Seed germination – what makes it work?

By Connie Mehmel
WSU Chelan/Douglas County Master Gardener

Newly germinated onions put out shoots.
Newly germinated onions put out shoots. If you use a heat mat, be sure to turn it off once seedlings have germinated. – Provided photo/Connie Mehmel
Connie Mehmel
Connie Mehmel – WSU Extension Chelan/Douglas County Master Gardener – Provided photo

Winter solstice has passed, and the days are getting longer. Now is the time to get out your saved seeds, or order new ones, and prepare to propagate vegetables and flowers for your garden.

Starting plants from seeds is exciting stuff for a gardener, but it has its challenges. Often you will hear someone say, “I never have any luck starting (name your problem seed).”

What makes for successful germination? Good-quality seeds, good starting mix and proper moisture are essential, but often overlooked is the importance of temperature. Germination results from a series of chemical reactions in the seed, and those reactions can only take place if the temperature is right.

Seeds may germinate at a wide range of temperatures, but best success is achieved at the temperature when germination is fastest.

Slow germination increases the likelihood that seeds will rot in the soil, or seedlings will be killed by fungal pathogens. If germination takes too long, seeds may use up their stored carbohydrates before the plants can begin photosynthesis, resulting in plant stunting or failure.

Seventy to 75 degrees Fahrenheit is a good soil temperature for most seeds, though warm-season crops like tomatoes and peppers germinate faster at 80 degrees.

If you are starting seeds in your house, consider investing in a seedling heat mat. These mats cost about $25 and are available from seed catalogs and many garden stores. When placed under a seedling tray, they increase the soil temperature by 10 to 20 degrees above air temperature. If you don’t have a mat, you can try starting seeds on top of your refrigerator and take advantage of the excess heat produced by the motor.

What about cool-season crops like lettuce and peas? The seeds may be able to germinate in soils as cool as 40 degrees, but germination will be slow with all the associated risks. Lettuce and peas germinate most quickly at soil temperatures between 77 and 86 degrees. They grow best in cool air temperatures, from 60 to 65 degrees. When air temperatures warm, these crops go to seed. That is why they are started indoors (in warm soil) for spring planting and harvested before the heat of the summer.

In summer, lettuce and pea seeds can be planted directly into the warm outdoor soil to be harvested as fall crops, germinating in the heat and growing as temperatures drop.

For more information, I recommend you read “The New Seed-Starters Handbook” (revised 2018) by Nancy Bubel and Jean Nick.