VEGETABLE GARDENING IN DENTDALE

 

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RHUBARB

Rhubarb (Rheum x hybridum) is a rhizomatous perennial whose stems ('sticks') are harvested for culinary use, usually as a dessert. It has a long period of cropping and most cultivars are suitable for forcing. Expected yield is .-5. 13.5kg per 3m (10-30lb per 10ft) row. Crowns ('sets') can be cropped for ten or more years, though division may be necessary after five or so years. Rhubarb has a cold requirement of between seven to nine weeks below 3°e (37°F) depending on the cultivar to bring it out of winter dormancy and is commercially grown mostly in the north of England.

Site and soil
Rhubarb is fully hardy but avoid frost pockets which will damage stems at the start and end of the season. An open site is needed as rhubarb does not grow well in shade. Most soils are suitable including sand, day and acidic soils provided they are fertile and well drained. Waterlogged soils cause rotting of the crowns.

Preparation of the soil
Although the large foliage can help smother weeds, the ground should be free from perennial weed5 before planting as these will be difficult to control once the crowns are in the soil Incorporate plenty of well-rotted organic matter, especially to light soils.

Planting
Plant crowns in November or December. If necessary planting can continue up to the beginning of March. Buy named cultivars or choose a division from a strong, healthy looking plant.

 Plant the crown with the growing point at, or just below, the soil surface. On wetter soils planting with the buds just raised out of the soil may help prevent rotting.

If planting more than one crown, space plants 1m (3ft) apart, with 1-2m (3-6ft) between rows.

Cultivation
In hot summers, if the ground becomes dry, growth will slow down and even stop. A spring mulch will help to retain moisture but do not bury the crowns. Plants will also respond to watering during the summer. Apply a high nitrogen feed such as sulphate of ammonia in spring or summer at 70g per m2 (20z per square yard).

Allow the foliage to die back naturally in autumn, then cut away the old leaves to expose the growing points to winter cold. There is no harm in adding these . leaves to the compost heap as the poisonous ·oxalic add contained in them breaks down during decomposition. Compost made from them makes a good soil conditioner.

Propagation

 By Seed
Sow seed in spring (March/ April) 2.5crn deep in a seedbed or individually in modules. If in a seedbed thin seedlings to 150n apart, choosing the most vigorous seedlings. The resulting plants will be more variable than named clones. Plant out in autumn or the following spring.

By Division
Division will guarantee a plant identical to the parent and is the most common method of, propagation. It is also good practice to divide established crowns about' once every five years if they have become weak or overcrowded. Lift crowns between autumn· and early spring (usually in November);' forking well away from the crown to avoid damaging the large root system.

Use a spade to divide the crown into sections, each retaining a portion of the rhizome_ (thickened root) and at least one growing point. Sections from the outer part are better than the centres of old plants. Discard any old or decayed parts of the crown. Replant straight away or wrap in damp sacking until ready to plant.

By Tissue Culture
Due to the prevalence of virus in many rhubarb stocks micropropagation techniques have been applied to obtain disease-free plants. New plants are grown from cells taken at the growing tips (meristem). This is carried out by commercial companies or research stations and is not possible to replicate in the home. Plants raised by tissue culture are not usually more sharply flavoured than conventionally propagated clones.

Harvesting
Stems can be picked from the early cultivars from March to April. Crowns that have been forced for earlier harvest should be left without pulling for the rest of that season. Do not harvest in the first season after planting and harvest only lightly in the second season to avoid weakening the crowns. From seedling plants, harvest in the second season after planting or in the first season after division.

Stems should be pulled rather than cut to prevent rotting of the remaining stump. Pull stems when they are between 23-30cm (9912in) long, holding them at the base and pulling gently outwards. Take no more than half the total stems at anyone time.

The last harvest is usually in late summer, around July or August, though growth may have stopped before this if the weather is very hot. Concern is sometimes expressed over the concentrations of oxalic add building up as the season progresses. However, this build-Up is mostly in the leaves which are not eaten and the amount in the stems is not sufficient to have a toxic effect.

Forcing
Earlier cultivars tend to be the best for forcing as these have a lower cold requirement and can be started into growth by midwinter.

Forcing results in earlier stems which are more tender and pinker than. unforced rhubarb. Cover the crowns in December or January With a layer of straw at bracken and cover over with an upturned bucket or a traditional clay rhubarb pot to exclude light. Stems will be ready to pull two to three weeks later.

For an even earlier harvest, lift some roots in November. If there has been insufficient cold it is possible to leave the lifted roots outside for up to two weeks prior to potting to expose them to more cold. Then pot up with compost and bring into a cool. room or greenhouse at a temperature of between 7-16°C (45-60°F). Exlude light with buckets or black polythene over crates. Alternatively, pack the roots into a large black bin and replace the lid, uncovering at night to provide ventilation. Keep the roots damp but not wet. Stems can usually be harvested in five weeks. Crowns forced in this manner are usually much weakened and therefore discarded after harvest.

Problems

Flowering
Some cultivars can be more prone than others. The cultivar 'Prince Albert' is particularly susceptible. Remove flower stalks as soon as they appear to prevent them weakening the crowns. Flowering is usually worse after wet summers or where high nitrogen feed has been overused.

Thin, weak stems
Lots of thin stems indicates that the crown is losing vigour and needs to be divided (see 'Propagation'). Increased feeding may also help.

Split stems, sometimes exuding sticky sap This is sometimes caused by lat~ frosts but is often an indication of erratic growth due to seasonal conditions. Cool or dry periods followed by moist or mild-weather means that the hard outer growth splits when the new rapid growth occurs. Mulching and feeding may help to avoid the worst damage.

Green, poor quality stems
Warm, dry summers can give rise to poorly coloured, bad tasting stalks. Try to harvest earlier while the days are cooler and moister.

Slow or no growth
Rhubarb will stop growing if the temperature rises above 32°C (90°F). This can happen in hot summers. Growth can also slow or stop if the plants are under drought stress so watering may help.

Dieback of crowns
Waterlogged soils or a very wet season can cause rotting in the crown. Lift and move to a better drained site or incorporate plenty of gravel to improve drainage. Discard any rotting roots. Rhubarb is also susceptible to the root disease honey fungus and a bacterial crown rot. If either is suspected lift and destroy all affected crowns and replace soil before replanting.

Recommended Cultivars

Early
'Appleton's Forcing' - deep carmine red base changing to green, speckled red higher up stem. Flesh pale green with red tinge. Leaves heart-shaped, smallish. Good for forcing. Came high in Harlow Carr taste tests.

'Early Champagne' - even cardinal red colour throughout length of stem. Green flesh, pink at edges. Good, strong, reliable cultivar.

'Early Cherry' ..:. bright reddish-pink base with red flecking on green towards top. Flesh green. Thick stems.

'Goliath' - very dark red base of stem paling to green at the top. Good, strong weed suppressing cultivar. Good rating in Harlow Carr taste tests.

'Grandad's Favorite' - currant red stems with some flecking and green streaking at top. Green flesh. Heavy cropper.

'Hawke's Champagne' - even currant red colour throughout length of stem. Flesh pale green through to pink. Leaves heart-shaped and attractive. Vigorous. Reliable early variety.

'Reeds Early Superb' - a good red stem along most of the length. Leaves glossy dark green. Good for forcing.

'Stein's Champagne' - very attractive, slender, deep red stems. Dark red flesh throughout.

'Stockbridge Arrow' - cardinal red stems with some lighter streaking and flecking. Flesh pink with white background. Thick stems. Small, triangular, red-veined leaves .

Good for forcing, producing. - deep red, straight sticks. Heavy cropper.

'TImperley Early' - base deep red, passing to light green with red flecking. Does not hold colour well. Green flesh with reddish tinges. Heavy cropper. Good for forcing. Disappointing in Harlow Carr taste tests. Frost susceptible.

 

Mid-season
'Canada Red' (mid- to late season) - stems currant red and of even colour. Very dark flesh throughout.

'Cawood Delight' (mid- to late season) (good deep red stems of high quality. Poor cropper.

'German Wine' - deep pinkish-red stems with some flecking at top. Red flesh. One of the best for colour.

'Strawberry' - vermilion base deepening to currant red at top. Pink-tinged flesh with white middle. Very attractive colouration. Good rating in Harlow Carr taste tests.

'The Sutton' - currant red base of stem becoming green with red flecking. Green flesh with some red tinges. Thick stems. Good for forcing.

'Zwolle Seedling' - greenish-pink young sfems maturing to carmine red. Light green flesh. Good rating in Harlow Carr taste tests.

 

Late
'
Barker's All Season' - cardinal red, slender stems. Red-centred flesh. Weak growth. Long cropping season. Frost susceptible. 'Valentine' - good variety for general use. Stems fiery red at base with strong colour up to the leaf. Green flesh. Heart-shaped leaves. Similar to the Canadian cultivar 'Macdonald' in-habit and stem colour. Came top in Harlow Carr taste tests.

'Victoria' - cardinal red stems with flecking at top. Red flesh tinged green. Very thick stems. Popular old variety.

 

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