Rhubarb Production in Alberta

Rhubarb is a perennial plant ideally suited to the varied climatic conditions characteristic of the Canadian prairies. A relatively short, cool growing season followed by a long, cold winter results in strong growth and high yields of good quality stalks.

Description

Rhubarb is a hardy perennial with a very short, semi-woody stem or crown. From this crown, large, thick fleshy storage roots protrude downwards and sideways. Buds develop on the top of the crown, first at the centre and then laterally as the plant ages. The best productivity occurs in the first 8 to 10 years.

Aged crowns produce poorly as vigour declines, resulting in thin stems that are weak and tough. If the crown is left undisturbed, it grows outward while the centre tissue dies and rots. After a number of years, a rhubarb crown can have a large, rotten, unproductive centre.

Buds for the following season develop during the summer and begin with new growth the following spring when the soil temperature exceeds 7 to 10øC. The most vigorous buds are found along the outer edge of crowns, particularly in young plants. Vigour is best maintained by replanting every 8 to 10 years.

The edible portion of the plant is the very elongated and thickened leaf stalk. The length, width and colour of leaf stalks vary greatly among varieties (see Varieties).

The leaf blade contains a high content of oxalic acid, characterized by soluble salts, which can be quite poisonous. In the stalks, oxalic acid is present in smaller amounts and is largely in an insoluble salt form.

At relatively low temperatures for growth, stalks have been observed to develop more red colour, while at high temperatures, the green colour predominates. Stalk colour is largely variety specific.

Seedstalk development is normal in all varieties, but it is more common when cool temperatures occur in spring. The frequency of development varies greatly with the variety. Some varieties are not recommended because of their tendency to produce seedstalks continually. Seedstalks begin to appear very early in spring, and if not cut off, the food energy drawn by the seedstalk will inhibit the production of leaves and petioles, thereby reducing vigour overall.

Varieties

The selection of varieties is limited by what nursery outlets or established growers currently supply. The following list and comments on varieties are based on observations made in southern Alberta at the Crop Diversification Centre South in Brooks.

1.	German Wine - the most vigorous variety that produces very large, green stalks; suitable only for juicing and wine products; second harvest in the same season is the highest in this variety; stalks are easy to pull.
2.	Mcdonald - moderately red stalks; very good vigour; excellent for pie-filling products.
3.	Early Sunrise - very similar to Mcdonald but vigour is moderately less; red is intense.
4.	Victoria - not recommended because of its frequent habit of producing seedstalks; used almost exclusively for wintertime indoor forcing.
5.	Cherry Wine - bright red stalks; almost free of seedstalk development; has moderate vigour.
6.	Valentine - bright red stalks make this variety the most attractive; vigour is lacking somewhat; almost free of seedstalk development.
7.	Crimson Red Cherry - not recommended because of its frequent habit of producing seedstalks.
8.	Cherry Red - bright red stalks; similar to Cherry Wine; the plant lacks vigour when harvested twice per season; almost free of seedstalk development; stalks are hard to pull.
9.	Holstein Red - bright red stalks; the plant lacks vigour and produces seedstalks continually; not recommended.
10.	Canada Red - good vigour along with moderately red stalks; similar to Mcdonald.
11.	Oregon Red - not recommended because of its frequent habit of producing seedstalks; weak red in stalks.
12.	Strawberry - stalks are very light red; almost free of seedstalk development; has moderate vigour.
13.	Simonette - a very intense red-stemmed variety; field evaluation is incomplete.
14.	Honey Red - a nice red-stemmed variety; field evaluation is incomplete. It has been generally observed that the redder the stalks, the less vigourous the plant. These varieties with the weaker red color seem more susceptible to a disease common in Alberta called redleaf (see Diseases of rhubarb).

Culinary Qualities

Well established varieties at the Crop Diversification Centre South have been analysed for culinary qualities. In terms of moisture content, all the varieties recorded moisture in the range of 92.2 to 94 per cent. The pH, or measure of acidity, ranged from a low 4.87 in Cherry Red to a high of 3.94 in Valentine. Sucrose readings averaged close to 5 per cent in all varieties.

Both juice content and red colour intensity varied more widely. The following table lists varieties from the most to least intense red colour and the corresponding juice yield.

Variety Red Colour Rating (from most to least)	Juice Yield %
Valentine	76.6
Cherry Red	79.4
Early Sunrise	82.6
Cherry Wine	79.7
Mcdonald	77.7
Canada Red	79.7
German Wine	82.5
Strawberry	81.2

Soil and Site Preparation

Soils used for growing rhubarb must have a capacity for retaining moisture and must also drain freely to prevent waterlogged conditions. Sandy soils are therefore not recommended. Rhubarb is tolerant to a wide range of soil pH and does well even in moderately alkaline soil conditions characteristic of most areas in southern Alberta.

Organic matter stimulates vigorous growth. Annual spring or fall application of well-rotted barnyard manure is recommended at a rate of 2 to 6 tons per acre. Before planting at a new site, up to 25 tones per acre should be applied and ploughed into the soil. In place of barnyard manure, commercial fertilizer application on an annual basis is recommended (see Fertilizer).

Fertilizer

Fertilizer trials in the United States have shown that rhubarb responds primarily to nitrogen provided that adequate levels of phosphorous and potassium are present. A base level of 150 lbs per acre nitrogen (N), 150 lbs per acre phosphorus (P) and 300 lbs per acre potassium (K) should be maintained from year-to-year. An additional 50 lbs per acre of nitrogen should be applied after each harvest. Annual soil tests are recommended in establishing fertilizer needs. Micronutrient requirements have not been documented.

Irrigation

Application of irrigation water is essential to maintain vigorous growth. Rhubarb thrives in moist soil conditions. Once every week to 10 days, an application of approximately 3 inches of water is recommended, particularly during July when summer temperatures are the highest. Water can effectively cool down rhubarb. This result is important because foliar growth declines when the temperature exceeds 27øC. A thorough soaking through September to early October provides for better overwintering crown strength and early spring growth.

Planting

New rhubarb plantings are most successfully established through the use of root or crown divisions. Select only healthy plants for propagation. When the plant is approaching full dormancy, usually in late September, a sharp spade can be used to slice-off a section of crown, or the entire crown can be lifted and cut into sections. Each section of crown should contain at least two eyes and some storage roots.

Old crowns are usually lifted entirely and split into sections. Crowns that are two to three years old are better if left undisturbed; only one or two crown sections are sliced off with a sharp spade, leaving the major part of the crown undisturbed.

Rhubarb crowns should be planted immediately. Rows should be at least 4 feet apart and crowns spaced no closer than 3 feet apart in the row. Row spacing should be designed to accommodate tillage equipment when the plants are fully grown. Rows are usually spaced 8 feet apart, and plants in the row are spaced 3 feet apart.

A furrow 12 to 15 inches deep should be opened, and crown sections hand-planted at the desired spacing. Place each piece in an upright position and firm it in place with soil. Each crown should then be covered with soil so that the crown tip is at least 1 to 2 inches below the soil surface when the ground has been levelled. The soil should be firmed around each crown.

One good irrigation after planting is recommended. Irrigation helps improve soil-root contact and also stimulates the production of new roots before winter sets in.

Seed should not be used for propagation as seedlings will not resemble the variety from which seed was harvested.

Harvesting and Yield

No harvest should be taken from plants in the first summer following planting. During the second season, one harvest can be made, and in subsequent years, two harvests can be made each season.

The number of harvests made each season should be guided by the overall vigour of plants. Every second or third year, only one harvest each season should be made if vigour is declining. A first harvest taken in early to mid-June and a second in late July are suitable for southern Alberta. Some variation of this harvest schedule should be made for more northern regions.

When harvesting, stalks should be pulled from the crown and not cut. Cutting makes open wounds, which serve as entry points for disease infection. Essentially, all leaves can be pulled at each harvest; however, there is some benefit to leaving the lowest leaves attached to the crown. These attached leaves will continue to supply strength to the crown until new leaves form.

Yield varies considerably with variety and plant density. In southern Alberta, average yield over two harvests per season can range from 3 to 7 tons per acre. It is important to reduce the number of harvests when there is a noticeable decline in yield. Reduced harvests allow crowns to build up strength for future production.

Weed Control

No herbicides are registered for weed control in rhubarb in Alberta. Perennial weeds must be eliminated before establishing a rhubarb planting. Annual weeds are best controlled through cultivation, and planting distances between rows should be designed to accommodate tillage equipment.

A good shallow cultivation very early in spring and after each harvest keeps weeds under control. Avoid deep tillage over the crown as buds are concentrated near the soil surface.

In difficult perennial weed problem situations, an application of Roundup works well only after harvest when every leaf is removed from all rhubarb crowns. The herbicide can be applied directly over crowns. The absence of leaves attached to crowns minimizes herbicide uptake, yet perennial weeds are fully exposed for maximum contact with the herbicide.

Diseases of Rhubarb

Leaf spot, or more commonly called redleaf, is the only major disease that affects rhubarb in Alberta. Redleaf first appears as small, greenish-yellow areas on the upper surface of the leaves. These change to circular or somewhat angular-shaped spots having white centres with wide, reddish margins. In severe cases, the red colour becomes very prominent so that the plant soon loses vigour and leaves droop to the ground. Eventually, the crown will die, usually into the second or third season after infection occurs.

To control leaf spot, remove all infected leaves during the growing season and destroy by burning or burial. If symptoms continue to appear, dig up the crown and burn or bury all infected tissue. The disease is spread by aphids feeding on infected plants and then subsequently feeding on healthy plants.

The disease is often confused with natural leaf dieback that occurs quickly at the end of the season. Redleaf-like symptoms occur in fall in response to low temperatures. True redleaf symptoms appear in June and July, slowly beginning with a few leaves showing symptoms and progressively getting worse each season.

Storage and Marketing

For the fresh market, rhubarb is generally trimmed to a 15 inch stalk length. Bunches of 5 to 6 stalks tied with string or ribbon are acceptable for direct-to-consumer sales.

The wholesale market requires boxes that hold 20 pounds of loosely arranged stalks. A plastic liner helps retain freshness by minimizing dehydration. Refrigeration also retains freshness, and high relative humidity maintains crispness. Rhubarb can be stored under these conditions.

Source: Agdex 254/20-1. August 2002.