Seed starting soil temperatures can be overlooked as gardeners focus on air temperature. This causes many gardeners to miss out on a few days, or maybe even a month, of growing time.
Seed-starting basics include what seeds need to break their dormancy including temperature, moisture, light and oxygen. Once these basics are met, watching this process happen will always be a miracle to me! Being a part of bringing seeds from dead-looking inanimate objects to living, photosynthesizing, food and pollen-producing plants is a source of joy and sustenance for more than our bellies as most gardeners know.
Seed Starting Soil Temperature vs. Air Temperature
Seed starting soil temperatures tend to warm slowly but consistently and can depend heavily on microclimates within your growing area. While air temperatures fluctuate much more, they’ll also catch up in the weeks it takes the seeds to germinate and get high enough above ground to be more affected by air than soil temperatures.
Some of the easiest seeds to get growing early in your garden require a lower seed starting soil temperature than you might think to germinate. As gardeners, we know that the health of our soil is key. Remember walking on and working in waterlogged or still frozen soil can leave you with compacted soil and damage the existing soil life, so make sure the garden space and soil you’re working in has completely thawed out and dried out a little from snow melt.
When pushing the seasons as early as possible remember you’re playing with Mother Nature. If you decide to plant at the minimum temperatures required for germination, and there’s an unexpected cold swing for a few days that brings soil temperatures back down, you may lose some seeds to rot, but if things go as planned, you’ll be eating from your garden weeks earlier than if you’d waited. If you choose to wait until the soil is even 5°F warmer than the lows listed below, you’ll end up with a higher percentage of seeds that germinate.
Having a soil thermometer or two on hand to be able to leave in place and get an accurate reading of soil temperatures both in the morning and afternoon.
Crops to Start as Early as Possible
- (35-70°F) Spinach – from 40-50 days to maturity – low temp threshold 30°F
- (35-75°F) Lettuces – from 45-70 days to maturity – low temp threshold 26°F
- (40-70°F) Peas – 50-60 days to maturity – low temp threshold 28°F
- (40- 80°F) Beets – from 50-70 days to maturity – low temp threshold 28°F
- (45-70°F) Onions – 100 days + to maturity – low temp threshold 28°F
- (45- 70°F) Arugula – 40-50 days to maturity – low temp threshold 28°F
- (45- 70°F) Kale – 45-70 days to maturity – low temp threshold 24°F
- (45-80°F) Radish – 30-40 days to maturity – low temp threshold 30°F
- (45- 80°F) Carrots – 60-85 days to maturity – low temp threshold 28°F
- (45- 85°F) Broccoli – 50-70 from transplant – low temp threshold 28°F
- (45- 75°F) Cauliflower – 50-70 from transplant – low temp threshold 28°F
- (55- 75°F) Cabbages – 60-90 days from transplant – low temp threshold 24°F
Tips for Getting Seed Starting Soil to Warm Faster
There comes a time when we all just want to start planting in our gardens again, and a low tunnel provides this earlier-is-better option.
- Cover the soil with plastic like a low tunnel, a cold frame, or individual cloches
- Choose a spot that gets full sun.
- Add a layer of dark compost to increase heat absorption.
This article about seed starting soil temperatures by Michelle Bruhn was written for Hobby Farms magazine online. Click here to subscribe to Hobby Farms magazine.
