The giant-rhubarb (Gunnera manicata) with its huge leaves is an absolute eye-catcher in the garden. Especially the oasis of calm at the garden pond is a great way to put the spotlight on it. The plant is easy to grow, so that you will soon enjoy this perennial herbaceous plant.
Even though the plant is very easy to care for, you should still avoid some mistakes. These include too much planting density, too shallow a planting depth and insufficient watering. If you follow our tips, nothing will stand in the way of successful cultivation.
Profile of giant-rhubarb:
Scientific name: Gunnera manicata
Plant family: Gunneraceae family
Other names: Brazilian giant-rhubarb
Sowing time: autumn
Planting time: spring, after frost
Flowering period: July to August
Location: sunny (but no direct sun) to partially shady
Soil quality: stony to sandy, nutrient rich, humus rich
These information are for temperate climate!
Use in: single position, pond planting, Mediterranean garden, natural garden, park, water garden
Winter hardiness: hardy, USDA Plant Hardiness Zones: 8 (-10 °C / +15 °F)
Bee and insect friendly: Yes
Plant characteristics and classification of giant-rhubarb
Plant order, origin and occurrence of giant-rhubarb
The giant-rhubarb (Gunnera manicata), also called Brazilian giant-rhubarb, belongs to the Gunneraceae family and is one of the largest perennials that can be cultivated in gardens. The giant-rhubarb is originally native to the cool and humid regions of southern Brazil, where it occurs in moorland and swampland, along rocky streams or on the edge of water bodies. Meanwhile, however, one finds giant-rhubarb also more and more frequently wildly growing in the English county Devon. The name “giant rhubarb” comes from the fact that the foliage of the two plants actually resembles each other at first sight – from a botanical point of view, however, the two are not related in any way.
Characteristics of giant-rhubarb
The giant-rhubarb develops large clusters from strong rhizomes. Huge leaves, which really do credit to the name “giant-rhubarb”, sit on yellowish-green stems up to 250 centimeters (100 in) long, which are covered with soft light red spines.
The leaves of the giant-rhubarb are round to kidney-shaped and have a lobed, irregularly serrated edge. The diameter of a single leaf can be up to 200 centimeters (80 in).
The flowering period is from July to August. Then the giant-rhubarb shows impressive reddish flower flasks, 120 centimeters (48 in) in size, initially upright and later almost horizontal, which consist of many inconspicuous individual flowers. In their overall shape they are arranged conically to narrowly ovate.
After self-pollination, giant-rhubarb produces fleshy fruits that contain the seeds.
Giant-rhubarb – cultivation and care
Partially shady in damp soil is the ideal location for the giant-rhubarb. The plant tolerates sun well, but not permanently. The swamps and cloud forests from which the plant originates rarely offer full sun. A lot of space is also important. Due to the size of the leaves, other plants can be overshadowed and can hardly receive any nutrients. Put the root well deep into the soil and make sure that the ground is large enough to solidify and provide sufficient support for the large plant.
The perennial can also be grown in a large flower pot. In this case, however, it will not reach such an impressive size as in a spacious garden location. It should be kept in a pot with sufficient moisture and placed in a partially shady to sunny spot on the porch.
- do not place it in the blazing sun, partially shady is appropriate
- must not be placed in a permanently shady place
- keep at least one and a half to two meters distance to other plants
As a forest and marsh plant, the giant-rhubarb absolutely needs moist soil. It can be planted well near garden ponds or water ditches, but must not be planted directly into the water. The soil should be nutritious, fertilization should be done with an acidic fertilizer, mulching can also be helpful to bring the leaves to their full beauty.
The giant-rhubarb forms a symbiosis with blue-green algae, so-called cyanobacteria, which settle at the base of the leaf stem. They produce the nitrogen necessary for the plant and in return are supplied with carbon. These algae should not be fought, they are important for the growth of the plant. Due to the large leaf surface, a lot of moisture evaporates, so it is useful on warm days to daily check if there is enough moisture.
Usually, the giant-rhubarb is obtained as an offshoot of an existing perennial. It should be planted in April or May when there is no longer any fear of frost. The planting depth should be at least half a meter, rather a little more. Water with plenty of water. Whoever has pulled young plants from the seed should make sure that the soil is not too wet, otherwise there is a risk of mold.
After flowering in autumn, the leaves should be cut off, they die off and do not form a beautiful appearance. Despite their size, the leaves are extremely stable. The leaves themselves are leathery and very firm, they have stabilizing ribs on the underside. This means that they are not susceptible to damage even in heavy rain and hail. Due to this stability, they are ideal for protecting the rootstock in winter.
Care / Watering / Fertilization
Giant-rhubarb evaporates large amounts of water through its huge leaves, so it needs to be watered abundantly during dry periods or when it is very hot. In autumn, when the leaves fall, the stems are also cut back. Fertilizer or some compost will help the perennial to bud in spring. If the soil is too dry, mulching helps to keep the moisture in the soil longer.
The giant-rhubarb can be propagated simply by dividing an existing plant. This is done by dividing the mother plant in spring. This can be done simply by digging out and cutting the perennial, then putting the mother plant back in place. The shoot should have at least one existing leaf node. It is recommended to grow the young plant in a pot for a few days. With moist garden or pond soil the shoot usually develops quite well and can be put in its final place after about 2 to 3 weeks.
Propagation can also be done by seeds and is promising. To do this, remove the seeds from the inflorescence in autumn and remove the flesh, place in a flower pot with moist potting soil and cover only slightly. Cover with a hood or foil to ensure high humidity.
- the soil must not dry out, even a single drying phase can prevent germination
- use only mature seeds
- fresh seeds germinate within a few days, dry seeds may take several weeks
- germination can be completely absent with dry seeds
- if condensation forms in the hood or foil, ventilation is required to regulate the humidity
Diseases and pests
Nothing is known about diseases on the giant-rhubarb, but care should be taken when watering to prevent rot. Voles or snails can appear as pests. Snails in particular find the large leaves and inflorescence a special treat, so regular checks should be made to ensure that there is no infestation. Also, before covering in autumn, it must be checked whether snails may have settled down. They could permanently damage the plant.
Voles can be a plague in any garden, if they eat the roots, the growth can be impaired. Other rodents could also select the giant-rhubarb in search of winter quarters and nibble the root system. In such cases a wire mesh helps to prevent infestation. However, the wire should be placed in a wide area so as not to hinder the spread of the roots.
The giant-rhubarb is not winter-hardy without sufficient protection. After flowering the leaves must be cut off, also the stems should be cut back to strengthen the plant. The above-ground parts all die before winter, but timely pruning prevents too many nutrients from being consumed. Since the stems can become up to 10 cm thick, the cutting tool should be sufficiently large and sharp. The dry leaves as a whole can be used well to cover the perennial. Everything should be done by November at the latest. The root perennial itself is not very sensitive, but it can die in permanent frost, so covering it is absolutely necessary.
It is essential to place the cut leaves on top of the perennial plant with the stems pointing upwards and weigh it down so that nothing can blow away even in an autumn storm. The winter protection should be removed gradually in spring so that the plant can get used to the changed temperatures. Only remove everything after the last frost. If a frost is imminent, the giant-rhubarb must be covered again.
- Thoroughly clean the ground before covering, leaves or dead plant parts could lead to rotting
- Pests such as snails should be removed by all means.
- Brushwood or coconut matting are also ideal for covering
- pay attention to air cushions when covering, they are important so that the plant can still generate enough heat of its own
Use in the garden
Due to its imposing size and attractive leaves, the giant-rhubarb is particularly effective in individual positions in open spaces. It is usually planted in large gardens as a solitaire on extensive lawns. In small gardens, however, giant-rhubarb can also be used as a design element and provide structure in the garden instead of a woody plant. Very typical is a planting place at the edge of a pond, close to watering places or along streams. Either way, the giant-rhubarb brings an exotic flair to any garden and also gives Mediterranean gardens that certain something.
Varieties of Gunnera manicata are not known.
Frequently asked questions
Is the giant-rhubarb poisonous?
No. However, the taste is bitter and sour in most species, so that consumption is hardly an option.
Does the Gunnera cause allergies?
Not as far as is known.
Can the giant-rhubarb be put directly into a pond?
No. Although the plant requires a lot of water, it is not an aquatic plant and would not survive a winter directly in water.
Why does the giant-rhubarb not bloom?
There may not be enough nutrients or the soil may not be acidic enough. It is possible that the roots have been eaten by rodents. It is advisable to check the soil, fertilize sufficiently and possibly put up a wire fence against rodents.