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Soil Temperature – It Matters!

April 16th, 2014

In the spring, many of us are excited to get planting and we see the phrase “as soon as soil can be worked” on our seed packet and plant things a little too early. Even though the soil can be worked, it’s cold, this causes delayed germination and in some cases seeds will rot in the ground before sprouting. It pays to wait an extra week before planting things like beets and peas.  Beets in particular seem to be very picky about soil temperature.
planting peas 2
Lettuce can be sown early, it will take longer to germinate than it does in slightly warmer temps, but the germination rates aren’t as drastically reduced as they are for other types of seeds since they’re so close to the surface. In most cases waiting a week won’t put you any farther behind as far as harvest scheduled go, since the seeds often take longer to germinate they end up coming up at the same time anyways, you just have less risk of seed damage or birds eating the seeds if you wait.
arugula seedlings
In my 5×5 Challenge Garden out front the arugula seeds have started to germinate. None of the lettuces have yet. It’s been a week since I sowed the seeds, the ones I planted in a seed flat indoors germinated in 2-3 days. This shows you how soil temperature affects seed germination.

Have you noticed differing germination rates of some things in the spring when the soil is cold? 

10 Comments to “Soil Temperature – It Matters!”
  1. Nebraska Dave on April 16, 2014 at 9:40 am

    Susy, soil temperature does make a difference. This is the first year that I’ve been watching the soil temperature with a soil thermometer. However air temperature makes a difference as well. We had rain that turned into 1.5 inches of snow here on the western bank of the Nebraskan Missouri river. The total moisture was one inch. It was sorely needed that’s for sure. It put about 200 gallons of water in my rain water collection tank. That night the low was 24 with wind chill of 17. The cabbages made it through the hard frost but I’m thinking the onions will need another replant. It’s just been a yoyo year for weather here. More rain and cold is predicted on Friday. I’m running out of transplants for the onions. I never thought that would happen with 50 starts. I’m going to transplant more into the bigger cups and wait for sunny days in the future. I can still replant the cabbages again if needed. It’s a good thing I started four times as many seeds as I needed to plant.

    Looks like your 5X5 is off to a good start. I bet it’s good to have all the snow melted and warmer weather. The grass is starting to sprout here so soon (like next week) mowing will begin. Have a great day in the garden.

    Reply to Nebraska Dave's comment

  2. Justin on April 16, 2014 at 10:35 am

    I’m actually considering direct-seeding some tomatoes and peppers this year. I know it goes against everything ever written on the subject, but I have a really hard time starting seeds in my house. Everything just gets leggy and falls-over and dies before getting a true set of leaves.

    In the past, I’ve had some really successful tomato plants germinate themselves from the compost pile or places where tomatoes fell the previous season. The plants seem heartier and usually fruit well even if I transplant them. So, I’m wondering if I just do it intentionally if it’ll produce better plants, even though I may get a lower germination rate. I can live with that.

    If anyone has any suggestions for this (like when to plant), I’d love to hear them. There’s literally no info on the subject in common places (back of seed packets, seed catalogs, etc). Everything says “start indoors 8-10 weeks before…”

    Reply to Justin's comment

    • mary on April 16, 2014 at 10:56 am

      Justin – Winter Sown is a great resource for outdoor planting, and tomatoes are one of things they encourage.
      Happy gardening.

      Reply to mary's comment

    • MN Reid on April 16, 2014 at 11:56 am

      Hi Justin. I too have had luck with self sown tomatoes, but I had to wait until September to get any fruit. Cherry tomatoes are quicker. Can you tell us about your seed starting techniques? Have you read Susie’s eBook on seed starting? The biggest cause of failed seedlings inside is lack of sufficient light, and too much water.

      Your lights, full spectrum shop lights need to be about an inch above the seedlings at all times. Use water sparingly. Use a small vessel to water so you have to really spread it out.

      You can let your tomatoes get root bound before you pot them up. Then, bury them up to their neck, and pluck off all of the leaves except the top ones. Roots will grow from the stem.

      For peppers, as soon as they have their second leaves, you can pot those up.

      Good luck! Let us know if you have any more questions.

      Reply to MN Reid's comment

    • Deb on April 16, 2014 at 1:18 pm

      I agree, I have volunteer tomatoes come up and their tomatoes actually come on and ripen faster than what I start in my greenhouse. I would suggest planting seeds out after all chances of frost are past. Good luck. I let tomatoes grow on purpose to get an early start where they fall or transplant to a better place.

      Reply to Deb's comment

    • Justin on April 16, 2014 at 10:41 pm

      Thanks for the tips, everyone! I’ll check-out that wintersown site.

      MN Reid, I’ve read a *lot* about seed starting, including Susy’s info. I think there are a couple of things that contribute to the problem and they’re not all easily solvable with my working space and very limited budget:
      1) I don’t have a place with enough natural light. Those people who can “put the pots or trays by a sunny window” as instructed on the back of the seed packet and get instant awesome results…well, they suck and I’m jealous of them. ;-)
      2) I have an inexpensive shop light that may or may not have a full-spectrum light (I didn’t pay attention when I bought it), but I don’t have a place where I can lay numerous flats and then hang a light 1-2 inches above them. My garage is nearly outside temperatures and my basement most likely has radon and definitely has mold/mildew odor problems, so I avoid putting anything edible down there. That leaves a small corner of the living room that has to be contained, else I upset the wife. I have one of those bookshelf-style “greenhouses” with the clear plastic tent and it works well in the space, but the light is hung vertically behind the unit so that each shelf can share the light. This sets-up a situation where the seedlings start “reaching” backwards as soon as they sprout. I try to rotate the trays, but that only partially mitigates the problem. I also have a heat mat, but only one…
      3) I’ve always had a hard time with even mositure. The room they’re in is heated with a wood pellet stove with a blower and a timer-based thermostat that goes cool-hot-cool throughout the day. This year, I started fussing with them less and watering the trays below the cells and letting the water wick-up and it seems to be working much better than direct watering. In fact, I’m seeing roots sneak out of the bottoms of the cells, which I think is a good sign. *crosses fingers*
      4) In the past, I think they’ve been fed too soon. I was mixing my own seed starting mix and I didn’t realize that the pearlite I was using was coated in miracle-gro. I rectified that, then got a little too zealous with the fish emulsion.

      All in all, I’ve learned from my lessons and this year’s trays look much better than last year’s. So there is hope…

      Reply to Justin's comment

      • Susy on April 17, 2014 at 8:13 am

        Unless you live in a southern area a window will never provide enough light for seedlings. Some people say they do, but the seedlings have a tendency to become leggy. The glass will often block out some parts of the sun’s rays which makes a big difference. I have never found fluorescent grow lights to be that effective with seedlings (hence finally purchasing a real metal halide light).

        Even moisture isn’t really necessary, letting things dry out in between waterings can help seedlings greatly. We heat with wood and it can be a bit dry.

        In your situation a seedling heating mat could be a fantastic resource. I find most seeds germinate better with it. We keep our house quite cold during the day and really cold at night. I love my heating mat.

        to Susy's comment

  3. MN Reid on April 16, 2014 at 11:58 am


    I have found that soil temps do really make a difference. Peas that I put outside about three weeks ago took about two to germinate. Peas I started inside, only took about 5 days.

    Reply to MN Reid's comment

    • Susy on April 16, 2014 at 6:43 pm

      Beets are no different, sometimes seeds planted with two weeks in between germinate at the same time. I usually wait and plant my peas a little later for that very reason, I still end up with harvests at the same time as those who planted earlier.

      Reply to Susy's comment

  4. Deb on April 16, 2014 at 1:26 pm

    In 30+ yrs. of gardening on my own and my parents have gardened for 54 yrs. on their own and never have any of us checked soil temps. it isn’t really necessary and I’ve never had seeds rot due to planting before the soil temp. is considered ‘correct’. I plant when the soil works up perfect and when I have time. With multiple beds if I waited till temps were ‘correct’ then I’d not get everything planted in time. I have many early crops out here in NW central Ohio, none up yet but we had snow and way below freezing temps so I figure it will be a bit longer. That’s OK, those beds are done and will come up when the temps are correct. Now I can go on to planting more beds. With 13 of them and only myself planting it’s a necessity to plant when soil and time permits. I’m glad things are doing well in my green house and soon things will be up in my garden outside. Perennial veggies are starting to green up now too. I don’t have money for a soil thermometer anyway but wouldn’t use it if I had one. Thanks for the post.

    Reply to Deb's comment

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This is a daily journal of my efforts to cultivate a more simple life, through local eating, gardening and so many other things. We used to live in a small suburban neighborhood Ohio but moved to 153 acres in Liberty, Maine in 2012.

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