When I think of planting ferns I usually think
of shady, moist conditions. But that’s
not all. There are ferns that do well in
dry conditions and some that can handle a fair amount of sun. One aspect of fern selection I had not
thought of much was soil pH (or acidity).
My only knowledge here was that maidenhair fern, Adiantum capillus-veneris, needed alkaline conditions, but that was
about it. (However, Northern maidenhair
fern, A. pedatum, is less demanding,
preferring neutral conditions.) My eyes
were opened a bit further when I read an article in Fine Gardening by C.
Colston Burell on Hardy Native Ferns. In it is a table listing light and soil
tolerance of about 25 native ferns. I
thought that I now had a definitive decision making tool. However, as I did some more research for this
blog post I started finding conflicting information on what conditions
particular species preferred. One reason
for the confusion may be the difference between ‘preferred conditions’ and
‘conditions tolerated.’ While some
species are very particular about what they need to grow, others are more
tolerant of variation. Lacking a full
understanding how growing conditions were measured and of what truly unacceptable
conditions are, I am inclined to treat published growing conditions as more
what you’d call guidelines than actual rules.
With that in mind, the table got me to thinking
about the ferns I had planted that were not doing very well. Three particular ferns that are not excelling
in my plantings are royal fern, cinnamon fern and hay-scented fern. The soil in my area is neutral to alkaline
and these struggling ferns are listed in Burell’s table as preferring acidic
soils.
The royal fern, Osmunda regalis, is in a moist shady spot. I was hoping that this taller fern would form
a diffuse back ground to highlight smaller plants planted in the
foreground. After three years it has not
grown more than 6 inches tall and has only put out a few stems. This year I will try transplanting over some
ostrich ferns, Matteuccia struthiopteris,
into that area. These are listed as growing
in alkaline conditions and they have a very strong vase shape, though they are
visually more dense than royal ferns.
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Ostrich fern grows to about 3' tall with a strong vase-shaped habit, reaching full size by mid-spring. Its preference for neutral to alkaline conditions makes it a good choice for foundation plantings. |
The cinnamon fern, Osmunda cinnamomea, is doing somewhat better. It’s planted in part sun with similar moist
soil. I was hoping that the light green
fronds of the vise-shaped fern would stand out on the edge of a shrub
border. So far the plants are just
holding on. Perhaps here I could add
some sulfur to make the soil just a bit more acid. (That would also benefit the nearby hollies.)
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These 2 cinnamon ferns were chosen for there bright green color and vase-shaped form which contrasts with the darker green of the hollies behind. All of these plants would benefit from a bit more soil acidity. |
The third fern that is under-performing my
expectations is hay-scented fern, Dennstaedtia
punctiloba. My understanding was
that this aggressive fern would grow just about anywhere. That’s why I am using it in my vinca
replacement project where the conditions are dry shade. Three years in and only one clump has spread
a little, and two others have died.
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These hay-scented ferns, growing in a gravel bed, are just unfurling in mid-April in the Boston area. They will grow to be about 2' tall. |
Considering that my soil is pretty much
neutral (6.8-7.2) I should avoid ferns that need acidic conditions and focus
more on those that are happy with neutral conditions.
Ferns suited to neutral to alkaline conditions include: ostrich
fern, Matteuccia struthiopteris, marginal
wood fern, Dryopteris marginalis, lady
fern, Athyrium filix-femina and sensitive fern, Onoclea sensibilis. One uncommon fern I found growing in the
woods was blunt-lobed cliff fern, Woodsia obtusa, which prefers
alkaline conditions.
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This cliff fern is growing on a east facing rocky slope. |
Ferns that I have found noted as indifferent to
soil pH are Christmas fern, Polystichum acrostichoides,
and ebony spleenwort, Asplenium
platyneuron. These are both very common
ferns on my property.
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Christmas fern is evergreen and has a more relaxed upright habit. Even in January this fern looks pretty good. |
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Ebony spleenwort is a smaller fern more useful as a ground cover component than as a feature plant. This specimen was growing in full shade and was about 8" tall. In sunnier locations it stays closer to the ground. Ebony spleenwort will grow on a variety of soils whereas other species of spleenworts prefer neutral to alkaline conditions. |
Maybe it's best to proceed cautiously when there
is a question. If you’re uncertain,
rather than doing a massive planting, put in one or two and see how they
do.