Heat Treatment: A Yerba Santa Story
Those who are familiar with using cold-stratification for triggering native seed germination may enjoy trying other types of seed treatments. Experimentation with heat treatments for fire adapted species can be really interesting and fun, and can help deepen your understanding of plant and fire ecology.
There are many methods of heat treatment, including hot water, oven, and direct heat. Using an oven or hot water are some of the easiest heat treatment methods. For Yerba Santa (Eriodictyon californicum), however, our experimentation has found that direct heat works the best to break down the tough seed coat.
Yerba Santa: The Holy Plant
Yerba Santa has a rugged beauty that’s hard to beat. The aromatic evergreen foliage of thick, leathery and resinous dark green leaves, combined with gorgeous white to lavender, trumpet-shaped flowers in late spring that are often adorned with bumble bees and other pollinators, is the perfect combination for a drought tolerant native planting project. Yerba Santa typically inhabits dry, sun-blasted slopes and ridges, often in rocky soil, but can be found in a variety of habitats, from disturbed sites, valley bottom grasslands, foothill chaparral and woodlands, to high elevation rocky ridgelines. It can grow from 2-6′ tall, depending on the location and growing conditions, and when mature it can spread by woody underground rhizomes and form clonal stands.
Yerba Santa is native to California and southern Oregon, where it is adapted to the Mediterranean climate and is a “fire-follower,” germinating readily by seed after wildfire events, and sprouting from underground rhizomes. Yerba Santa is an important medicinal plant with significant ethnobotanical uses. The name Yerba Santa means holy plant in Spanish. Many Native American tribes and modern herbalists have long-used various parts of the plant for a wide range of ailments.
Yerba Santa is currently classified in the Boraginaceae (Borage) family, and was formerly classified in the Hydrophyllaceae (Waterleaf family). It is deer resistant, drought tolerant, and is best grown in full sun in well drained soil.
We currently have a limited amount of seed packets available of Yerba Santa seeds, but we hope to expand our availability for this species in years to come.
Direct Heat Treatment of Yerba Santa Seed
Many people know and love Yerba Santa but have a hard time growing it from seed. The trick: heat! Using a metal pail and a handheld propane torch, a flash burn of Yerba Santa seeds has worked well for us for successful seed germination. We place uncleaned Yerba Santa seed in the metal pail and use the propane torch to light the material on fire for a quick, flash burn.
If needed, a small amount of wadded newspaper will help to ignite the material. Remember that this will get hot and can be dangerous. This heat treatment method should be conducted with safety in mind, and should only be used outdoors, away from combustible material and on a calm day with no wind, preferably during the rainy days of late fall or early winter when fire danger is at a minimum.
If you are unable or uncomfortable using direct heat to treat Yerba Santa seeds, alternatively, you can heat the seeds in an oven for 5 minutes at 190 °F, or scarify the seed coat by rubbing the seeds between two pieces of sandpaper to help break down the tough seed coat.
Remember that seeds are living organisms that may not always behave in the way you expect! That’s why experimentation is at the foundation of seed germination success!
Seed pot of Yerba Santa, grown from heat treated seed.
Yerba Santa seedlings ready for transplanting.
Transplanted Yerba Santa seedlings.
Seed Coat Dormancy
In general, there are two types of seed dormancy: seed coat dormancy and internal dormancy. Internal dormancy is most often overcome using cold stratification methods. Seeds with seed coat dormancy usually have a tough seed coat that is impermeable to oxygen and/or water, allowing the seed to stay dormant, sometimes for a very long time, until an external mechanism (e.g. fire, animal digestion, chemical reaction, etc.) cracks the seed coat and allows for oxygen and water to permeate the seed coat and trigger seed germination.
Using Charate for Fire Adapted Species
For some species, just adding charate (charred wood containing leachable chemicals) to the soil alone can stimulate seed germination. Heat treatment is a mechanical treatment that can crack the seed coat and allow for germination; whereas, using charate or liquid smoke is a chemical treatment. Chemical treatments mimic the signals fire adapted species get when plants on the soil surface have burned in a wildfire and open ground is available for seed germination success.
In other words, the chemicals from charate signal to the seeds: “Hey, there’s great post-fire conditions up on the soil surface for successful seed germination, growth, and plant establishment. You should germinate now while there’s room to grow and mineral rich ash to help you grow healthy and strong!”
If you want to experiment with using charate for a natural chemical treatment for fire adapted species, use the following method: Char small branches with a propane torch until blackened through — do not burn to ash — or use charred wood from a campfire or woodstove. Finely grind the charred wood to a powder. Mix the powder into your soil. If you don’t have the materials to grind the charred wood, soak it in water for 24-48 hours and use the extract to water the soil. Char wood only from species that don’t have allelopathic properties.
Using an Oven for Fire Adapted Species
Mimicking fire through the use of an oven can be used to heat seeds and open serotinous cones of species such as knobcone pine. Serotinous cones are sealed with a resin that must be melted for seed dispersal. This adaptation allows species to exploit the favorable conditions of the post-fire environment, and cones can remain sealed for over twenty years. Just place cones on a lined baking sheet and heat them briefly in the oven until they open.
For heat treatment of seeds in the oven, just place the seeds on a tray and place in the oven at the recommended temperature and for the recommended duration. You may need to do some research prior to heat treatment, to find the recommendations for the species you are treating. As mentioned above, for Yerba Santa seeds, the recommendation is to heat the seeds in an oven for 5 minutes at 190°F.
If the temperature recommended for heat treatment is between 180°- 200°F, it is possible that a hot water treatment of the same temperature and for the same duration would give comparable results.
Using Hot Water for Fire Adapted Species
In general, when using hot water for fire adapted species, bring water to a boil, take the water off the flame, immerse the seed in the hot (not boiling) water, and let the seeds soak overnight.
More specifically, seeds should be placed in about six times their volume of water that is between 180°- 200°F. They should be left to cool and soak in the water for 12 to 24 hours, after which they are ready for sowing. The seeds should be sown promptly.
Although many species germinate readily after hot water treatment, we haven’t found this method successful for Yerba Santa seeds. Perhaps they need to be boiled for a short time for this method to work, instead of just placed in hot water, but we found a method that works for us — direct heat using a propane torch as described above — so we stopped experimenting with hot water treatment for Yerba Santa. We do use hot water treatments regularly for other species, however, with great success!
Small batches of larger sized seeds can be scarified by hand, using a file or knife to make a nick or slice in the seed coat, or by using a rock tumbler to allow for the penetration of oxygen and water through the tough seed coat. Some care must be taken to avoid injuring the interior radicle of the seed. Smaller seeds can be rubbed between sandpaper. Yerba Santa has smaller seeds, so the best way to scarify the seed would be with sandpaper.
2021 New Year’s Resolution: Experiment with Heat Treatment!
Whether you’re germinating Yerba Santa seed or seed of other fire adapted species, the treatments mentioned above will help you start experimenting. Make a New Year’s resolution to try one type of treatment for a fire adapted species in 2021! Seed treatments are like food recipes, there can be many different methods to achieve similar results, and experimentation is the key! And like food recipes, once you get the hang of a new trick, it becomes second nature.
Many of the plants we know and love in the West are not only adapted to wildfire, but they need periodic wildfire to reproduce and/or thrive. Whether their seed germination is stimulated by wildfire, prescribed fire, Indigenous burning, or seed germination methods such as those described in this blog post, understanding how fire adapted plant species reproduce is an important part of understanding native plant communities and conservation.