I have a 35-year-old retaining wall running the length of my driveway behind my house. It holds back a massive “hill” in which the road towers about 20 feet above my home. The former owners planted rhododendrons that have since slowly died above the retaining wall. I believe they are dying because the former owners allowed ferns to fill the entire area, creating about 1′ depth of fern matting. Nothing but invasive wisteria will grow in this deep woodland mat. I would like to get rid of the ferns and plant something to attract birds/bees/etc. How can I do this without having to dig up and disturb the soil sitting above the retaining wall? I have read ferns produce chemicals that ensure some plants cannot grow in the soil around them. Are there native shade wildflowers I can sow there, and if sown, would I have to till up the soil and disturb it? Do you have any other suggestions for this area? Maybe native flowering shrubs to appeal to birds and bees and me?
∼ Leslie Duthie
The Rhododendrons probably succumbed to Phytophthora, a fungal-like infection affecting Rhododendrons in the Northeast.
The fern that has taken over is aggressive, as you know, and is probably hay-scented fern (Dennstaedtia punctilobula), which probably took hold as the Rhododendrons were dying from the fungus.
You are correct. The hay-scented fern is allelopathic – it exudes a chemical from its roots that deter other plants from growing. That chemical will not kill other plants; it just discourages them. It is a good fern for difficult areas as it will thrive almost anywhere, but I can tell you would like to be rid of it, and this is no easy task.
So how do we go about removal? You can repeatedly mow the ferns or cut them off with a weed whacker to deplete their energy throughout the summer. You can also “solarize” the soil by mowing the site down and then covering it with a sheet of plastic. The plastic will have to be held down very well at all edges to retain the heat. This method could be most effective in killing the fern and providing you with a better opportunity for re-planting the site. My third option is to cover it with cardboard (break down your Amazon or shipping cartons) and lay the cardboard over the area where the ferns grow. Top the cardboard with soil or compost, or natural mulch.
The fern will try to escape from all of these options by growing out into the area beyond. Hay-scented fern spreads by rhizomes, and those underground stems will travel to a place where they can re-emerge. Be consistent and vigilant. Mowing around the patch will help to deter them from spreading further.
Replanting: As you can see, these methods take some time. If you just mow or cut them, the area might be ready for new species next spring, though- you may still need to be vigilant and remove ferns that return. I also have some additional suggestions.
If you solarize, your site may be ready for planting later this season in late summer and early fall, which is great for seeding. If you cover with cardboard and top it with soil, you can plant directly into the new soil on top, and as the cardboard disintegrates, you will have plenty of room for your plants to root down into the old area. Although this seems like the fastest way to get something new, the ferns can re-emerge through the cardboard. (I had a similar experience with abundant violets. A although I covered them with cardboard and compost, they came back, and I still needed to dig some out. But the cardboard and compost made a looser, softer soil bed, and it wasn’t as challenging as I expected). If you seed heavily with an annual crop (see below) the first year, this may be sufficient to deter most of the fern.
Once the ferns have been knocked back,you can seed. Seed likes a nice bed, so I suggest top dressing with compost, or you can bring in soil and top dress. If the site is open, you might try a meadow mix with partridge pea (Chamaecrista fasciculata.)
Ernst Conservation Seeds or Prairie Moon nursery both offer a good seed mix with lots of perennials and some grasses for New England. They both offer partridge pea as well.
In the case of the cardboard and compost or soil, partridge pea is an annual, and you can sow it when you put the cardboard down. It will grow up and take over this year. (And we are hoping the cardboard and the lush growth of the annual will smother the fern.) It will help your soil recover because it is a legume and fixes nitrogen. In the cardboard scenario, you can select perennial seed and sow that in fall as the partridge pea is dying back. Since partridge pea is an annual, it will re-seed and come up the following year and continue to be a cover crop – growing for the first season while your other perennials take hold but declining over time as the perennials fill in.
In the case of solarization or mowing (which I still think is best for killing most of the fern), you can sow the partridge pea with other perennials in the fall. Remember to top-dress the area with some good soil for the seedbed. The partridge pea will germinate in spring, but it will not deter good perennial plants from germinating and starting growth. It does provide color and cover as your perennials start their growth and become established. It will also deter the fern as it will compete for light and nutrients at the site.
I hope this is helpful. It is a lot of information, but it will give you the result you need. Both of these seed sources have lots of choices, so you can pick and choose what you like. Make your own mix, but that partridge pea will help recover your area and help make a suitable site for new native species.
Good luck.
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