Drosera filiformis

Leaves like red threads

Drosera filiformis looks unlike most sundews. Its leaves are upright threads 8 to 25 cm long, occasionally 30 cm, and only about 1 mm wide, with no separate leaf stalk. They are covered in red to reddish purple glandular hairs that dry dark brown. The leaf bases swell into a woolly, bulb-like base 1 to 2 cm long, and in winter the plant retreats to a bud (hibernaculum) ^[2]. From late May to September it sends up stalks 6 to 26 cm tall with 4 to 12 (up to 24) pink to purple-lavender flowers 10 to 20 mm across ^[2]. Barry Rice notes that the only other sundew even remotely similar in form is the South American D. graminifolia ^[3].

One species or two

The plant most likely to be confused with it is the Gulf Coast thread-leaf sundew, D. tracyi. The two differ mainly in size and color ^{[2, 9, 3]}:

Botanists have argued for a century about whether these are one species or two. Rafinesque named D. filiformis in 1808. In 1906 Diels treated the larger, greener Gulf Coast plants as a variety, and Macfarlane published them as the species D. tracyi in 1914. Floristic works and databases such as Flora of North America and NatureServe now treat them as two species, while carnivorous plant growers have tended to keep one. The two have identical seeds and form fertile hybrids, including in the wild, but Rice, after reviewing the evidence in 2011, concluded that differences in leaf size, flower size, color and leaf proportions justify two species ^[3]. ICPS notes that many plants grown as D. tracyi may have some D. filiformis in their parentage ^[7]. One practical difference: D. tracyi and most of the hybrids have longer styles and self-pollinate less readily than D. filiformis ^[10].

The Florida panhandle plants are smaller and redder than those of the Atlantic coast ^[7]. Barry Rice named them D. filiformis var. floridana in 2017, in the second volume of Drosera of the World ^{[10, 6]}, separating them by uniformly orange-red to bright purplish red leaves, shorter flower stalks and smaller flowers ^[6]. The ICPS Seed Bank has distributed their seed as “Florida all red”, “Florida red” or simply “Florida” ^[7].

Four separate ranges

In 2011 Rice described the species’ range as four widely separated segments ^[3]:

• Nova Scotia: discovered in Shelburne County in 1977, known from only five locations, and at least 4,240 years old there according to peat studies. Its presence stopped one bog from becoming a peat mine.
• Mid-Atlantic: Massachusetts to New Jersey, with a Maryland site of uncertain native status. In New York it occurs only in Suffolk County on Long Island; it has vanished from many former Massachusetts sites, the single Rhode Island population died out around 1990, and the Connecticut sites are probably gone.
• North Carolina: reduced to as few as nine populations.
• Florida: a cluster of sites about 20 km across in Bay and Washington counties.

In these places it grows on wet sand along pond shores, in seepage bogs and fens, between dunes, in coastal peat bogs, Carolina bays and even roadside depressions and borrow pits, from sea level to 40 m ^{[2, 4]}.

Horticulturists have planted it outside its range. Flora of North America treats the Virginia and West Virginia sites as introduced, and NatureServe treats Maryland, Pennsylvania and Virginia as non-native ^{[2, 4]}; WCVP still includes Pennsylvania and West Virginia in the native range ^[1]. IUCN has assessed the species for the Red List; see its Red List assessment ^[5]. NatureServe counts about 250 occurrences and lists threats from development, forestry, peat mining, off-road vehicles, fire suppression and overgrowth by shrubs and invasive plants. Canada lists it as Endangered under the Species at Risk Act ^[4].

Growing the northern and Florida forms

Flora of North America calls it very easy to cultivate ^[2], and ICPS agrees as long as you match the form to your climate. Atlantic coast plants need a real winter. ICPS suggests growing them outdoors all year within their natural range of USDA zones 6 and 7, moving them to shelter after they go dormant in colder areas, and protecting them from late spring frosts. In zones 8 and 9 they should do well but may go dormant too early, and in zone 9 and warmer ICPS recommends growing the Florida variety outdoors instead ^[7]. A few selected Florida clones survive outdoors in zone 7, but most will not survive a zone 8 winter outside ^[7]. In a 2018 study of clones grown outdoors in Ashland, Oregon, John Brittnacher found some Florida clones did not go dormant enough to face frost, while one survived outside all winter without protection ^[10]. See the dormancy guide.

For a terrarium, ICPS recommends the Florida variety: its seed germinates without cold treatment, some plants never go dormant, and most make only loose buds for about a month. Feeding more makes dormancy less likely, although it may not color up as it would outdoors ^[7].

ICPS grows the plants in large, tall pots of 50:50 peat and sand, always standing in water. The buds can rot in winter if the soil surface stays too wet, so add extra sand to the top 1 cm or a layer of pure sand as deep as the buds ^[7]. Moss is the other long-term problem: large species such as this can usually go two years between repottings in early spring, and a pot left a year too long produced smaller buds under a 2 cm mat of moss ^[11]. See repotting. If the plants catch few insects, ICPS suggests rehydrated dried bloodworms or a dilute liquid foliar fertilizer ^[7].

Propagation and hybrids

Clumps can be divided while dormant ^[2]. For leaf cuttings, the long leaves can be cut into shorter segments, and D. filiformis is one of the species that can sprout quickly in the second month ^[12]; see sundew leaf cuttings. Seed of Atlantic coast plants needs about four weeks of damp cold, in the refrigerator or outdoors in winter. ICPS prefers sowing outdoors in winter so the seedlings stay in step with the seasons, with the seed left on the surface of a 50:50 peat and sand mix ^[7]. See cold stratification.

Hybrids with D. tracyi are easy to make when both bloom together, and ICPS says they are usually easier to keep than either parent ^[7]. ‘California Sunset’ came from a 1973 cross and has leaves up to 0.5 m long with light red to deep pink tentacles and rose pink flowers ^[13]. The natural hybrid with D. intermedia, D. ×hybrida, is sterile and can only be propagated by leaf cuttings ^[12]. To compare it with other northern sundews, see the round-leaved sundew and the genus overview.