Pyrethrum is a white‐flowered, herbaceous, perennial plant belonging to the same family as the chrysanthemum, the garden flower. The plant is thought to have originated in Dalmatia, and isgrown in many parts of the world including Southern Africa and Eastern Africa, Kenya being a major producer. The crop is grown commercially for the production of the insecticides called pyrethrins which are contained in the flowers. The pyrethrin, cinerin and jasmolin, are essential oils and their advantages are:
They kill a wide range of insects very rapidly;
They have a very low rate of toxicity for mammals; and
Very few insects have developed any type of resistance to pyrethrins.
Their disadvantage is that once they have been applied to a crop, they break down very rapidly and there is no residual effect. The prospects are good because of the recent discovery of the harmful effects of D.D.T. which has been the major insecticide used in the past. In America, pyrethrum is the only insecticide on the market bearing the label ‘Harmless to Humans’.
The flowers from the plants are dried, ground and the powder produced contains the pyrethrum flower, which is the insecticide. Any increase in the area of the crop grown would be greatly assisted by the establishment of a factory to extract and refine the pyrethrum powder.
2. CLIMATE AND SOILS
Pyrethrum will grow in many areas of the world, from temperate regions to the tropics and at altitudes from 0 to 2 700 metres. However, it does have certain requirements, one being a cool period during the year which is needed to stimulate the production of the flower buds.
Temperatures should not be too high. A maximum of 24⁰ C for a week can reduce flowering later. The crop is resistant to cold and can survive several degrees of frost because established plants become dormant in cold Winter weather. Young seedlings should not be exposed to frost and the crop is best grown in areas higher than 1 500 metres.
Although pyrethrum is very drought‐resistant and able to survive long periods without water, it produces a poor crop under those conditions and does best with a rainfall of 1 000 to 1 100 mm
distributed evenly throughout the year. In most areas the crop requires at least supplementary irrigation during the winter and drought periods during the summer to get the best yields.
Pyrethrum will grow on any soil that is deep and free from waterlogging. The crop is not sensitive to pH so that only very acid soils need to be limed. The soil should be ploughed and cultivated to a depth of 300 mm to allow the deep roots to become established and on heavy soils the plants should be grown on ridges to prevent waterlogging.
Once established the plants stay in the land for 3 ‐ 4 years and as the pickers pass up and down the rows many times the soil structure can become damaged. The crop should be grown in a rotation with other crops such as a legume or a grass ley that will improve the structure of the soil.
Pyrethrum can be propagated by sowing seed or planting splits which are obtained by digging up a mature plant and dividing it into several pieces, each with some root material attached. As far as possible both seed and splits should be selected from plants which have the following characteristics:
Plants with flowers which have high pyrethrum content;
Plants that have produced a large number of flowers e.g. a high yield; Plants that have produced large flowers;
Plants that have grown vigorously and are upright rather than spreading; and Disease‐free plants.
The new grower will have to start his crop by sowing seed, but, once established, can be propagated vegetatively by taking slips from selected plants.
This is sown in nursery beds and the seedlings planted out when they are 3 – 4 months old. The Seed can be sown in August and the seedlings planted out with the rains, or sown at the end of the rainy season in March, and the planting out done with irrigation. Seedbeds measuring 1 metre by 25 metres should be made in a sheltered site that is free from frost and supplied with water. About 95 m² of seedbeds will provide enough seedlings to plant one hectares of land. Because pyrethrum is very susceptible to damage from nematodes (eelworms), the seedbeds should be fumigated with Methyl Bromide or EDB in exactly the same way as tobacco seedbeds. A fertilizer with ratios of (N 6; P 18; K6) should be broadcast over the beds at the rate of 3 – 4kg per 25 m² and worked into the soil.
The seed has a poor germination rate, 20 – 30% and should be sown at the rate of 60 g for 10m², either broadcast or in drills mm apart. Immediately after sowing, a thick mulch of fumigated grass should be spread over the beds and the soil kept moist by watering twice a day if necessary. Germination of the seeds is very variable and can be from 7 days to 4 weeks after sowing. As the seedlings emerge the mulch should be thinned out and the plants will be ready for planting out into the lands after 3 – 4 months.
Mature plants can be dug up and divided into well‐rooted portions for planting out into new land. The number of splits will vary from 4 ‐ 15 from a single plant although this number can be increased by earthing up around the parent plant 1 month before they are to be dug up. Plants which are used to supply splits should be selected carefully for desirable characteristics. Allowing for the rejection of poor parent material, 1 hectare of mature plants should provide enough splits for 5 hectares of replanting.
Both seedlings and splits should be planted out as soon as possible after lifting. The land should be well moistened either by rain or irrigation and marked out before planting. The young plants should be placed 500 mm apart and in rows 1 metre apart to allow access for the pickers. Once the crop is mature, the plants should meet in the rows and almost meet across the rows so that a good cover is provided to keep down weeds. Planting out is done by hand using a team of 1 planter and 1 waiter to carry the plants. Plants should be placed in a hole big enough to take the root without bending, and the crown should not be below the soil surface.
Each plant should be well firmed down after planting and watered in. Any gaps in the rows can be filled up after a few weeks. The plant population will be about 44 000 plants per hectare.
A general recommendation for fertilizing the crop is the following:
AT PLANTING OUT
|N||20 – 30 kg/ha|
|P||60 – 80 kg/ha|
|K||20 – 30 kg/ha|
This can be put on in the form of a blended fertilizer or as straights and should be broadcast over the land and disced in. This dressing of fertilizer should be repeated at the end of each Winter and spread down the rows between the plants.
A top dressing of nitrogen should be given to the crop in November each year to bring the total of nitrogen applied to the crop up to 100 kg/ha. Too much nitrogen should not be given, as this causes an increase in leaves and decrease in flowers.
Although pyrethrum once established is a very drought resistant crop and will survive the Winter without additional water the yield will be greatly reduced. The Spring flush, when flowers are abundant and the pyrethrin content is highest, takes place in September and October when the weather is hottest and just before the rains begin. Production starts to decrease at the end of November until the end of January when the crop is cut back. Flowering is largely controlled by temperature. Water will not bring plants into flower again once they have finished for the season. Due to these factors the crop should be treated as a fully irrigated crop, following a full irrigation schedule based on rainfall records and an evaporation pan. Moisture stress must be avoided in September and October and if plenty of water is available then the plants will flower profusely.
5. PESTS AND DISEASES
Although it has insecticidal properties the crop does suffer from some pests. Thrips can attack and should be sprayed with chlorpyrifos. Red Spider Mite can be controlled by spraying with Tetradifon. The main pest is the eelworm nematode which can be prevented by using sound rotations and fumigating seedbeds with Methol Bromide. Never grow pyrethrum in the same rotation as sunflowers or tobacco. Buck can damage the crop when grazing the plants.
This is a major problem of pyrethrum in other parts of the world. The disease causes the flower stem to shrivel just below the head, causing it to hang down. Some plants show a natural resistance and splits should be taken from these plants for planting out.
The pyrethrum content of the flowers increases as they open and develops seed, and is highest when the white petals are horizontal. When harvesting, pickers remove all the flowers, together with those that have gone past the horizontal stage.
This picking is carried out every 2 ‐ 3 weeks although it may pay to pick more often, getting higher pyrethrin content and a lower weight of flowers.
The flowers should be picked without any of the stalk attached with only the head being harvested. Pickers should use both hands and the flower heads dropped into a container slung from the shoulders or tied around the waist. Picking rates will vary according to the picker and the state of the crop. A fair daily average is 11 – 13 kg of fresh flower heads a day. Good pickers working in a crop in full flower can pick up to 27 kg a day.The number of flower heads in a kg will be between 2 000 and 2 200.
At the end of the flowering season, all the old flower stalks should be cut but the leaves of the plants should not be damaged.
These depend on climate, the type of plant and management of the crop. Yields of dried flowers can vary from 500 to 1 000 kg/ha, and an average pyrethrum content of the flowers would be 1.5% although up to 2% can be obtained. Under good management both the yield of the flowers and pyrethrum content can be improved by carefully selecting the slips for propagating. Slips should be taken from plants with a high yield and high pyrethrum content.
As soon as the fresh flowers have been picked they should he dried to prevent them from fermenting or going mouldy. The flowers are dried down to a moisture content of 7 ‐ 12%, losing about 75% of their fresh weight. Small crops of flower heads can be dried by spreading on trays made from wood and chicken wire and leaving them in the sun. The trays should be covered with a plastic sheet at night and during rain and flowers should be turned at least twice every day. Drying is complete when 4 out of 5 flowers can be crushed between the finger and thumb. During fine weather drying takes place quickly but if overcast or humid, it may take up to 15 days because the partly dried flowers rapidly re‐absorb moisture from the air.
Artificial drying can be carried out in tobacco barns and is much quicker and a more efficient process than natural drying. The flowers should not be exposed to too much heat and the maximum temperature should not exceed 60o C.
The dried flowers should be sent for sale as soon as possible because the pyrethrum content will fall if they are stored. When storage is necessary the bags of dried flowers should be kept in a cool, dry, dark place to prevent any deterioration.