Not all plant seeds are ready to sprout as soon as you put them in the soil. Some hard-shelled seeds need a bit more coaxing and some need a temperature change to trigger the end of dormancy. Scarification (cracking the hard outer shell) and stratification (fooling seeds into thinking they've been through winter) are two simple techniques that will save you a lot of frustration when starting seeds.
Stratification and Scarification in Nature
Stratification and scarification occur naturally when seeds stay outdoors through the cold winter.
Some seeds, like morning glories and lotus, have outer shells that are extremely hard and don’t allow water through. This is one way a seed stays dormant in the fall and winter until growing conditions improve.
Animals can also scarify seed by eating the hard seeds and digesting them. This is how strawberries can make their way around your yard.
Another way hard seeds can be cracked open is by leaving them outdoors throughout a cold winter. The constant freezing and thawing will be enough to get them to eventually crack. This process is generally referred to as stratification or cold stratification. Some seeds are not hardy enough for cold winter temperatures, but many perennial plants are started this way.
How to Stratify Seeds
Stratification is a means of simulating the chilling and warming that seeds would endure if left outdoors for the winter in their native climate.
Some seeds will stay dormant until triggered by a certain amount of time in cold temperature or warm, damp conditions. This will occur naturally if the seeds are left outdoors or in a cold frame throughout the winter. Gardeners can break the dormancy of these seeds by mimicking the required conditions indoors.
To stratify seeds, start by placing them in some moistened peat, sand, or paper towels in a closed container or sealed plastic bag. For cold stratification, place the container in the refrigerator. For warm stratification, store it somewhere where the temperature remains between 68 and 85 degrees Fahrenheit.
The length of time needed to stratify depends on the type of seed. Check every once in a while to make sure there is still some moisture in the container.
Selecting Seeds to Stratify
Seeds that benefit from being stratified tend to be perennials. It is a means for them to survive the winter and germinate when conditions are more favorable. This includes a lot of trees and shrubs along with perennial flowers such as apples, bugbane (Cimicifuga), butterfly weed (Asclepias), cranesbill geranium, daylilies (Hemerocallis), Delphinium, False Indigo (Baptisia), False sunflower (Heliopsis), fuchsia, Lady's Mantle (Alchemilla mollis), Monkshood (Aconitum), perennial sunflower (Helianthus), poppies, and turtlehead (Chelone).
Some seeds, like Baptisia, may need both stratification and scarification. They need to have their outer shell opened before water can penetrate.
How to Scarify Seeds
Gardeners can scarify seed by gently rubbing the seed with something coarse, like sandpaper or a file, or by making nicks in the shell with a knife. You have to be careful when doing this. You only want to crack the shell, not damage the seed inside or your fingers. Work gently. Some seed coatings are so hard to crack, many gardeners can’t scarify them without crumbling the whole seed.
Selecting Seeds to Scarify
Large, thick seeds such as morning glory, moonflower, nasturtiums, and purple hyacinth bean are the most likely candidates for scarifying. Although edible beans are large seeds, they will not need scarification.
Starting Plants Indoors from Seed. University of New Hampshire Extension
Scarification and Vernalization. Oregon State University