Water Arum
Calla palustris
Water Arum (Calla palustris) is a perennial aquatic or semi-aquatic herbaceous plant belonging to the family Araceae. It is the sole species of the genus Calla found in the wild in the Northern Hemisphere, distinguished from the more commonly known 'calla lily' (Zantedeschia), which belongs to a different genus within the same family.
• Native to cool temperate regions of Europe, Asia, and North America
• Grows in shallow water, bogs, fens, and along the margins of ponds and slow-moving streams
• Produces a distinctive white spathe surrounding a yellow spadix, typical of the Araceae family
• After flowering, develops a striking cluster of bright red berries that rise above the foliage
• Deciduous in winter, dying back to a submerged rhizome
Taxonomy
• In Europe: ranges from France and Scandinavia eastward through Russia
• In Asia: extends through Siberia to parts of northern Japan
• In North America: found from Newfoundland and Labrador west to Manitoba, and south to Virginia and Minnesota
• Fossil evidence suggests the genus Calla had a broader distribution during the Tertiary period, with related fossil species found in regions where the genus no longer occurs
• The current disjunct distribution reflects post-glacial range contractions and the species' preference for cool, wet habitats
Rhizome & Roots:
• Thick, creeping, fleshy rhizome that grows horizontally in mud or shallow water (up to ~15 cm deep)
• Rhizome is cylindrical, 1–2 cm in diameter, with prominent leaf scars
• Produces adventitious roots at nodes that anchor the plant in soft substrate
Leaves:
• Basal, arising directly from the rhizome on long petioles
• Petioles are long (10–30 cm), allowing leaves to reach the water surface or rise above it
• Leaf blade is cordate (heart-shaped), 6–12 cm long and 4–11 cm wide
• Texture is thick, glossy, and leathery; margins are entire (smooth)
• Dark green on the upper surface, paler beneath
Inflorescence:
• Spathe is white, ovate to elliptic, 3–6 cm long, partially enclosing the spadix
• Spadix is shorter than the spathe, cylindrical, 2–3 cm long, densely covered with small yellow flowers
• Flowers are bisexual (perfect), containing both stamens and pistils — unusual among aroids, which often have unisexual flowers
• Blooms from May to July depending on latitude and altitude
Fruit:
• After pollination, the spadix develops into a dense cluster of fleshy red berries
• Each berry is ~8–12 mm in diameter, containing several seeds
• The bright red fruiting spadix is highly conspicuous and persists into autumn
• Berries are buoyant, aiding in water dispersal
Habitat:
• Shallow standing or slow-moving water (typically 5–30 cm deep)
• Peat bogs, fens, marshes, swamp margins, and muddy pond edges
• Often found in partially shaded locations under alder (Alnus) or willow (Salus) canopies
• Prefers substrates rich in organic matter (peat or muck)
Water & Light Requirements:
• Requires consistently wet or submerged conditions; cannot tolerate desiccation
• Grows in full sun to partial shade; in warmer parts of its range, it benefits from afternoon shade
• Water temperature preference: cool, typically below 25°C
Pollination & Seed Dispersal:
• Pollinated primarily by small flies and other insects attracted to the spadix
• The spadix may generate mild thermogenesis (heat production), a trait common in Araceae, which helps volatilize scent compounds to attract pollinators
• Seeds are dispersed by water (hydrochory) due to their buoyancy
• Also potentially dispersed by birds and mammals that consume the berries
Associated Species:
• Often grows alongside Sphagnum mosses, sedges (Carex spp.), marsh marigold (Caltha palustris), and skunk cabbage (Symplocarpus foetidus)
• In North America, it shares habitat with its relative Lysichiton americanus (western skunk cabbage)
• Chewing or biting raw plant tissue causes intense burning, swelling, and irritation of the mouth, lips, tongue, and throat
• Ingestion can lead to nausea, vomiting, diarrhea, and difficulty swallowing
• The bright red berries, while visually attractive, are particularly dangerous as they may be mistaken for edible fruit, especially by children
• Despite its toxicity, the rhizome was historically rendered edible by thorough drying and cooking by some Indigenous peoples of North America, which breaks down the calcium oxalate crystals
• Not to be confused with Zantedeschia species (commonly sold as 'calla lilies'), which contain similar toxins
Light:
• Full sun to partial shade; in warmer zones (USDA 7 and above), partial shade is preferred to prevent overheating
Water:
• Plant in shallow water 5–15 cm deep, or in permanently saturated boggy soil
• Cannot tolerate drying out; substrate must remain waterlogged year-round
• Suitable for planting at the margins of garden ponds or in constructed bog gardens
Soil:
• Rich, organic, acidic to neutral substrate (pH 5.0–7.0)
• A mix of peat, loam, and sand works well
• In pond settings, plant in aquatic planting baskets filled with heavy loam
Temperature:
• Hardy in USDA zones 3–7
• Requires a cold winter dormancy period; not suited to tropical or warm subtropical climates
• Rhizomes can survive freezing when submerged or buried in saturated mud
Propagation:
• Division of rhizomes in early spring before active growth begins
• Seed propagation is possible but slow; seeds require a period of cold stratification (2–3 months at 2–4°C) to germinate
• Germination may take several weeks to months; plants grown from seed may take 3–5 years to reach flowering size
Common Problems:
• Generally pest- and disease-free in appropriate conditions
• May fail in warm climates where summer water temperatures exceed 25°C
• Slugs and snails may damage young leaves
• Insufficient water depth or drying of substrate is the most common cause of decline
Fun Fact
The name 'Calla' comes from the Greek word 'kallos,' meaning 'beauty' — yet for centuries, botanists confused this wild aquatic plant with the popular 'calla lily' sold in florists. • The familiar white 'calla lily' of the floral trade is actually Zantedeschia aethiopica, a South African species only distantly related to Calla palustris • The true genus Calla contains only one widely accepted wild species: C. palustris • Carl Linnaeus originally described Calla palustris in his landmark work Species Plantarum in 1753 Water Arum's fruiting strategy is a masterwork of evolutionary design: • The bright red berries are produced on an elevated spadix that rises above the waterline, making them highly visible to birds and mammals • The berries contain seeds that are buoyant and coated in a waxy substance, allowing them to float and disperse via water currents long after being dropped • This dual dispersal strategy — animal ingestion plus water flotation — maximizes the plant's ability to colonize new wetland habitats The plant's calcium oxalate crystals are a remarkable natural defense: • The raphides are stored in specialized cells called idioblasts • When plant tissue is chewed, the needle-like crystals are forcibly ejected from the cells under pressure, physically piercing soft tissues in the mouth and throat • This mechanical irritation is compounded by chemical irritants, creating a powerful deterrent against herbivory • The same defense mechanism is found in many other members of the Araceae family, including taro (Colocasia esculenta) and philodendrons
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