Among the plants sold during the Holiday season, the so-called “frosted fern” is certainly the most mysterious, because not only is there is so little information about it, but most merchants don’t even seem to know what to call it!
You see, this plant is not a fern at all, but what is known as a spikemoss (Selaginella), a primitive plant not really that closely related to true mosses (bryophytes) either, but rather to clubmosses (Lycopodium and others). So you could call it a “fern ally”, but still, it’s neither a fern nor a moss.
The plant in question is Selaginella martensii ‘Frosty’, but it is generally sold, when indeed any semblance of a botanical name is used at all, as S. kraussiana ‘Frosty Fern’. The real S. kraussiana, though, is a low-growing and distinctly creeping plant, rarely more than 2 inches (5 cm) tall, that forms a spreading carpet, whereas ‘Frosty’ is very different and clearly a variant of Marten’s spikemoss (S. martensii) with its more upright habit and tufted growth.
Selaginella ‘Frosty’ can grow to be up to 1 foot (30 cm) tall, but only under very good conditions. 6 to 8 inches (15 to 20 cm) is more likely in most homes. It forms a dense clump of stems that are upright at the base, then arch outwards. They then branch repeatedly, forming a flattened fan, and what with their covering of tiny scalelike leaves, make the plant look very much like an itsy bitsy flat-topped arborvitae (Thuja). Curiously, ‘Frosty’ occasionally produces long stiltlike aerial roots that descend from the stems towards the soil.
The name Frosty comes from the coloring of the new growth at each stem tip. It is white at first, contrasting with the darker green color of older sections, and this does gives a frosted look to the plant.
S. martensii ‘Frosty’ differs from another variegated selection of Marten’s spikemoss, S. martensii ‘Variegata’ (‘Albomarginata’), in that its variegation is limited to the tips of the stems. In ‘Variegata’ the white coloring is more sporadic (only a few stems are affected), but since they doesn’t green up over time, the stems that inherit the discoloration keep it, creating patches of white. As for S. martensii ‘Jori’, another cultivar sometimes seen, it’ very similar to ‘Frosty’, but the stem tips are yellow rather than white.
Neither of these are as commonly seen as ‘Frosty’.
Greenhouse Conditions are a Must
Although you’re likely to see selaginella ‘Frosty’ sold in nurseries as a holiday plant, it actually doesn’t do at all well in the average home. If it holds up in the greenhouse section of plant nurseries, it’s because of the high humidity that prevails there, but when you bring it home, the plant tends to die rather quickly… unless you know what to do.
You see, it needs very high humidity, of the order of 70%, otherwise it dies back little by little, starting at the tips. That’s far more humidity than just about any household can provide, at least during the winter heating season.
So when you bring this plant home, don’t just set it on a windowsill and think your job done. You’ll need to grow in on a humidity tray or inside a terrarium, either open or closed. Or seal it inside a clear plastic bag for the winter. It will also perform well in a very humid greenhouse. And before you even ask, no, misting it on a daily basis will not help at all, while running a room humidifier will only help a bit, as most humidifiers don’t produce enough humidity to meet the needs of this plant that is dreaming of its homeland, the steamy jungles of Mexico.
Once the Humidity Problem is Fixed…
Once you meet its requirements for high atmospheric humidity, ‘Frosty’ selaginella is much more accommodating.
When it comes to light, for example, it is satisfied with medium or even low lighting. No direct sun is necessary or even desirable, although northern gardeners shouldn’t be afraid of exposing it to direct sunlight during winter, when the sun is very weak.
The soil must remain moist at all times, so water as soon as the soil appears to be dry to the touch. It may be necessary to water this plant more than once a week if it’s exposed to the air. Plants in a terrarium, though, won’t need as much attention, as they lose less water to evaporation. In a closed terrarium or a sealed plastic bag, your plant may not need more than a spoonful of water two or three times a year!
Don’t worry if your plant loses its frosted coloration in the summer. It does so because daytime temperatures are too hot (the white cells turn green at temperatures greater than 70˚F/21˚C). Its frosting will return when temperatures drop in the fall.
Be careful when you fertilize this plant: products too rich in nitrogen (the 1st number) can also make the white parts turn green. The easiest solution is to rarely fertilize it and, when you do, only with a very diluted fertilizer. One or two annual applications of a very dilute soluble fertilizer (1/8th of the recommended rate) is plenty.
Other selaginellas have turned out to be invasive when planted outdoors. I don’t think this one would be, but still, if you plant it outside in a humid tropical climate where it might escape, like Southern Florida (hardiness zone 10 or above), keep a watch on it. It won’t be invasive in temperate climates, as its frost resistance is nil.
Finally, our little selaginella seldom suffers from insect or disease problems. And there’s no need to be concerned about the health of your kids, your pooch or your kitty: selaginella ‘Frosty” simply isn’t poisonous.
Multipiying selaginella ‘Frosty’ by stem cuttings is a snap: simply pluck a section of stem and press it into a pot of moist soil, then maintain high humidity by covering the container with a transparent dome or plastic bag. It’ll root within a week or so.
So there you go: a more complete picture of a widely available Christmas plant that has pretty much gone under the radar until now. Best of luck with it!