Many seeds with hard shells need to be treated to allow water to penetrate them to germinate the embryo inside, a process called scarification. While you can tumble or scrape seeds with sandpaper or a file to help water penetrate their hard exteriors, hot water may also work for some seeds. The water should be close to boiling, but you should not boil the seeds.
Examples of Hard Seeds
Legumes, or beans, typically have to be scarified. This includes the annual pinto bean (Phaseolus vulgaris var. pinto).
Flower seeds that need scarifying include hibiscus, such as Chinese hibiscus (Hibiscus rosa-sinensis), which grows in U.S. Department of Agriculture plant hardiness zones 9 through 11 and zonal geraniums (Perlargonium x Hortorum), which grow in USDA 9 through 11.
Hot Water Scarification
There is no one hot water treatment that applies to all seeds. In addition, recommendations about treating with hot water differ.
- One method is to put them in a cup and pour on the four to five times their volume of water heated to 200 degrees Fahrenheit. Water boils at 212 degrees Fahrenheit. As they cool, the seeds should swell from the water that has penetrated their hard exteriors.
- Alternatively, dip the seeds in boiling water for 10 seconds to three minutes then allow to cool.
- The Permaculture Research Institute recommends putting the seeds in a pot of 180 F water and letting them soak. Remove them when the water cools to room temperature.
- As a variation to this, North Carolina State University recommends placing the seeds in boiling water and letting them soak while the water cools to room temperature.
Other Ways to Scarify Seeds
Other ways to scarify seeds with hard shells that are impermeable to water is to scratch or nick the exterior. Carefully scrape the outer coat with a file or rub it lightly with sandpaper. For seriously hard seeds you may have to nick seeds one by one with a knife.
Shake larger amount of seeds in a can lined with sandpaper or rub them between two boards covered with sandpaper. After treating the exteriors, soak the seeds in water until they swell.
If they don't swell, scarify them again and soak them again. If you soak seeds first, some of them may swell without scarifying.