Seeds don’t sprout while they’re still in their packets at the garden center for a reason. Most seeds possess internal dormancy, meaning they require specific conditions to germinate. Optimal temperatures, moisture, oxygen, and light (or darkness, depending on the plant species) are necessary for seeds to begin their growth. While some seeds have shallow internal dormancy and will readily sprout under favorable conditions, others with more complex dormancy may need assistance.

Certain seeds have external dormancy, indicated by hard, impermeable seed coats that prevent water and oxygen from reaching the embryo. In nature, freezing temperatures or intense heat from wildfires can naturally break the seed coat and initiate germination. Gardeners can employ various methods to scarify or weaken the seed coat. If the coat isn’t excessively hard, soaking the seeds in warm water for up to 24 hours may suffice. Tougher seeds might require an overnight soak in vinegar, using white vinegar with at least 5% acetic acid. Rinse the seeds with water before sowing, and avoid over-soaking to prevent oxygen deprivation, which can harm the seeds.

Some seeds need further encouragement and may require specific tools. Fine sandpaper can be used to gently scour certain varieties, while others may need to be nicked with a hammer or sharp knife (caution should be exercised to prevent injury). Regardless of the scarification method chosen, it’s essential to avoid damaging the seed embryo and compromising its viability. After scarification, promptly sow the treated seeds, as they won’t store well once altered.

Farmer’s Almanac has this to say:

Seeds To Scarify

Nasturtium
Morning glory
Castor bean
Beets, Swiss chard
Parsley
Sweet peas
Edible peas
New Zealand spinach
Canna

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 1 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

Strawberry
Dogwood
Crabapple
Southern magnolia
Delphinium
Columbine
Clematis
Monkshood
Lady’s mantle
Alstromeria
Butterfly bush
Sunflower
Geranium (perennial)
Penstemon
Lavender
Hibiscus
Heuchera
Rudbeckia
Sedum
Viola
Pincushion flower
Globeflower

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.