Five Tips for Successful Indoor Seed Starting

Now is the time to start early vegetable crops indoors as we wait for more soil warmth and longer days. I’ve been tinkering with my seeds and equipment in the basement, and am starting on my second tray of seedlings.

Here are five tips to help get better seed-starting success indoors:

1. Use fresh seed. This year I did a test. Cleaning out my seed boxes, I found some Little Gem romaine lettuce seed from 2011. It’s seed from my own plants, and I’ve been growing it each year. But lettuce seed is delicate, and often lasts only 1 to 2 years.

I am storing it in the dark, in glass jars with a desiccant pack, in my basement, which keeps a pretty consistent temperature in the mid-50s F. Still, it’s old enough that I should probably toss it into the compost.

But I also had some Little Gem seeds from last year. I decided to plant some from each batch in starter cells in my indoor seed-starting station. My approach is to pinch 5 or 6 seeds from the packet for each cell, then thin them if too many come up.

New and old seed
Here’s an example of new and old seed. Lettuce from 2011 in front did not come up nearly as
thickly as the 2014 lettuce in back.

As you can see in the photo, the 2011 seeds in front germinated at a much lower rate than the new seeds in back. Another thing that happens with old seed is that it is not as vigorous as new seed, so the plants themselves might not be as large or robust.

Time for me to say goodbye to that 2011 seed.

Some seeds, like beans, have a high germination rate even after many years. It does depend on how you store them. You can do a germination test to see if seeds are still viable.

2. Use bottom heat. Some seeds, like lettuce, will sprout in 40 degree F. soil. But that’s not the optimum sprouting temperature. For lettuce, that’s closer to 60. The soil in indoor seed trays will be cooler than the room they’re in. So bringing the soil temp up will cause faster, more robust sprouting.

You do that with a germination heat mat. The electric mat is set up to be water-resistant, so careful watering won’t damage it or cause electrical problems. It will generally heat the soil to about 10 degrees above the ambient room temperature.

To control the soil temperature even more, plug the heat mat into a thermostat. The heat-mat thermostat comes with a soil probe that tells it what the soil temperature is. When you set the thermostat for a particular temperature, like 68, it will monitor the soil and keep the heat mat working to maintain that temp.

3. Use supplemental light. Our Seattle days are getting longer, but still pretty short, and sometimes a dim grey when there’s heavy cloud cover and rain. Supplemental light will give your seedlings another good start.

You don’t need light until the seeds pop out of the ground. Once they’re up, they can be moved off the heat mat and put under light. The light should be 4-6 inches from the plants, and moved up as they grow. Use a cool light, like fluorescent, to not add heat to the plants. Heat from lights can burn or dry out the tender young plants, or at the very least dry out the soil too fast.

I use regular fluorescents, but there are “grow light” types that are much more expensive but will boost plant growth.

4. Keep a close watch. When seeds are sprouting, the soil needs to be kept evenly moist, but not wet. Once the seedlings are up, they need to be thinned, the plastic cover removed as they get tall enough to hit it, and the lights set in the proper position. And regular water is essential.

That means a visit at least once a day to your seed-starting station in order to maximize the process.

5. Don’t over do it. Too many starts too quickly will overwhelm your seed-starting system, and also your available time.

I often start one tray of salad greens in mid-January and grow them to 2 sets of true leaves, then sprout my second tray of them 3-4 weeks later (mid-February), and throw in some brassicas (kale, broccoli, etc.). I will be setting out the first tray of greens into a cold frame or cloche by the time the second one needs the light.

In another 3-4 weeks after that (early March), I’ll sprout a tray of warm-season crops like tomatoes and peppers.

Starting seeds indoors gives my garden an early start. It also scratches the itch I get as the days start to get warmer and longer early in the year.

Seed starting station

Here is my seed-starting station: lights on a timer, seedlings on top shelf, sprouting seed tray on heat mat (with thermostat) on the lower shelf. All electrical plugged into a power strip.

One Response to “Five Tips for Successful Indoor Seed Starting”

  1. Cindy Riskin says:

    Thanks for the can-do information, Bill. I’ll set up my growing station this week.

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