Juncus filiformis (Thread Rush)

Plant Info
Also known as:
Genus:Juncus
Family:Juncaceae (Rush)
Life cycle:perennial
Origin:native
Habitat:sun; moist to wet sandy or peaty soil; shores, stream and river banks, wet meadows, wet ditches
Fruiting season:June - July
Plant height:8 to 30 inches
Wetland Indicator Status:GP: OBL MW: FACW NCNE: FACW
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: 6-petals Cluster type: panicle

[photo of primary bract] Branching, somewhat compact cluster at the top of the stem, the branches ascending to spreading with 3 to 12 flowers. The cluster is subtended by an erect bract that is round in cross-section and appears to be a continuation of the stem, with the cluster erupting from the side of the stem. The bract is about as long as or longer than the stem.

[photo of budding flower cluster] Flowers are single, not in heads of 2 or more, on stalks of varying lengths, with 6 tepals (petals and similar sepals) in 2 layers, 2.5 to 4.2 mm long, the inner and outer tepals about equal in shape, sharply pointed at the tip, and the inner tepals slightly shorter than the outer. Tepals are light brown to green with translucent, white papery edging. Flowers have a 3-parted style and 6 yellow stamens, the anthers (tips) about as long as the filament (stalk) or shorter.

Leaves and stems: Leaf attachment: basal Leaf type: simple

[photo of lower stem and sheath] A flowering stem has no leaves to speak of, just one to several bladeless sheaths at the base. Sheaths are light brown at the base and green to brown above. Stems are medium green, round in cross-section, smooth, erect, unbranched, and create colonies from creeping rhizomes.

Fruit: Fruit type: capsule/pod

[photo of fruit] The tepals persist and become light brown and slightly spreading in fruit. Fruit is an oval to nearly round, 3-chambered capsule 2.5 to 3.2 mm long, greenish brown to tan when mature, as long as or slightly longer than the tepals, rounded at the tip, sometimes with a minute beak at the top. Inside the capsule are numerous tiny seeds, about .5mm long, amber colored when mature, a wrinkled texture across the surface, and with no elongated tails.

Notes:

Juncus filiformis is a circumboreal species, native to northern latitudes and upper altitudes of North America as well as parts of Europe and Asia. There are 3 Juncus species in Minnesota that have a lateral cluster with the bract appearing to be a continuation of the stem. Of the other two, both Juncus effusus and Juncus arcticus have a much shorter bract, not more that 1/3 as long as the stem. Also, Juncus effusus is strongly clump-forming and has more numerous flowers, and Juncus arcticus has tepals that are often dark purplish brown with a green midrib and white edging, and the stems often arise in straight lines from the rhizome.

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More photos

Photos by K. Chayka and Peter M. Dziuk taken in Cook County.

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