Garden Q&A: What’s creating pellets under my fern?
Q: I grew my potted Boston fern outside this summer, and it did well. Now that it’s been inside for a while, I’m seeing a few dark pellets sprinkled on the table under its fronds. Where are they coming from? The plant still looks fine.
A: This sounds like frass (the term for insect poop), and I suspect it’s coming from a Florida fern caterpillar (Callopistria floridensis) feeding on the fronds since relatively few insects bother ferns. I’ve experienced the same situation with ferns I have summered outdoors and can attest that finding larvae can be challenging since they tend to be well-camouflaged. They come in shades of brown, near-black, and green, and predictably, the green ones are particularly hard to spot. Caterpillars tend to feed at night and on the most tender foliage, hiding closer to the soil during the day.
Fortunately, Boston fern is so resilient that these nuisance pests usually don’t cause much damage aside from some cosmetic chewing and the gifts of frass, which at least can serve as houseplant fertilizer. This moth species is native to much of the Americas and is routinely encountered in Maryland.
If you give the foliage a rough shake over a bathtub or trash can, you might be able to dislodge any larvae not hunkered down near the soil. I’ve found that an ultraviolet flashlight — an overall great toy, ahem, tool, for curious insect observers and gardeners – can help you spot a green caterpillar among the leaves if you inspect the plant in a darkened room. (Their green bodies look more periwinkle-blue under UV light, and the foliage redder, creating enough contrast for better visibility.)
If you suspect there are multiple caterpillars and the fern is at risk of becoming defoliated, and if that degree of temporary damage is intolerable, then you could consider using a low-toxicity insecticide if the product is labeled for indoor use. (Otherwise, you might need to wait for a mild day to treat it outside.)
Active ingredients Bacillus thuringiensis (Bt; make sure it’s a strain targeting caterpillars and not mosquito larvae or beetle grubs) and spinosad are insecticide candidates, with the former being most effective only on young caterpillars. Treated foliage must be ingested by the insects to be killed, so thorough coverage of the foliage is important. Ferns are sensitive to many pesticides, so the popular houseplant pest controls of either horticultural oil (like neem) or insecticidal soap might not be good choices in this case. You can test for phytotoxicity reaction (plant tissue damage from chemical exposure) by treating a portion of the foliage and waiting a few days to see if symptoms like browning appear.
If left untreated, the worst that would likely happen is that the caterpillars will run out of food when the plant is denuded, an outcome that is only likely if there are a lot of them and the plant is small. If the life cycle is broken at that point because they starve, regrowth from the fern should be pest-free. A plant that was thriving from luxuriating in a summer outdoors should have good root energy stores to fuel regrowth once the pests are gone.
Q: I want to try out winter sowing this year, but am overrun with some seeds and am lacking in others for species I want to try. Is there a cheap source of seeds, like for native plants or vegetables?
A: Free seeds are even better, and you’re in luck as National Seed Swap Day (Jan. 27) is fast approaching. Fellow gardeners are usually eager to give away extra seeds, and swaps can be a great way to find varieties that do well locally or species that are more obscure and hard to find. Plus, you won’t be committed to buying a packet containing way more seeds than you may need. (Or, split the packet contents with friends.)
Check out your local Master Gardener programs, libraries, and public gardens for planned seed swap/giveaway events. While the seeds themselves might be free or low-cost, some gatherings might charge a registration or admission fee if the venue is also hosting speakers for the event or providing other activities and take-home goodies. If you use social media, inquire with area gardening groups about events; not all swaps will necessarily take place on that exact Seed Swap day, so you’ve got some wiggle room.
You also have the option of taking up offers posted on social media for up-for-grabs seeds from people in a similar situation (too many to store or sow later) or who have native plants in their garden beds they have not cut back yet, assuming seeds in the pods haven’t blown away or been eaten by birds.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.