Drosera rotundifolia

Round blades on long stalks

Drosera rotundifolia forms a flat or slightly raised rosette, usually 4 to 10 cm across. Each leaf has a flat stalk 1.5 to 5 cm long ending in a nearly round blade 4 to 10 mm long and 5 to 12 mm wide, broader than it is long ^[2]. A blade carries roughly 200 reddish tentacles, 1 to 5 mm long and longest at the edge ^[3]. From June to September the plant sends up leafless flower stalks 5 to 35 cm tall, with 2 to 15 small white or pink flowers whose petals are 5 to 6 mm long ^{[2, 3]}.

In North America it often grows with other sundews, and the Flora of North America key separates them by the leaf blade ^[9]:

• D. rotundifolia: blade nearly round and broader than long; seeds finely striped.
• D. anglica: blade obovate to long spoon-shaped, clearly longer than broad.
• D. intermedia: blade longer than broad on a thread-like, hairless stalk, stipules free of the petiole, and a plant that develops a leafy stem 1 to 8 cm (rarely 20 cm) tall.

Natural hybrids are rare and usually sterile, but this species is a parent of several: D. ×obovata with D. anglica, D. ×beleziana with D. intermedia, and D. ×woodii with D. linearis ^[9]. ICPS adds that a cross between D. linearis and D. rotundifolia gave rise to D. anglica itself, and that clones of D. ×beleziana are usually larger and more vigorous than either parent ^[8].

Darwin’s sundew

This is the carnivorous plant Charles Darwin studied most, and it fills most of his 1875 book Insectivorous Plants ^{[2, 10]}. Darwin reported that the touch of a small part of an insect on a single tentacle can start the tentacles moving ^[9]. He also noticed that a drop of water does not make them bend. A 2023 study on a related species traced that to the jasmonate signaling pathway, which water drops do not switch on ^[11]. The cultivar ‘Charles Darwin’, bred by Ivan Snyder from two Californian forms of this species and registered with ICPS, is named after him ^[10].

Life on a sphagnum hummock

Baranyai and Joosten call it probably one of the most widely distributed carnivorous plants. It spans the boreal and temperate zones of Europe, Asia and North America and is scattered through the mountains of the Mediterranean ^[3]. Flora of North America describes it as much more common in the north and rarer in the south, from sea level to 3000 m ^[2]. In Europe it reaches 2,000 m in Austria, France and Italy and 2,100 m in Spain ^[3].

Its main habitats are acidic bogs and poor fens dominated by sphagnum moss. Among European sundews it grows highest on the bog surface, on sphagnum hummocks, where its stem lengthens fast enough to keep its leaves above the growing moss. It avoids places with permanent standing water, and extra nitrogen and phosphorus make it smaller, with fewer leaves and flowers ^[3]. In North America it also colonizes beaver ponds, wet sand in old sand pits, sandy shorelines and seeps ^[2].

The flowers open only in direct sun, for two or three hours on a single day. The first flowers are often cleistogamous, staying closed and pollinating themselves, and in the wild most seed comes from self-pollination. Capsules in the populations studied held on average about 70 to 90 seeds, which can travel up to 10 km on the wind or float on water for months, and stay viable for one to five years ^[3].

Picked, protected and declining

Round-leaved sundew has been gathered from the wild since medieval times as a herbal cough remedy, traded as Droserae herba. The destruction and eutrophication of mires have shrunk European populations. The plant has been protected in most European countries since the 1980s, which made wild plants largely unavailable to the pharmaceutical industry, although wild harvest continues in Finland, where pickers are asked to leave 5 to 10 flowering plants per square meter ^[3]. France protects all its native sundews, and Germany bans commercial collection ^[3]. German law lists all native sundews as specially protected species, covering wild populations ^[6].

Globally the picture is better: NatureServe ranks it secure (G5) ^[5]. IUCN has also assessed it; see its Red List assessment ^[4]. It is nonetheless ranked critically imperiled (S1) in seven US states, including Alabama, Kentucky and Tennessee ^[5]. Buy nursery-propagated plants rather than collecting.

Growing it means giving it a winter

ICPS advises that if you live within its natural range, grow plants from your own climate zone outdoors all year. It expects a definite winter and spends about six months of the year as a hibernaculum, a tight resting bud, which makes it tricky in a greenhouse or a terrarium ^[8]. During that rest, keep the bud cool and just damp ^[8]. Selections from coastal northern California, southern Europe and possibly Japan may cope better with mild climates, and an Austrian form pictured by ICPS tolerates warmer temperatures than the Californian plants it was compared with ^[8]. ‘Charles Darwin’ grows continuously in a terrarium, like the Hawaiian form of D. anglica, but still makes a winter bud outdoors ^[10]. See the dormancy guide.

In the growing season the pot should always stand in water, and ICPS suggests flooding it from time to time in hot weather. A 50:50 peat and sand mix works; live sphagnum, though natural, can overgrow the plants in a pot ^[8]. Use low-mineral water: one review of indoor culture recommends water of 50 to 100 µS/cm conductivity ^[3]. Water for carnivorous plants explains how to check yours.

Sources differ on light. ICPS puts most sundews at 15,000 to 25,000 lux under lamps ^[7], while the indoor-cultivation literature reviewed by Baranyai and Joosten gives 10,000 to 15,000 lux in summer ^[3]. Outdoors, the ICPS author found D. rotundifolia did better partly shaded in Sarracenia pots than in its own pots beside them ^[12]. One option, then, is to grow it in the same pot as a Sarracenia such as the purple pitcher plant, or in a bog garden.

Moss is the slow killer in pots. The ICPS author repots small temperate sundews every spring, just as the buds open, and sets the hibernacula so their bases sit in the soil under a thin layer of sand ^{[12, 8]}; the repotting guide covers the steps.

Seed and cuttings

Seed needs a winter before it will sprout. ICPS suggests sowing in autumn and leaving the pots outdoors, or giving seed four weeks in damp, chopped sphagnum at 1 to 4 °C in the refrigerator during your own winter, then spreading it on the surface of the mix ^[8]. Baranyai and Joosten add that the seed also needs light and 20 to 25 °C. On peat it usually sprouts within 20 to 30 days, and germination drops sharply if seed is buried deeper than 5 mm ^[3]. See cold stratification.

The plant also buds naturally from its leaves, mostly old leaves that have partly or fully come away from the stem. Buds usually form in the middle of the blade and take 14 to 30 days to develop ^[3]. That makes leaf cuttings a practical way to copy a good clone.