scarifying seeds

Stratification and Scarification of Seeds

Give your seeds a head start

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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, day lilies (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.

Learn about stratification and scarification, two simple techniques that trick plants into germinating.

Seed Scarification: How, When and Why to Scarify Seeds

Steph is a certified Square Food Gardening Instructor who has been gardening for more than 10 years in Canada where the winters are long and cold, and the summers are unpredictable. She is a volunteer for her community’s Incredible Edible project. In the past she created an educational gardening space for seniors and taught classes at a local community center where she created her own curriculum and activities. She participated in several local municipal garden days where she set up a booth to educate citizens about the joy of gardening.

Have you ever read on a packet that a seed needs scarification before planting? While it’s not a common instruction, it’s a handy trick to know because it speeds up and improves germination rates.

So what in the world is scarification?

Seed scarification involves weakening the coating of a seed to encourage sprouting. This can be done in a number of ways, but most common is mechanically breaking a seed’s shell.

So when do you need it and how do you make it happen? Here’s what to know.

Why Do Some Seeds Have a Hard Coating, Anyway?

First things first, why do seeds need scarification? The reason some seeds have a hard coating is to prevent them from germinating in poor weather conditions. In some climates and situations, early sprouting can be disastrous for a tiny, vulnerable sprout.

If seed interiors are exposed to water at the wrong time, like right before winter arrives, for example, they might sprout and die before they even have a chance to survive. The hard coating on some seeds ensures seeds sprout at the optimal time to ensure survival.

How do seeds with tough outer coatings manage to sprout in nature if they’re so impenetrable? Natural scarifying occurs over time, usually throughout the winter season as the ground freezes and thaws over and over.

Seeds are slowly scarified over time and the outer coating is eventually weakened enough to let water and air through, which leads to germination.

It also happens when a bird eats a seed and then poops it out later.

Manual scarification recreates these conditions and preps the seed for germination, especially in seed types that are tough to grow, such as asparagus. There’s evidence that with stratification seed germination rates increase significantly.

Annual plants are less likely to require scarification compared to perennial plants.

Types of Scarification

There are three main types of scarification. Let’s go over them:

Mechanical involves physically opening the seed coating to allow air and water to enter.

Chemical involves the use of chemicals to weaken the seed coating and encourage germination. Sulfuric acid is a commonly used chemical for this method.

Thermal involves brief exposure of seeds to hot water.

Step-by-Step Strategies for Scarifying Seeds

There are a few easy ways to scarify seeds:

  • Soak seeds in water for at most 24 hours.
  • Use a sharp implement to pierce the seed’s hard outer coating.
  • Lightly apply pressure to seeds to break or nick the outer coating.

You can also sow seeds in the fall, as opposed to the spring, to promote scarification rather than physically opening them yourself.

Careful when scarifying seeds. One must be gentle enough to pierce the outer coating and allow air and moisture in. The pressure or element applied cannot be used so forcefully as to damage the interior of the seed.

Don’t scarify seeds until you plan to sow them. Seeds should be used promptly once scarified since they quickly lose viability.

Here are some step-by-step instructions for scarifying seeds.

Sandpaper Method

  • Cut a small piece of sandpaper.
  • Gently rub seeds with the sandpaper to erode the strong outer coating.
  • Stop rubbing when it’s clear the outer coating has been penetrated. You should notice a lighter color starting to show through.
  • You don’t need to remove the coating on the entire seed, just in one small area.
  • Plant seeds right away.

Water Soaking Method

  • Prepare a bowl of warm water prior to planting.
  • Submerge seeds in the water and let soak for 8-12 hours.
  • Stir seeds a few times during soaking.
  • Check for any visible swelling. Some seeds may also sprout during soaking. Remove any that split or sprout.
  • Plant straight away.

File or Clipper Method

  • Use a nail file to rub a small nick in the side of the seed. Alternately, you can use a pair of nail clippers to make a small nick.
  • For smaller seeds, you may need to hold them with a pair of tweezers.
  • Stop when you see the lighter inner part of the seed.

Fridge Method

  • Dampen paper towel.
  • Place seeds on the wet surface and fold over the paper towel.
  • Press down so that seeds are in contact with towel on both sides.
  • Store towel inside a plastic bag.
  • Label bag if you’re doing this with multiple seed types.
  • Put bags inside the fridge and leave them there for about three months.
  • Plant after three month period is up.
  • Some seeds may rot during this process. If they smell odd or have brown markings, toss them.

Hot Water Method

  • Bring a pot of water to just under boiling. The water should be about 180°F.
  • Remove the water from heat and add the seeds.
  • Leave the seeds in the water until it cools.

What’s the Difference Between Stratification and Scarification?

There are two methods for breaking seed dormancy and encouraging germination. Stratification is slightly different from scarification in that the seed needs moisture and/or a change in temperature to let it know it’s time to sprout.

Seeds that require stratification need a period where they’re exposed to cold, moist conditions. In the natural world, the period of cold is the winter. Other seeds need exposure to heat. Still others need a combination of heat, cold, and moisture.

How Can You Promote Stratification?

Here are a few ways to promote stratification:

  • Sow seeds in the fall so that they must go through a period of dormancy in the winter.
  • Put seeds in your fridge to mimic winter stratification.
  • Soak seeds in cold water.
  • Soak seeds in warm water.

Types of Seeds That Require Scarification

Most common vegetable seeds for a home garden don’t have hard outer coverings. If you’re wondering which seeds need a little extra helping hand, here’s a brief list:

  • Many trees, like horse chestnuts, black walnuts, redbuds, crabapples, hickories, and maples.
  • Perennials like butterfly weed, lupine, moonflowers, lotus, Joe Pye weed, columbine.
  • Some annual flowers, such as nasturtiums, morning glory, and milkweed.
  • Many native flowers
  • Okra
  • Bean seeds
  • Purple hyacinth beans
  • Strawberries
  • Spinach
  • Winter squash
  • Plants in the tomato family, like chayote, eggplant, and tomatillo.

Basically, if a seed has a thick outer coating, it might be a good candidate for scarification. For instance, if you look at nasturtium seeds, it’s clear that they have a thick, wood-like shell.

Some plants don’t require scarficiation, but they’ll germinate faster if you do. Beans, for instance, will sprout without being scarified. But they’ll pop up a lot faster if you give them a little nick.

You’ll also see a higher germination rate.

Scarification is a Handy Skill to Know

If you love nasturtium or lupines and you want to start them from seed in your garden, then you’re going to want to learn how to scarify seeds. The process isn’t difficult, but it pays off big with faster and better seed germination.

A quick rub between some sandpaper, and suddenly your seeds will be growing better than ever.

You may have heard that you need to scarify seeds. So how do you do it? And when? What seeds need scarification? Morning Chores explains.