Plants / Frosty fern
Frosty fern care: keep Selaginella martensii humid, not waterlogged
Check the plant’s identity first, then separate its humid air requirement from the water around its roots.
By Terrarium GuideSources & editorial method
Botanical identity: Selaginella martensii
The short answer
This Frosty fern page covers Selaginella martensii, a spikemoss rather than a true fern. It needs a humid position, partial shade, and moist, well-drained medium. Keep the potted roots above any water collected in a tray or glass display.
Missouri Extension describes about 65–70°F (18–21°C) and at least 70% relative humidity for its indoor example. Measure at the plant and keep maintenance access. These source recommendations do not identify every retail Frosty fern or establish a permanent sealed-jar result.
Retain humidity while keeping roots inspectable
| Arrangement | How to use it | Check before choosing |
|---|---|---|
| Pot inside an openable humid enclosure | Retain humidity around foliage; lift the pot for watering and inspection. | Roots must remain above collected water, with room for the expanding plant. |
| Ordinary dry heated room | Measure the actual air conditions before buying. | The humid-air requirement may be difficult to meet without an enclosure or room humidification. |
| Small decorative holiday jar | Treat the nursery specimen as a growing plant. | It may be sold at 4–6 in. tall but reach 10–12 in.; allow removal and later division. |
| Wet base with immersed roots | Change the setup. | A moisture reservoir is not permission to leave the rootball standing in it. |
Scope and important limits
A useful fit when
- Choosing, planting, and maintaining the named plant in the conditions described here.
Use another path when
- Transferring these conditions to an unidentified retail plant or assuming a permanent sealed display.
Published horticultural guidance and its application to display care; no Terrarium Guide growing trial is claimed.
Confirm what the Frosty fern label refers to
The name is used here for the S. martensii plant described by Missouri Extension. Ask a seller for the botanical identity and cultivated-form label rather than identifying the plant from white tips alone. Selaginella kraussiana has its own profile and is not the substitute identity for this page.
S. martensii spreads through branching stems with upward-turning tips and can need a broader footprint as it develops. RHS gives a broad 0.1–0.5 m spread band; the Missouri article describes eventual height of 10–12 in. These different measures are planning context, not a single bottle-size promise.
Inspect beyond the decorative wrapping
- Ask for the scientific nameA seasonal Frosty fern tag does not verify the identity or establish a dwarf form.
- Inspect roots and mediumRemove decorative water traps or sleeves for the check. Avoid a specimen with soft growth or a rootball sitting in old water.
- Plan the next containerMeasure the current plant and space for upward tips. Choose an opening through which the whole pot can leave again.
- Check the retail identity before ordering Keep the identity and specimen checks above; compare seller information, arrival forms and delivery before ordering.
Use a reachable pot above collected water
Choose partial shade and a stable indoor position
Avoid cold drafts, heating blasts, and direct sun on glass. Measure temperature and humidity near the foliage.
Prepare moist, well-drained medium
RHS describes two parts peat-free loam-based potting compost and one part leafmould for S. martensii under glass. Use potting compost rather than dense garden soil; dampen and loosen it before placing the existing rootball. Keep the original planting level and green stems exposed. Do not pack the mix or use a reservoir as the root zone.
Position the pot for lifting
If water is retained below pebbles or a support, keep its surface below the pot’s roots. Test this after watering rather than assuming the starting level will stay safe.
Water the medium and inspect foliage
Water the medium gently until a removable pot drains, then empty surplus before returning it. In a shared bed, add small amounts while watching the rooted layer and stop before water pools. Keep roots moist and above collected water; inspect again before adding more. Continually wet foliage is not the target.
- Prepare the vessel and root arrangement Choose the whole display around this form’s accessible clump and root requirements.
Use humidity measurements; feed sparingly
Check root moisture and surrounding air separately. An enclosure can retain moisture, yet roots still need drainage. Lift the pot to inspect whenever the medium remains saturated or growth starts to soften. The Missouri example supports an enclosure for humidity; it does not supply a universal opening schedule.
The same article describes very weak monthly feed, about one-eighth of its referenced indoor-product recommendation, and reports loss of white tips with excessive nitrogen. Product strengths vary: do not pour a full houseplant dose into a small wet display. Check the plant’s identity, new growth, and feeding history before treating greener tips as disease.
Evidence[1]
Divide a crowded clump instead of burying its tips
Lift the plant where you can see the rootball
Work outside the display and support the stems so separation does not pull on neighboring plants.
Separate a healthy rooted portion
The Missouri guidance uses division when crowded. Repot the portion into moist, well-drained medium at its existing level and remove loose dead material.
Return only what the space can support
Retest headroom and the route for watering. A healthy smaller division is a new planting decision, not a reason to leave the rest crammed under a lid.
Use cuttings separately if propagating
The article also describes placing a stem end in moist medium under a clear bag. Check moisture and health while rooting; its reported rooting time is an example, not a guaranteed outcome in your conditions.
Match the action to what you observe
| Observation | Check | Action |
|---|---|---|
| Crisp tips or drying foliage | Root moisture and humidity at the plant, not across the room. | Correct drying or inadequate humidity while preserving root drainage. |
| Soft wet growth | Root immersion, standing water, and persistently wet leaves. | Lift for inspection, remove dead tissue, and restore drainage. |
| White tips become green | Identity, new growth, and nitrogen history. | Reduce unnecessary feeding; do not apply a disease treatment from color alone. |
| Clump reaches the enclosure | Whether the whole plant can still be lifted out. | Divide, repot, or move it before access is lost. |
- Continue checking soft or declining growth Retain the white-tip and feeding checks here; use the decline guide for additional physical checks.
Compare conditions and companions
Explore more in plants
Sources and review
- Frosty Ferns Warm Your Home Interior · University of Missouri
- Selaginella martensii — RHS Plant Profile · Royal Horticultural Society
- Flora of New Zealand — Selaginella martensii Taxon Profile · Manaaki Whenua — Landcare Research; Flora of New Zealand
- Selaginella martensii — Flora & Fauna Web · National Parks Board, Singapore
- WFO Plant List 2026-06 · World Flora Online
- How to Create and Care for a Terrarium · Iowa State University Extension and Outreach