Carex filiformis

thread sedge

Cyperaceae

Europe and northern Asia (British Isles east through Russia to Siberia)

Deer Resistant

SunFull Sun – Part Shade

WaterHigh

HardinessZones 3–8

Overview

Carex filiformis (often treated as a synonym of C. lasiocarpa in North American taxonomy) is a rhizomatous perennial sedge in the Cyperaceae family, reaching 18-36 inches (45-90 cm) tall and spreading by slender, far-creeping rhizomes to form loose colonies of indefinite width. Stems are slender, triangular, smooth below and slightly rough above. Leaves are narrow, 1-2 mm wide, grey-green, involute (rolled inward), wiry, and often exceed the stems in length. The inflorescence consists of 2-4 terminal spikes: 1-2 slender male spikes 1-2 inches (2.5-5 cm) long at the top, and 1-2 female spikes below, 0.5-1.5 inches (1.5-4 cm) long, cylindrical, with densely packed utricles. Utricles are 3-4 mm long, ovoid, densely pubescent (hairy) — the downy fruit surface is a primary diagnostic character separating this species from related Carex spp. taxa with glabrous utricles. The species is characteristic of fens, bogs, lake margins, and alkaline wetlands, where it often grows in shallow water or on floating mats of peat. In the Pacific Northwest, the closely related C. lasiocarpa is native in similar habitats. Plants tolerate calcareous and mildly acidic conditions but do not tolerate drought — permanent moisture or standing water is required.

Native Range

Carex filiformis is native across Europe, from the British Isles and Scandinavia south to central France and east through Russia to Siberia, where the species occurs in base-rich fens, transition mires, lake margins, and alkaline wetlands. The closely related C. lasiocarpa extends the range into North America.

Suggested Uses

Planted in constructed wetlands, fen restoration, bog gardens, and pond margins where permanent moisture is available. The species stabilises the margins of ponds and slow-flowing streams. Used in ecological restoration of calcareous fen habitats. Not suited to garden borders or sites that dry out in summer.

How to Identify

Identified by the very narrow (1-2 mm), grey-green, involute (rolled-inward) wiry leaves, slender creeping rhizomes, and densely hairy (pubescent) utricles on the female spikes. Separated from C. lasiocarpa by subtle differences in utricle beak length and geographic range — some authors treat the two as conspecific. Separated from C. rostrata by the much narrower leaves and pubescent (not glabrous) utricles.

Appearance

Size & Dimensions

Height1'6" - 3'

Width/Spread1' - 3'

Reaches mature size in approximately 3 years

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~3 weeks

Spring Summer

Flowering occurs from May through June. Male spikes at the stem apex release pollen by wind over a 2-3 week period. Female spikes develop below, with utricles ripening to light brown by July-August. Seed dispersal is by water and wind.

Detailed Descriptions

Flower Description

Brown (male spikes), light brown utricles (female)

Foliage Description

Grey-green

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade

Requires 4-10 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.5 - 7.5(Neutral)

357912

Soil Types

peat silt loam

Drainagewet

Water & Climate

Water Needs

High

Frost Tolerance

hardy

Time to Maturity

2-3 years

Care & Maintenance

Care Guide

Plants grow in full sun to partial shade in permanently wet to waterlogged, base-rich soil with a pH of 5.5-7.5. Plants are spaced 12-18 inches (30-45 cm) apart. The species tolerates standing water to 4 inches (10 cm) deep. In constructed wetlands, planting at the waterline or in shallow water matches the species' natural conditions. No fertilising is needed — the species is adapted to nutrient-poor fen conditions. The far-creeping rhizomes can colonise adjacent wet areas, so spread is monitored where containment matters.

Pruning

Dead foliage is cut back in late winter before new growth begins. No other pruning is required. Colonies are thinned by removing sections of rhizome with a spade where spread exceeds the desired area.

Pruning Schedule

early spring