Why Are My Microsorum Fronds Curling?

Low humidity

Low humidity causes microsorum fronds to curl at the edges and tips, progressively browning and crisping as humidity falls further below the plant's threshold. Central heating in UK homes in winter is the primary cause, reducing ambient humidity to 30 to 40 percent when microsorum requires at least 50 to 60 percent. Although microsorum's glossy, leathery fronds are somewhat more resilient to low humidity than the delicate fronds of adiantum (maidenhair fern), extended exposure to dry air will cause cumulative damage, particularly to the frond margins and tips. The crocodile fern (Microsorum musifolium 'Crocodyllus') with its deeply ridged fronds is somewhat better at tolerating humidity fluctuations than the broader, flatter-fronded species.

What to do

• Place microsorum in a naturally humid room such as a bathroom or kitchen, where ambient humidity is consistently higher than in centrally heated living rooms. The glossy fronds are ornamental in bathroom settings and the plant thrives in the steam-raised humidity of a used bathroom.
• Stand the pot on a pebble tray with water beneath to raise local humidity through continuous evaporation. Ensure the pot base sits above the water level to avoid waterlogging the roots.
• Use a room humidifier near the plant in winter when central heating runs continuously. Group microsorum with other humidity-loving houseplants to create a more humid local microclimate.
• Keep microsorum away from radiators, underfloor heating vents, and draughts from open windows, all of which lower local humidity and accelerate moisture loss from the fronds.

Underwatering

Underwatering causes microsorum fronds to lose their characteristic glossy turgidity, becoming dull, limp, and eventually curling inward as moisture stress increases. Unlike the dramatic, rapid collapse of adiantum fronds when dry, microsorum shows drought stress more gradually, allowing a wider window for corrective watering. The rhizome of microsorum has some capacity to store moisture, which provides a degree of buffer against brief drying. However, allowing the potting mix to dry completely between waterings regularly will cause progressive frond curl and eventually frond die-back.

What to do

• Water microsorum when the top centimetre of potting mix feels just barely moist, before it dries out completely. The potting mix should remain consistently moist rather than cycling between wet and bone dry.
• Use room-temperature water, as cold tap water can shock the roots of this tropical fern. Some growers prefer to use filtered water or water that has been left to stand overnight, which reduces the fluoride content that can cause tip browning.
• Flush the potting mix with a generous volume of water every few months to remove accumulated salts from fertiliser and fluoride from tap water, which can cause tip burn similar to drought damage.

Direct sun

Direct sunlight bleaches the glossy fronds of microsorum rapidly, causing the fronds to pale, curl, and develop dry, scorched patches. The plant is adapted to the dappled, indirect light of forest environments and should never be placed in direct sun indoors, particularly through south- or west-facing glass in summer. However, microsorum tolerates brighter indirect light than many houseplant ferns and performs well near a well-lit north- or east-facing window, or in bright, reflected light some distance from a larger south-facing window.

What to do

• Position microsorum in bright, indirect light. A position near a north- or east-facing window, or 1 to 2 metres from a south- or west-facing window behind a sheer curtain, provides adequate light without the risk of direct sun damage.
• Microsorum tolerates lower light levels than some houseplant ferns and can perform reasonably well in the middle of a room if ambient light is reasonable. Very deep shade will slow growth and cause fronds to become progressively paler and weaker.

Cold temperatures

Microsorum is sensitive to cold and does not tolerate temperatures below 10 degrees Celsius. Cold draughts from windows and exterior doors in winter cause frond curl and browning, particularly at the frond margins. Cold windowsills in winter may be significantly colder than the room air temperature, chilling the root system. Most microsorum species perform best in temperatures of 15 to 25 degrees Celsius year-round.

What to do

• Keep microsorum in rooms that maintain a minimum temperature of 12 to 15 degrees Celsius. Move it away from cold windows in winter and ensure it is not in contact with cold exterior walls.
• The kangaroo fern (Microsorum diversifolium) from Australia is generally somewhat hardier than tropical species, tolerating temperatures down to around 7 degrees Celsius in dry conditions, making it more suitable for cool conservatories and unheated rooms than other species.

Overwatering

Overwatering causes the epiphytic root system of microsorum to rot in persistently saturated, oxygen-poor compost. The rhizome turns dark and slimy at the soil surface, and the fronds gradually yellow and weaken before curling and dying back as the root system's ability to deliver water is compromised. Overwatered microsorum often confusingly shows frond wilt even in wet compost, as root damage prevents water uptake.

What to do

• Use a very free-draining, bark-heavy potting mix that replicates the epiphytic conditions of microsorum's natural habitat. A mix of fine bark, perlite, and a small amount of peat-free compost drains well while retaining some moisture.
• Ensure pots have adequate drainage holes and that water can drain freely. Never leave microsorum sitting in water in a saucer for extended periods.
• If root rot is suspected, remove the plant from its pot, trim away any blackened rhizome or root material, allow the remaining root system to dry briefly, and repot into fresh, bark-based compost in a clean pot.

Frequently asked questions

Why are my microsorum fronds curling?

Microsorum fronds curl most often from low humidity or underwatering. Microsorum is an epiphytic fern from tropical Southeast Asia and Australasia, where it grows attached to trees or rocks in humid forest environments. In UK homes, central heating in winter reduces ambient humidity well below the levels microsorum requires, causing the glossy fronds to curl, particularly at the edges and tips. Underwatering causes a similar response, as the rhizome dries out between waterings more rapidly than many gardeners expect, particularly in warm rooms.

What is microsorum and why is it called kangaroo fern?

Microsorum is a genus of epiphytic ferns native to tropical and subtropical regions of Asia, Australasia, and the Pacific. The most commonly grown species as a houseplant is Microsorum punctatum and the distinctive Microsorum musifolium 'Crocodyllus', whose broad, glossy fronds have a deeply textured, crocodile-skin surface that makes it one of the most unusual-looking ferns available. The common name 'kangaroo fern' is applied to Microsorum diversifolium from Australia, which has deeply lobed fronds on a creeping rhizome and is one of the hardier species, tolerating cooler conditions than most microsorum.

How do I care for microsorum in winter?

In winter, microsorum requires reduced watering (as growth slows and moisture evaporates more slowly), protection from cold draughts and low temperatures, and higher humidity than typical centrally heated UK rooms provide. Move microsorum to a naturally humid room such as a bathroom if central heating is causing low-humidity problems, or use a pebble tray with water beneath the pot. Continue to check the potting mix regularly and water when the top centimetre feels barely moist, but do not allow it to dry out completely. Keep microsorum in temperatures above 10 degrees Celsius in winter; most species prefer a minimum of 12 to 15 degrees.

Why are the tips of my microsorum fronds turning brown?

Brown frond tips in microsorum are typically caused by low humidity, underwatering, or fluoride or salt build-up in the potting mix. Low humidity causes the tip cells to desiccate and die first, as they are the last to receive water from the roots and the most exposed to dry air. Fluoride sensitivity is common in many ferns including microsorum: fluoride in tap water accumulates in the potting mix and causes tip burn. Using filtered water or leaving tap water to stand overnight before use can reduce fluoride damage. Flushing the potting mix thoroughly with water every few months removes accumulated salts.