Commercial Calendula Cultivation

By Conrad Richter

Calendula’s main commercial value lies in the flowers used in medicinal and cosmetic preparations. To a lesser extent, calendula is also grown for its edible fresh and dried flowers to add colour to foods. Farmers feed the flowers to laying hens to deepen the colour of the egg yolk. The potential of the seed oil for industrial applications has been explored and development of the crop for this purpose is close to commercialization.

Varieties

Erfurter Orange Calendula
Erfurter Orangefarbige Calendula

Of the more than 100 varieties of calendula known to exist, most are intended for the ornamental market. Historically, North American growers have chosen to grow ornamental varieties assumed to be bioactive. The Czech variety, ‘Plamen’, registered in 1941 and still cultivated as a medicinal crop in Europe, is said to be a progenitor of many early ornamental varieties such as the Pacific Beauty series. Neither ‘Plamen’ nor its improved large-flowered version, ‘Plamen Plus’, are available in North America. In Europe and North America the standard variety for commercial production is ‘Erfurter Orangefarbige’, a high-yielding, large, double-flowered type with high flavonoid and carotenoid content. Dry flower yields of 1.7 tonnes/hectare (1500 lbs/acre) have been recorded for this variety. Compared to ‘Erfurter Orangefarbige’, yields of ‘Resina’, an American variety with mostly yellow flowers, are similar, but flavonoid content is as much as a third lower. Both ‘Erfurter Orangefarbige’ and ‘Resina’ were evaluated for oilseed production but seed yields of 0.6-0.8 tonne/hectare are too low compared to the 3 tonnes/hectare of some European selections. A proprietary European variety, ‘Carola’, registered in 2005, is apparently the first to come out of European oilseed research; but it is not available in North America. ‘Regina’, an older European oil-rich variety is also not available to North American growers. For competitive reasons the use of proprietary selections appears to be common in the industry. For example, the German herbal medicine manufacturer, Dr. Theiss, registered ‘Rinathei’ in 1998 for its own exclusive production use. This variety is claimed to be rich in faradiol triterpenoids believed to be most responsible for calendula’s anti-inflammatory activity. Intriguingly, a readily available dwarf ornamental variety, ‘Calypso Orange’, is also rich in faradiols, having the highest content of 10 varieties tested, as much as a third more than ‘Erfurter Orangefarbige’.

Site Selection and Preparation

Cool temperate areas with mild summers are best suited for calendula. Where summers are too hot yields are depressed. Typical USDA zones where calendula is grown successfully are zones 2-9. Calendula prefers full sun, and will tolerate a range of soils if the drainage is good. It does not require a bare fallow period prior to planting. The ground should be finely worked to provide a smooth surface for sowing. Calendula is a hardy annual. Planting occurs in early spring or, where winters are mild, in the fall.

Direct Seeding

Calypso Orange Calendula

For rows 70 cm (28 in) apart with a final plant density of 5-7/m² (5-7/10ft²), the traditionally recommended seeding rate is 2-3 kg/hectare (2-2.5 lb/acre). One study suggests that dry flower yields can be tripled with broadcast sowing at 24 kg/hectare (20 lb/acre); but the decision to sow in rows or broadcast depends on weed control, harvest method and seed cost. Broadcast sowing is only feasible if herbicide use is planned. For oilseed production broadcast sowing is preferred. For oilseed production the seeding rate of 24 kg/hectare is recommended; this will result in plant density of about 60/m².

Indirect Seeding and Transplanting

Sow in plugs or seed beds 4-6 weeks prior to field planting. At the seed density 130 seeds/g (3700 seeds/oz) approximately 1 kg of seed is needed to plant 1 hectare (1 lb/acre). Plug trays with 128 cells planted 2 seeds per cell works well. Seeds need light to germinate so they should be covered very lightly or just pressed in. Transplant plugs or seedlings to the field in rows spaced 70 cm (28 in) apart with plants spaced 15-20 cm (6-8 in) apart within rows.

Fertilizer and Growth Control

Too much nitrogen reduces flower yields in favour of unmarketable foliage. If nitrogen is very low, up to 50 kg/hectare (45 lbs/acre) may be applied. Phosphate (P2O5) and potassium (K2O) may be applied at the rate of 100 kg/hectare (90 lbs/acre) and 50-100 kg/hectare (45-90 lbs/acre), respectively. Organic growers may apply compost or composted manure at 20 tonnes/hectare (10 tons/acre) if soil fertility is low. The microelements B, Mo, Zn, Mn, and Co are known to stimulate flower production and carotenoid content. Maleic hydrazide (50 mg/litre) induces branching and flower formation.

Irrigation

During dry periods calendula needs 2.5-4 cm (1-1.5 in) water per week.

Weed Control

If planted in rows, regular cultivation and hoeing is usually sufficient to keep weeds down to levels that do not interfere with flower production or harvest. A plastic weed barrier works for transplanted fields but may not be cost effective. Effective chemical controls include the preplant herbicides ethalfluralin (Edge) and trifluralin (Bonanza), the pre- or postemergent herbicide propyzamide (Kerb), and the postemergent herbicide sethoxydim (Poast Ultra).

Diseases and Pests

The main problems are powdery mildew (Sphaerotheca fuliginea and Erysiphe cichoracearum) which damages flowers during cool periods and aster yellows which causes stunting and deformation of flowers. For mildew, preventative applications of sulfur or baking soda sprays may help. Mild infections can be treated with neem oil or horticultural oil sprays. Picking flowers early and often avoids late cool season mildew. Aster yellows infection rates as high as 27% have been reported. There is no treatment other than to remove infected plants. It is spread by aster (six-spotted) leafhoppers so prevention centres on controlling leafhoppers. Contol measures include monitoring leafhoppers with sweep nets and treatment with neem oil, soap and pyrethrum sprays or with chemicals such as endusulfan (Thiodan, Phaser) and carbaryl (Sevin). Blister beetles, cabbage loopers, and caterpillars are the other main pests.

Harvesting

Flower heads can be harvested by hand-picking or by using a comb. Hand-picking results in a superior product as the comb method will capture buds, over mature flowers, and seed heads. The combed product either has to be picked over to remove the unwanted material or must be sold at a lower price. Fresh flower yields range 6-9 tonnes/hectare (2.5-4 tons/acre). A worker can hand-pick 12-20 kg (25-45 lbs) of fresh flower heads per hour. Harvesting labour can account for up to 80% of the total production labour required to produce the flower crop. For oilseed production the crop is dessicated with diquat (Reglone Dessicant) and then combined.

Drying

Calendula flower heads require shade, ventilation and artificial heat to dry. Because there are numerous sites on the flower heads where moisture can accumulate, drying must be done carefully. The flower heads should be spread on screens no more than one layer thick. A brief period of elevated heat at 50-60 ºC (120-140 ºF) to remove surface moisture is followed by a sustained drying period of not more than 35 ºC (95 ºF). If required product is to be petals only, then the petals can be rubbed off when the petals are dry and the centres are not yet hard. When centres are hard and no longer pliable flower heads are ready for processing or storage.

Processing

Depending on the intended market the product may or may not need further processing such as cleaning. For whole flower heads no further processing is usually required. For a petals only product winnowing may be required to clean the product


Conrad Richter is president of Richters Herbs, Goodwood, ON (www.richters.com). © 2007 Conrad Richter.
 
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