One of the factors that most influences the germination rate of seeds is how they are stored. Like the plants that produce them, seeds come in all sorts of forms and sizes. They are also variable in their longevity.
Vegetable seeds that are considered “long-lived” include the Brassicas (broccoli, Brussels sprouts, cauliflower, cabbage, collards, kohlrabi…) the chicory group (endive, escarole, radicchio), cucumber, kale, lettuce, melons, mustards, peppers, radish, rutabaga, sunflower, tomato and turnips. Stored correctly, these seeds should maintain a good viability for more than five years.
“Medium-lived” varieties include beans, carrots, celery, chard, eggplant, parsley, peas, pumpkin, salsify, and squash. These varieties, if properly stored, should last up to five years.
“Short-lived” seeds include corn, leek, onion, parsnip, and spinach. These are generally not recommended for more than one season, although they may maintain acceptable germination rates in the second year.
Some tricks to keeping seeds and getting the best germination:
The secret to successful seed storage is “cool and dry.”
As soon as your seeds arrive, store them immediately in an airtight container in a cool spot, away from any obvious heat source, and out of the sunlight. Try and store them at 4 – 10°C (40 – 50°F). The refrigerator is not ideal, as it tends to fluctuate in humidity. Freezing will kill many seed varieties. It’s true that government-run seed vaults freeze their seeds, but they do so in laboratory conditions with specialized equipment and controls that few of us could ever simulate at home. Some people like to include desiccant packets with their seeds to ensure a dry environment. One other trick is to wrap a teaspoon of milk powder in a piece of tissue, and use this the same way. This will absorb any available moisture in the airtight container.
Only ever use sterilized starting mix when starting seeds indoors. Regular soil is teeming with microbes, including certain fungi that can cause damping off of seedlings, and bacteria that can harm the seeds themselves.
Once the seeds are planted, keep the temperature constant. Aim for continuous temperatures of 20 – 24°C (70 – 74°F) both day and night.
Once the seeds are planted, never let them dry out. Seeds absorb oxygen as they germinate, so too much water can deprive them of this vital gas. Aim to keep the soil evenly moist. Seed starting soil is designed to provide both water retention and drainage.
Ventilate seedlings. One mistake that is easy to make is to leave the plastic dome over seedling trays on a hot day. If there is significant condensation on the plastic dome, remove it or prop it open to allow air to get in and out. On a sunny day, you can actually steam your seedlings.
Finally, harden seedlings off before transplanting them outdoors. Sudden plunges into cold, damp conditions can cause enough stress on young plants to cause severe stunting or death. Make the transition gradual. Using a cold frame or cloche works well.