The oldest discovered fossil of a fern is dated back to about 360 million years ago. The interrupted fern, Osmunda claytoniana, has not changed or evolved at all in 180 million years. It grows wild and rampant all over Northeastern America and Asia, exactly as it has for over a hundred million years. Many of the ferns we grow as common garden ferns are the same species of fern that have grown here since the Cretaceous period, about 145 million years ago. What this means for us is that Mother Nature has got fern-growing down pat, and no matter how much of a black thumb you think you have, you probably won’t kill them. That said, when it comes to fertilizing outdoor ferns, there are things you should know.
Fertilizer for Garden Ferns
About the most harmful thing you can do for ferns is too much. Ferns are very sensitive to over-fertilization. In nature, they get the nutrients they need from fallen leaves or evergreen needles and rainwater running off their tree companions.
The best thing to try if ferns look pale and limp is to add organic material like peat, leaf mold, or worm castings around the root zone. If fern beds are well maintained and kept free of fallen leaves and debris, it’s best to top dress the soil around your ferns each spring with rich organic material.
Feeding Outdoor Fern Plants
If you feel you must use fertilizer for garden ferns, use only a light slow release fertilizer. 10-10-10 is plenty, but you could use up to 15-15-15.
If the outer fronds or tips of the fronds turn brown, this is a sign of over-fertilizing outdoor ferns. You can then try to flush the fertilizer from the soil with extra watering. Ferns like a lot of water and should be fine with this flushing, but if tips turn black, decrease the watering.
Slow-release fertilizer for garden ferns should only be done annually in the spring. Container-grown outdoor ferns can be fertilized in spring, and again in midsummer if they look pale and unhealthy. Fertilizer is leached out of container-grown plants quicker than it is leached from garden soil.
Never apply garden fern fertilizer in the fall. Even ferns divided in fall will not need to be fertilized until spring. Adding fertilizer in fall can be far more hurtful than helpful. You can cover fern crowns with mulch, straw, or peat in late autumn though for a little boost of nutrients in early spring.