Members of the Gunnera genus are hardy perennials that can reach up to 1.8 metres in height, and are often wider.
They have very large leaves that are both lobed and toothed.
Though they bloom in summer and the start of autumn, it is the attractive combination of foliage and flowers that Gunnera plants are usually grown for.
The flowers grow to be cone shaped and may be red, yellow or green. A commonly grown member of the genus is Prickly Rhubarb.
Gunnera by saxifrage.
Common Names: Gunnera, Prickly Rhubarb, Giant Rhubarb.
Life Cycle: Hardy perennial.
Height: 100 inches (250 cm).
Native: Southern America.
Growing Region: Zones 6 to 9.
Flowers: Middle of summer to early autumn.
Flower Details: Red, yellow. Small. Cones. Massive.
Foliage: Herbaceous. Lobed. Toothed. Large leaves may reach over 9 feet (275 cm) in length. Succulent stalks.
Sow Outside: Cover seed. Following the last frost. Spacing 48 to 120 inches (120 to 300 cm).
Sow Inside: Germination time: two weeks to two months. Temperature: 60 to 70°F (16 to 21°C). Seven or eight weeks in advance. Transplant outdoors a few weeks after the last.
Requirements and care: Full sunlight or partial shade. Soil should be prepared beforehand so as to be deep. Boggy soil, Moist soil, rich soil. Regular watering during dry periods. Regular liquid feed. Leaves should be cut back to the ground in autumn. Then cover the crowns with a leaf mold. Propagate: by dividing in the spring.
Miscellaneous: Gunnera is considered to be an invasive species in many areas, especially parts of New Zealand.
When growing prickly rhubarb and other Gunnera plant species outdoors from seed sow just after the last frost; lightly cover the prickly rhubarb seeds once sown. They are very large plants; space normal Gunnera at 1.2 to 2 m apart; while gigantic Gunnera species need to be grown at least 3 metres apart. Plants are able to grow in both sunny and partially shaded parts of the garden. Due to their massive size it is important that Prickly rhubarb and similar plants have a very deep soil in which to grow; the soil should also be rich and moist.
If starting off indoors, sow Gunnera seeds about eight weeks in advance. The seeds should typically take from two to ten weeks to germinate at a temperature of 15 to 20 degrees centigrade.
As Gunnera plants such as Prickly & Giant Rhubarb are very large they require a lot of water and food: so water them frequently and supply regular feeds of liquid fertilizer. Once they have finished flowering in the autumn, cut off their leaves, and cover their crowns with a mulch of leaf mold. If you require more plants then propagate plants in the spring by division.
I hope that you enjoyed this guide on how to grow Gunnera plants. Updated September 2020.