Soil Temperature: The Science of Planting

March 4, 2014 | By | More

soil temperature

Yes, that’s snow…we have quite some time before planting.

One of the many things I love about gardening is that it’s art that’s backed by science. You end up with this beautiful thing to look at, maybe even eat, and it all happened according to a very precise natural schedule.

When I started trying to grow vegetables a few years ago, I based many of my decisions about when to plant on my first and last frost dates. There are lots of gardening resources out there with handy charts that tell you to direct sow carrots three weeks prior to your last frost date in spring or to transplant broccoli seedlings nine weeks before the first frost in the fall. But, what if the weather hasn’t been “normal” or predictable?

I’ve noticed that the weather is getting weirder all the time with late frosts, unexpected ice storms and epic snowstorms before Thanksgiving. Recently, I was reading about soil temperature and how much it can affect your gardening success. I read that, for example, putting peppers in the ground too soon can doom them to a life of stunted growth and reduced productivity. Right then, I thought back to my first year of vegetable gardening and remembered just that about my peppers, eggplants, and especially my tomatoes. I remember digging the hole for my tomato seedlings and noticing that the soil seemed very cold. Not knowing any better, I plopped the poor plants into the ground anyway and had disappointing results to say the least.

soil temperature

It’s hard to believe vegetables will be growing here soon!

This year, I’m going to take a much more scientific approach and plant according to soil temperature, using the weeks around my frost dates as rough guidelines. Not only do seeds have a range of ideal soil temperatures for germination, but transplants will also succeed at their favorite temperatures. I really think this will improve my harvest this year. I invested in a digital soil thermometer, and I’ve even organized my seeds according to temperature so it will be easier to pull the seeds out at the proper time.

Many plants have a wide range of suitable growing temperatures and some have ideal temperatures for germination that are just not practical. For instance, the optimum germination temperature for spinach is 70° but if you waited that long to plant it, your spinach would bolt right away. The key is to find a soil temperature that may not give 100% germination, but results in germination rates that are good enough while being optimum for plant growth. There are numerous sources online to find charts for the specific crops you’re growing, but here’s a list for some of the crops I’ll be growing this spring.

Soil Temperature

45° – Direct sow: lettuce, mustard greens, broccoli raab, spinach, chervil

50°- Direct sow: arugula, fava beans, peas, radishes,cilantro, parsley     Transplant seedlings: bok choy

55° – Direct sow: carrots, parsnips

60° – Direct sow: runner beans, bush beans, summer squash

65° – Direct sow: cucumbers, melons     Transplant seedlings: tomatoes

70° – Transplant seedlings: peppers, eggplant, basil

 

 

 

 

 

 

 

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Category: General, Home and Garden, Kitchen Garden

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