Plant of the Month April 2024
GENERAL INFORMATION:
Spring started early this year with some record high temperatures in Toronto, followed by severe frosts. Luckily most of our bulbs just fall flat and then revive. I go round the garden looking for emerging shoots. Small reddish leaf tips and buds mark the start of Jeffersonia dubia in the second wave of spring bulbs.
Jeffersonia dubia 16.03.24.
Synonym: Plagiorhegma dubium is the correct taxonomical name.
Common Name: Japanese Twin-leaf.
Cultivars: Jeffersonia dubia ‘Variegata’, Jeffersonia dubia ‘Dark Centers’, Jeffersonia dubia alba, Jeffersonia dubia ‘Flore-Pleno’.
Life Cycle: Rhizomatous herbaceous perennial.
Height: 20-30 cm.
Bloom Time: April to early May in the Toronto area. 
Jeffersonia dubia - early April.
Jeffersonia dubia - mid April.
Jeffersonia dubia - early May (Lilactree Farm).
Flower Colour & Size: Lavender-blue, up-facing, cup shaped, 3-5 cm across. Flowering stem 8-10 cm high, growing to 25 cm after petal drop.
Jeffersonia dubia - leaves.
Leaves: Long stemmed, reddish purple leaves emerging with the flowers, then dull green. Almost circular, with lobed edges, slightly cleft into two.
Jeffersonia dubia - capsule splitting, showing seeds.
Fruit: Several seeded capsule, dehiscing in June. Will self seed.
Range: Native to Korea, China, Russia. Much cultivated in Japan.
Habitat: Deciduous woodland.
CULTIVATION:
Light: Semi-shade to full shade.
Soil: Humusy soil, well drained, near neutral pH.
Water: Keep slightly moist.
USDA Hardiness: Zones 4-7b.
Jeffersonia dubia - with Sanguinaria, Corydalis, Anemone and Scilla.
Companion planting: Other rock garden plants, early woodlanders, spring ephemerals: Sanguinaria canadensis, Corydalis, Anemone, dwarf Epimedium.
Propagation: From fresh seed: Sow at 20°C for 6 weeks, then place @ 4°C for 6 weeks. Slowly raise temperature to 10°C for 6 weeks. If there is no germination, repeat the cycle. Will self seed. Clumps may be carefully divided after flowering.
Native alternatives: Plagiorhegma (Jeffersonia) diphyllum. White flowered with leaves cleft into two. See POM Jeffersonia diphyllum.
Pollinators: Bees.
References:
https://www.plantdelights.com/products/jeffersonia-dubia
https://onrockgarden.com/index.php/germination-guide
Text and images supplied by Anna Leggatt (Toronto Master Gardener)