Seed Scarification: How to Scarify Seeds For Better Germination

Nature has the beautiful ability to protect one of our most precious assets: the seeds that bring the next generation of food and beauty. Some seeds have extremely tough or waxy exteriors to help them survive dormancy through harsh winters. Dormancy is genetically built in, and the length varies from species to species.

Each seed, no matter how tiny or large, only has a finite amount of stored energy called the endosperm. The energy will be used on seed germination and growth until the seedling reaches sunlight past the soil surface. It then starts absorbing nutrients from other sources. 

Additionally, some seeds will not germinate even under ideal conditions. They have a tough or waxy exterior that must be softened or broken down. This process is called seed scarification. Home gardeners may have difficulty getting these seeds to germinate. With a little intervention, we can help them, and in turn, our gardens, to thrive. 

What Is Scarification?

Think of scarification as a pre-treatment for seeds before sowing. It’s our way to signal spring so their dormancy breaks and germination occurs quickly and more evenly. The process breaks the seed surface, creating a weak spot in the outer shell. This leads to increased water and oxygen absorption and better germination rates. 

Scarification can happen naturally. Seeds that have been left on the soil surface or buried below debris are exposed to temperature changes, rainfall and snow melt, and general wear and tear from being exposed to the elements. 

While scarification isn’t necessary with all seeds, some require it. With simple tools and a few extra minutes, you can scarify seeds before your next sowing session. Plus, it’s fun to play such an intimate role in the birth of your plant babies!

Now that you know what scarification is, let’s go through a few methods available and which ones are best for various types of seeds. 

Methods of Scarification 

Different species, sizes, and thicknesses respond differently to the various methods of scarification. Scarification can be done mechanically, thermally, or chemically. 

Water Softening

This method includes soaking seeds for 6-24 hours in lukewarm water. The longer the soak, the softer the outside shell will become. However, too long of a soak will cause seeds to become waterlogged, and they may rot. Each seed type will react differently, so do a test on a few seeds to determine the proper length of time. 

It’s as simple as it sounds. Just add water to a bowl and add your seeds. Place the bowl on your countertop, out of direct sunlight and where temperatures are mild. While some information you’ll find will say to discard any seeds that float as they’re unviable, the scarification process may make them just evenly weighted enough to cause them to float and they may not necessarily be unviable. Just a note for you to consider. 

Mechanical Abrasion 

Mechanical abrasion is likely the most popular method to scarify seeds for the home gardener, is the least risky, and is quite simple. It works well for species considered “hard seeds,” which are known to have a water-impermeable outer shell or coating, like lima beans, chickpeas, nasturtium, and sweet peas. The low moisture content of hard seeds makes their dormancy long and stable, with germination more spread out, which isn’t ideal for a gardener on a schedule. 

To make these seeds easier to manage in commercial cultivation, growers can sand, cut, or file a corner or edge. Aim for the end opposite the “eye” of the seed, which is where the root will emerge. Videos to watch are available online before getting started. 

For small seeds like white sage, use sandpaper with 100-200 grit or a simple nail file for safety and ease. Secure the sandpaper to a working surface and gently rub the seed on the sandpaper until the surface of the seed begins to change color. Alternatively, hold a file in one hand and gently rub the seed up and down on the file in your other hand. Don’t go too hard or you’ll damage the innards of the seed. 

For larger seeds like pumpkins or sunflowers, use scissors, nail clippers, a higher-grit file, or a pocket knife. Carefully hold the seed down on your working surface and cut the corner of the seed off or slice it along the seam. This process will be quick and safe with a clean, sharp blade. Germination can occur in as little as three days. 

Heat Exposure 

A study conducted to compare five different methods of scarification on the same cultivar of long bean seed concluded the most effective way was heating them in a 55°C oven for some time, with some positive results coming from sanding and wounding the seeds before sowing them. The study also showed that temperatures greater than 80°C resulted in decreased germination, and the ideal temperature for treatment was between 40-50°C. 

All of this is to say that each cultivar will react differently to these methods, so experimentation is recommended before scarifying all of your seeds. Humidity levels, temperature, growing medium, and minerals present in your water may all have effects on your experiment. 

Another way of using heat to scarify seeds is by placing your seeds in a heat-safe container,  pouring boiling water over them, and allowing them to soak for up to 24 hours. The amount of time seeds should sit in water depends on their type. Heating methods will have varying results based on timing, exact method, and temperatures of exposure. 

Freeze-Thaw

This method scarifies seeds by alternating exposure to extreme freezing temperatures, sometimes using liquid nitrogen, and then rapidly changing it to high temperatures. While this method can have success at breaking “hardseededness”, an increased rate of damaged or killed seeds increases. 

Some growers have success freezing seeds for 24-48 hours, sowing them into a growing medium, and pouring hot water over them to water in and assist in germination.

Others store their seeds in a refrigerator or freezer during the off-season to keep them cool and dry. Remove them when it’s time to sow, effectively fulfilling the thaw portion of this method. 

Sulfuric Acid 

In some studies, this treatment is one of the best ways to soften hard seeds’ outer shells. A key characteristic of hard seeds is a moisture content of 12% or below, creating an impermeable outer shell. Soaking seeds in a properly mixed acid bath leads to an increased permeability of moisture, softening the outer shell enough to lead to higher germination rates. 

Without careful monitoring and if over-soaking occurs during this process, seeds may be harmed, leading to decreased germination or death. Local extension offices and gardening clubs are good resources to check in with if you’re new to the process and have questions about using sulfuric acid to scarify specific types of seeds.  

Animal Digestive Enzymes 

If you notice seeds germinating in places you didn’t sow them, they may have been dropped there naturally by an animal who ingested them. Enzymes in animal and bird digestive tracts, just like humans, help break down food, releasing different nutrients from it. 

In many cases, only the outer shell of a seed is softened or broken down in the animal’s digestive tract before it’s excreted out, giving it the perfect conditions for germination. Hopefully, it lands somewhere suitable and the weather conditions are ideal.

What To Do After Scarifying

No matter what method you choose, continue with your normal sowing process after you complete scarification. Assuming germination will happen sooner after scarification, check on your seeds often. Adjust your watering schedule as needed and provide sunlight when you see the first signs of germination.