Garden Rhubarb

Family

Polygonaceae

Habit

perennial

Height

1.5 m

Spread

1.2 m

Growth

fast

Lifespan

15 years

First yield

after 2 years

Hardiness

zone 3–8

Shade tolerance

light shade

Soil

Sand, Silt

Soil moisture

semi-dry – very moist

pH

5.5 – 7.5

Synonyms

Cultivated Rhubarb, Garden Rhubarb

Times apply to the northern hemisphere in a temperate climate and shift with altitude and cultivar.

Edible: Only the fleshy leaf stalks are used, harvested from April into June. Malic and citric acid make them markedly sour, so they are usually sweetened and made into compote, cake, jam, juice, syrup and wine; botanically a vegetable, it is treated culinarily as a fruit. Red-stalked cultivars contain less fruit acid and taste milder, green-stalked ones crop more heavily and taste stronger. Harvest by twisting and pulling rather than cutting, so no stub rots in the crown, and never take more than two thirds of the stalks at once. The leaves are not eaten. The rule of thumb is to stop harvesting by Midsummer Day on 24 June, so the plant can rebuild reserves before winter.

Other uses: The oxalic-acid-rich leaves are too useful to throw away. Traditionally they were used to brighten copper and aluminium vessels, because the acid dissolves oxide layers, and a decoction of them counted as a household remedy against aphids; the latter is no longer covered by pesticide law and should be dropped. Their size is what makes them worthwhile: as a mulch layer the leaves cover large areas and suppress weeds, and they rot down in the compost without difficulty. In the ornamental garden rhubarb is planted for its foliage, and the cream-white flower panicle, up to two metres tall, is a striking sight if for once it is left standing. The red pigment of the stalks is occasionally used to tint food.

Site: Rhubarb is a hungry feeder that stands in the same place for decades, so it belongs at the edge of the vegetable garden rather than in the rotation. A mature plant needs around a square metre and shades everything beneath it with its leaves, which is useful as a living mulch but obstructive next to lettuce and carrots. It likes sunny to half-shaded positions on deep, humus-rich, water-retentive soil at pH 5.6 to 7.2, but will not take waterlogging. A cold winter is a requirement, not a risk: without the cold stimulus it shoots only reluctantly in spring. The leaves are high in oxalic acid and unsuitable for small livestock and poultry, worth bearing in mind near pens and free-ranging hens. Care: Cover generously with rotted manure or compost in autumn, which replaces feeding; water deeply in spring dry spells, as its water demand is high. Break out flower stems the moment they appear, or the plant's strength goes into seed and the crop collapses. Harvest from the second season, not at all in the first, and always by twisting out. Stop harvesting at the end of June, after which the crown recovers. Divide the clump every four to six years, otherwise the centre ages out and the stalks thin. For an early crop, invert a bucket or forcing pot over the emerging shoots in February. Mosaic viruses and root eelworm occur; only healthy planting stock and a fresh site at replanting help against them.

Garden rhubarb is propagated only vegetatively, since it is a hybrid and does not come true from seed. The route is division of the rootstock in late autumn after it dies back, or in early spring before growth starts. Lift the clump and split it with a spade or a strong knife so that each piece keeps at least one strong bud and a decent section of fleshy rhizome; smaller divisions take two years to the first harvest. Let the cut surfaces dry off for a day so they do not rot, then plant with the bud sitting just below the soil surface and no deeper. Commercial growers additionally use in vitro culture to obtain virus-free starting material.

In the garden rhubarb rarely gets to flower, because the flower stems are broken out to protect the crop, and with that its main contribution to wildlife usually disappears. If an older plant is allowed to bloom for once, it throws a cream-white panicle one to two metres tall with thousands of tiny flowers, heavily worked by honeybees, hoverflies, beetles and short-tongued wild bees; as a cultivated plant with no native relatives, however, it has no specialised species tied to it. The large, oxalic-acid-rich leaves are avoided by slugs and deer, which makes rhubarb one of the few reliably browse-free garden plants. Under the dense canopy of leaves, ground beetles, spiders and amphibians find cool, moist cover through the summer.

Notable: botanically a vegetable, treated culinarily as fruit; the Midsummer Day harvest stop on 24 June is both folk rule + sound practice, as the crown must build winter reserves afterwards; modern cultivars are tetraploid (2n=44), the wild species diploid; forced rhubarb grows so fast in the dark that you can hear it crack; a clump crops for 10-20 years.

A cultivated plant with no wild range, grown worldwide in cool temperate climates. Garden rhubarb is not a wild species but a product of breeding: it arose from crosses between Rheum rhabarbarum from southern Siberia and Rheum rhaponticum from the Balkans, which hybridise readily. Because cultivars were selected from open-pollinated seed over centuries, the exact ancestry can no longer be reconstructed. It has been grown as a vegetable in England and Scandinavia since the early 18th century, and commercially in Germany since 1848 at Hamburg-Kirchwerder. Today it is cultivated across Europe, North America, New Zealand and East Asia, occasionally escaping from gardens without spreading.

Garden rhubarb has no natural habitat; it occurs only in cultivation and on garden tips. Its requirements nonetheless betray the origin of its parent species in the continental steppes and mountains of Asia: it needs a pronounced cold stimulus in winter to shoot vigorously in spring, and tolerates frost well below minus twenty degrees without trouble, while hot, dry summers force it into premature summer dormancy. Where it thrives it stands on medium-heavy, deep, water-retentive soils at pH 5.6 to 7.2 with a high nutrient supply. It cannot be grown in subtropical climates because it lacks a winter.