It’s 11 weeks to the last frost in my neighborhood, and I’ve been gripped by seed-starting fever. My family will not thank me if I take up any more room in the refrigerator for my seeds than I already am, so before I can place orders with my favorite seed catalogues, I need to test the seeds’ viability.
Seed viability is another term for the likelihood of a packet of seed to germinate. Viability varies among genera. Many root crops’ seeds, such as onions or carrots, remain viable for one year, while some melons may be viable for longer than 4 years. If kept in proper conditions (low light, low temperature, and low humidity), seed may remain viable for many years past their average.
A simple way to test seed viability
To test whether your leftover seeds are viable, gather together the following supplies:
- The seed packet
- Paper toweling
- Plastic sandwich bag with zippable closure, or a clear plastic container with a lid.
- Misting bottle of clean water
- Masking tape and pen or pencil
- Lay out a sheet of paper toweling and place some seeds onto the towel. Ten seeds, or a multiple of 10 for small seeds, makes for convenient estimating.
- Lightly mist the seeds and the paper towel with water.
- Fold the towel in half, and then in half again.
- Lightly mist the folded towel one more time.
- Slide the folded towel into the sandwich bag or plastic container, and label it using the masking tape. Note the seed contents and the date on which you prepared the sample.
- Keep the bag or container in a warm place, but out of direct sunlight. After a few days (about five to seven, but not any longer), you’ll be able to check the germination rate.
Germination and potting up
Remove the paper towel from its bag and gently unfold it.
The photo above shows an excellent germination rate, and indicates the seed is still quite viable. But they’re not going to get much further growing on a paper towel, so it’s time to pot them up.
The seedlings at this stage have only their cotyledons, or seed leaves. As fragile as these seed leaves look, they’re more resilient than the stem, which will be easily crushed by handling. Grasp the leaves and gently but firmly pull them up from the paper towel. Some of the roots here have grown through the towel, so we’ll tackle the easy ones first.
- Prepare a clean seed flat with sterile seed-starting mix. Moisten the mix and tamp it down well.
- Make a slit or trench in the seed flat using a spoon or knife. Holding the seedlings by their leaves, lay them into the trench up to where the leaves fork from the stem.
- Gently firm the soil back over the stem and root. Follow the same procedure for additional seedlings, but don’t overcrowd the flat. I’ve allowed six seedlings to a flat 3 inches wide by 6 inches long.
- Keep the flat warm (65-70 degrees Fahrenheit) and well lit, either in a sunny windowsill or under a grow light. A fine dusting of sand, vermiculite, or even chicken grit can help to fend off damping-off.
If any of the seedlings have grown through the substrate (paper towel), there’s no need to wrestle them. Using a pair of scissors, trim small seed mats and handle the seedlings by them. The mats can be planted along with the seedlings, and will eventually compost.
- As before, prepare a seed flat with moistened, sterile seed-starting mixture.
- Guide the blade of the knife or scissors between the sprouted seeds. Cut a small section of towel to support the seedling. You can hold up the towel to a light if necessary, to help you find the roots’ paths.
- Plant the seedlings, giving adequate space to each one. Don’t overcrowd the flat. Gently firm the roots against the soil.
- Top the seedlings off with fresh seed-starting mix, up to the base of the seed leaves.
- Water gently and set in a bright, warm space.
Check the seedlings daily and do not let them dry out. Watering from the bottom will help to prevent damping-off disease. When the seedlings have developed one or two sets of true leaves, they may be potted up again.
Good luck with the growing season ahead!