What's the Best temperature for starting seeds?

After being nestled away in worm casting seedling mix, we all wait (not patiently) for that amazing moment of having our plant baby sprout through its worm casting seedling mix and join us for the first time. It truly is a special feeling - so how do we ensure it happens as frequently as possible?

Why does temperature affect the germination of seeds

When it comes to starting seeds, there are a number of factors at play that will impact how many of our seeds germinate:

  • Age: Seeds will germinate best in their first few years and then their germination rate will begin to diminish after 3-4 years
  • Storage: Ensuring seeds are stored in a cool, dark, and dry location will help preserve their ability to germinate
  • Moisture: For seeds to germinate they need to interact with moisture and remain consistently moist. To ensure they stay moist, choose a seedling mix with worm castings as it will retain significantly more moisture than sterile seed starting mixes. 
  • Lighting: Some seeds prefer a dark environment for germinating - ensure that these are started outside of your seed starting area. 
  • Temperature: Lastly, many seeds require a consistent minimum temperature to germinate.

When we think about Spring, it is as the temperatures warm up that we notice more and more new growth emerging from the surface. And conversely, as the temperatures dip in the Fall we notice our plants beginning to struggle before ultimately succumbing to the cool weather of the Fall. 

picture of early spring growthWith the warmer spring days we'll start to see our first signs of growth. 

As such, we want to pay close attention to the temperatures when we're starting our seeds to give them the highest likelihood of germinating.

So, what temperature is best?

Seed germination temperature experiment

As you know by now, I'm a huge fan of experiments! 
To determine the impact of temperature on seed starting, I decided to run a little experiment, which consisted of:
  1. Selected 3 sets of 5 different seeds: Zucchinis, Cucumbers, Tomato, Basil, and Kale.
  2. Started each set of seeds in our worm casting seedling mix and in 3" seed cells. Followed the steps in our Seed Starting guide for layering and moistening the seedling mix.
  3. Utilized the same grow light and amount of light per day (12hrs) for each set of seeds.
As such, every variable was the exact same except for one: temperature

Picture of a thermometer for starting seeds

The last thing that I did was find three significantly different temperature zones in my house:
  • 50 Fahrenheit: I used my unheated garage as the coldest environment which was consistently below 50 Fahrenheit
  • 60 Fahrenheit: My downstairs living area was selected as the second space and consistently sits at 60 degrees.
  • 70 Fahrenheit: Lastly, I used a temperature controlled space heater in my seed starting station to keep the environment consistently at 70 Fahrenheit. 

image of seeds starting in 60f environment

A set of seeds sitting in a room at 60F

Lastly, I let these seeds simply sit under their grow light for 30 days to see what impact temperature would have on both germination and how well they grew. 

What temperature do seeds germinate at

The results of this experiment were truly shocking:
comparison picture
From left to right, the seeds that were started at 50F, 60F, and 70F.
As we can see, the plants absolutely thrived in the 70 Fahrenheit environment while struggling a bit in the two colder environments. Specifically:
  • Kale: Successfully germinated in all 3 environments but really struggled to grow in the 50F and 60F environments while putting on numerous sets of leaves in the 70F environment. 
  • Tomato and Basil: Neither of these germinated in the 50F environment. While they did both germinate in the 60F and 70F environment, neither were growing overly vigorously - similar to how the Kale germinated in the cool environment but was not growing well. 
  • Zucchini and Cucumber: Lastly, two of our warmest weather crops, Zucchinis and Cucumbers only germinated in the 70F environment and grew extremely well in this environment. 

The key takeaway from this experiment is that temperature plays a VERY significant role in how many seeds will germinate and how well they will grow.

Heating mats for plants

So the question then becomes, how do we ensure the seeds have a warm environment?

This is one of the biggest reasons why I love starting seeds indoors. If we were waiting for those requisite warm temperatures outside, we wouldn't have any of our seeds germinating until some of the warmer days of the year - and that's when we want them already at maturity to be making the most of the long summer days.

To ensure our indoor environment is sufficiently warm, there are three ways that we can go about accomplishing this:

  • Thermostat: The first option is simply to adjust your home or apartment thermostat to at least 70F while starting seeds. This ensures the ambient temperature is sufficiently warm.
  • Space Heater: If you'd prefer to only heat a small area (like myself), you can create a dedicated seed starting station and enclose it with a sheet. Once that's complete, put a small space heater into the bottom of this area to heat it up to at least 70F. 
  • Heating Mat: Lastly, heating mats are a great space friendly option for applying heat directly to the seed cells. Simply slide it underneath and it will warm up the seedling mix. Keep an eye on the mix as it will also dry out more quickly than ambient temperatures! 

Keep those temperatures nice and warm and before you know it you'll have a full family of plant babies pushing through the surface!

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