms the air. So, even if there are not 70°F (21°C) in the air, there is a chance of being so in the upper layers of soil. Just measure it like Graham says, at the depth you want to place the seeds. – Alina Feb 12 '18 at 7:50

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