Royally large indeed, Osmunda regalis or royal fern is a large deciduous perennial that can bring character to your garden. It is known by many other names, such as old-world royal fern, blooming fern, kidney fern, and water fern.
This elegant species is a notable member of the Osmundaceae family of flowering ferns, which are relatively large. Many species appear to be living fossils as their features are largely conserved through the fossil record. O. regalis is one of just a handful of species under the Osmunda genus. It is native to Asia, Africa, and Europe.
The grandest of the European ferns, this “blooming” species doesn’t actually produce flowers. Its rust-colored spore-bearing fronds simply have an uncanny resemblance to them. Apart from these fertile fronds, the rest of the leaves are sterile. They can grow remarkably wide and tall, reaching a height of 5 feet (160 cm) and a width of 1.3 feet (40 cm) at maturity. The fronds are bipinnate, meaning their leaflets occur opposite one another, with up to 9 pairs of pinnae each. Each pinna is further divided into pinnules, making the entire frond look extremely textural and broad.
The royal fern is a perfect plant for the partly shaded edges of water features. Its fronds are great for naturalizing both wildlife and ornamental ponds, adding dimension to your garden, affording shade and cover to visiting wildlife, and providing visual interest from spring through fall. A notable cultivar is ‘Cristata’ which, along with the parent species, has received the RHS Award of Garden Merit.
Royal Fern Growth, Hardiness & Climate
As Osmunda regalis can grow quite large, it must be given a lot of space so that its fronds can expand to their maximum spread. A clearance of about 2 – 3 feet (61 – 91 cm) should suffice. This species can take 5 – 10 years to reach its maximum height under optimal conditions.
Attention must be given to the substrate, as it can influence the plant’s rate of establishment. Slightly acidic, consistently moist soil is preferred. Alkaline soil is also acceptable, as long as it is thoroughly incorporated with fibrous compost, pine needles, or shredded leaves. Both poor and well-draining soil with a high ratio of either loam or clay is tolerated as well.
A cool, lightly shaded location would be preferred, though one with full sun exposure can also benefit mature plants. Hardy to USDA zones 3 – 9, the royal fern is well-suited to seasonal fluctuations in temperate zones. It may need additional protection through exceedingly warm summers or chilly winters, however. Note that temperatures between 62 – 70˚F (16.7 – 21˚C) are best and extreme temperatures will likely cause the fronds to die back.
How to Plant Royal Fern
Royal fern cannot be grown from seed, as it produces spores that germinate into prothalli. The prothallus is basically a small heart-shaped proto-plant that must undergo fertilization before developing roots and leaves. The process of germinating prothalli can be quite complex for a beginner gardener as it requires sterilization, the use of an appropriate germination base, and consistent temperatures.
Once ripened and dried, sterilized spores should be sprinkled onto a fibrous seeding medium mixed with vermiculite and kept in a moistened glass or plastic container. The container should permit entry and exit of air, but should mostly be able to help with moisture retention. It should be kept in a greenhouse or cold frame where temperatures are maintained at around 68˚F (20˚C), for around 2 – 6 weeks or until successful germination occurs.
Once prothalli are formed, they must be transferred to a fine, lightly sifted, soil mixture in another container. Developed young fronds indicate that the plant should be potted in coarser, organically rich soil. Throughout the process, the juvenile fern should not be exposed to direct sunlight.
If you’re unable to create a proper setup for spore germination, spreading the plant via division is more ideal. First, dig out the entire root system of the plant so that you can divide the root ball. A good time to do this is when your plant is due for repotting in spring. Use a sterilized knife to cleanly separate the root ball in half. If it’s large enough, you can separate it into quarters, making sure each has its own set of leaves. Plant each division under at least 2 inches (5 cm) of moisture-retentive soil. Ensure that moisture is constantly provided until the divisions are established.
How to Care for Royal Fern
This species is remarkably easy to maintain as long as it is provided with enough moisture. Ensure that the fronds are provided with shade if sun exposure is severe in the summer. Remove any fronds that have died back or become yellow as these may attract pathogens and pests. Spent fertile fronds may also be pruned back prior to winter. If potted, try to repot your royal fern every three years or whenever it has outgrown its container.
How to Winter Royal Fern
In hardiness zones 3 – 9, the hardy royal fern is able to survive outdoors through winter as long as temperatures don’t drop to below -30˚F (-37˚C). It is a deciduous species, however, and its fronds will likely die back as temperatures decrease. If located in a subtropical or tropical zone, it may stay green throughout the year.
Before the first frosts occur or when the fronds show signs of deterioration in fall, cut them back until just a few inches of upright shoots jut above the soil. Place a layer of mulch to provide the roots with protection and warmth through winter. Come spring, pink to wine-colored fiddleheads should rise up and out of the ground, unfurling in warmer air. If located in areas with extreme winter temperatures, you will have to overwinter your fern indoors.
Is Royal Fern Invasive or Toxic?
Although Osmunda regalis produces thousands of spores each year and tends to grow in large colonies in nature, it is not known for being an invasive species. Some cultivars have become naturalized in North America, where they don’t outcompete native plants. Their preference for consistently wet soil limits potential sites of colonization and successful spore dispersion. Additionally, royal fern is not known for harboring toxins. Nonetheless, caution should be taken with plants that are exposed to fumes as ferns can filter chemical gases from the air.
Is Royal Fern Edible? Will Animals Eat it?
Like those of its close relatives, the unfurling fronds or fiddleheads of royal fern are edible. They supposedly taste like asparagus. Its close cousin, Osmunda japonica (potentially a subspecies of O. regalis) or Japanese royal fern, is used in a savory dish. The fiddleheads are thoroughly seasoned and served in a nutritious dish, called ‘Royal Fern Namul’.
Deer, rabbits, and squirrels are known for grazing on the fronds and roots of royal fern. This species is safe for herbivores and is able to withstand moderate grazing.
Where to Buy Royal Fern & Seeds? (UK & US)
Osmunda regalis and its cultivars can be purchased from plant nurseries and garden shops as potted plants. In the US, the ‘Spectabilis’ variety (known as the American royal fern) may be more readily available due to its naturalization in several states. Online plant portals should also carry this well-known species.
