Friday, April 2, 2021

Picking the Right Ferns

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.

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.)

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. 

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.

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.

Christmas fern is evergreen and has a more relaxed upright habit. 
Even in January this fern looks pretty good.

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. 

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