There is a reason seeds don’t sprout while they’re still in their seed packets on the shelves at the garden center: most possess a degree of internal (endogenous) dormancy, which means the conditions have to be just right for them to germinate. Seeds will only get started if they are given optimal temperatures, moisture, oxygen, and light (or darkness, depending on the plant species). In many cases, the degree of internal dormancy is considered shallow, and seeds will readily perform once conditions are favorable, but for plants with more complex dormancy, it may be necessary to give them a helping hand.
Some seeds have external (exogenous) dormancy. Their hard, impermeable seed coats are inaccessible to water and oxygen without physical alteration. In nature, freezing temperatures or the extreme heat from wildfires might split the seed coat and promote germination. Gardeners can try several methods to scarify or wear down the seed coat. If the seed coats are not extremely hard, soaking the seeds in warm (not hot) water for up to 24 hours may be sufficient. Tougher seeds may need to be soaked overnight in vinegar. Use white vinegar containing at least 5% acetic acid, and don’t forget to rinse the seeds with water before sowing. Never soak seeds for too long, as they can “drown” without oxygen.
Some seeds need a bit more encouragement, and it will be necessary to bring out the tools. Fine sandpaper will scour some varieties, but others may need to be nicked with a hammer or even a sharp knife. (Always exercise caution to avoid injury). No matter what method you use to scarify your seeds, be careful not to damage the seed embryo and lose viability. Always sow your newly-scarified seeds as quickly as possible, as they will not store properly once treated.
Seeds to Scarify
Beets, Swiss chard
New Zealand spinach
Cold stratification or moist pre-chilling is a method of treating seeds that exhibit a deep internal dormancy. Stratifying seeds is easy, but requires advance planning: most seeds need about 3 to 4 months of chilling before sowing. Place seeds in a plastic bag along with a handful of damp coconut coir, perlite, vermiculite, or sterilized sand and seal tightly. Label the bags with the plant name and store them in the refrigerator. Don’t forget to mark on your calendar when it is time to remove the seeds for planting!
Seeds to Cold Stratify
Another way to cold stratify seeds without using your fridge is to plant them out in the fall, either directly into the bed where they will grow or into a nursery bed to be transplanted at a later date. A pot sunk into the ground will work as well, especially if you want to move the seeds in the spring. If your winters are cold over a period of several months, cold stratification will occur naturally.
There are also seeds that require warm-moist stratification, such as those of portulaca species. Still others need to go through a combination of warm, cool, wet and dry conditions before they are ready to germinate. Some — such as baptisia seeds – must undergo both scarification and stratification for success. It is easy to see why some research may be necessary before deciding on a pre-treatment for your seeds. Many seed companies will print the information on their seed packets, and there are numerous books and online resources about propagation that can offer assistance. If in doubt, the general rule is that seeds that set in late autumn usually require cool-moist stratification. (These are mostly perennials, as well as trees and shrubs). Seeds to scarify may be identified by their hard or otherwise impermeable seed coats.