International Institute of Tropical Agriculture

About this booklet

This booklet is one in a set of field guides prepared by the International Insti-tute of Tropical Agriculture (IITA) to increase the technical knowledge of exten-sion agents and enhance the integration of plant protection and plant produc-tion practices in farmers’ efforts to grow a healthy crop of cassava. The bookletis based largely on the extension and farmer training experience of the regionalproject “Ecologically Sustainable Cassava Plant Protection” (ESCaPP), 1993–1997. ESCaPP was executed by IITA’s Plant Health Management Division(PHMD), in collaboration with national agricultural research and extension sys-tems in Bénin, Cameroon, Ghana, and Nigeria, and funded by the Division ofGlobal and Interregional Programmes of the United Nations DevelopmentProgramme (UNDP).IITA is one of 16 nonprofit international agricultural research and training cen-ters supported by the Consultative Group on International Agricultural Re-search (CGIAR). Their shared mission is the alleviation of hunger and poverty intropical developing countries by generating appropriate plant production andprotection technologies which benefit the poor and enhance agricultural pro-duction while preserving the natural resource base. At IITA, PHMD is dedicatedto sustainable plant protection of primary food crops in Africa. The division’sresearch philosophy is to identify and correct the ecological imbalances in agri-cultural systems causing pest problems and to provide environmentally andeconomically appropriate options for integrated pest management. (IPM)

For more information contact:The Director

IITA Plant Health Management DivisionBiological Control Center for Africa

08 B.P. 0932Cotonou, Republic of Bénin

Fax: (229) 35 05 56Tel: (229) 35 01 88

E-mail: IITA-benin@cgiar.orgOr visit IITA’s website at: http://www.cgiar.org/iita

Alex Melifonwu, Braima James, Kouessi Aïhou,Stephan Weise, Emmanuel Awah, Brice Gbaguidi

Weed Controlin

Cassava Farms

Weed Controlin

Cassava Farms

Alex MelifonwuFederal College of Agriculture, Umudike,Nigeria

Braima JamesInternational Institute of Tropical Agriculture, Plant Health Management Division,Cotonou, Bénin

Kouessi AïhouInstitut National des Recherches Agricole du Bénin, Niaouli, Bénin

Stephan WeiseInternational Institute of Tropical Agriculture, Research and Crop Management Divi-sion, Mbalmayo, Cameroon

Emmanuel AwahInstitut de Recherche Agronomique et du Développement, Ekona, Cameroon

Brice GbaguidiInternational Institute of Tropical Agriculture, Plant Health Management Division,Cotonou, Bénin

Weed Controlin Cassava FarmsIPM Field Guide for Extension Agents

Contents

What are the objectives of this guide? ------------------------------------------ 4

Introduction -------------------------------------------------------------------------- 4

What are the common weeds in cassava farms? ------------------------------ 6

How do weeds spread in cassava farms? --------------------------------------- 10

How do weeds become a problem in cassava farms? ------------------------ 14

How can I best control weeds in cassava farms? ----------------------------- 16

Summary ----------------------------------------------------------------------------- 22

5

IPM Field GuideWeed Control in Cassava Farms

4

Introduction

Weeds are plants growing where they are notwanted. Many different types of weeds (forexample, Figures 1, 2, and 3) occur in cassavafarms and cause considerable losses to thefarmer. This is because weeds compete withthe cassava crop for nutrients, sunlight, andspace. Weeds may harbor pests and diseasesor physically injure cassava plants and storageroots. Weeds may also harbor natural enemiesthat control pests, so certain weeds can beleft on cassava farms, provided they are notmany enough to compete with the crop. Forthese reasons close attention should be paidto weed control in the farmer’s effort to growa healthy crop and obtain high yields of cas-sava.

What are the objectives of thisguide?

This field guide has been prepared to help youto:

• identify common weeds in cassava farms,

• identify the structures with which theweeds reproduce and spread,

• specify how weed plants become prob-lems in cassava farms, and

• combine the most appropriate culturalpractices to control weeds in your cassavafarm. Figure 1: Spear grass, Imperata cylindrica

Figure 3: Siam weed, Chromolaena odorata

Figure 2: The sedge, Mariscus alternifolius

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IPM Field GuideWeed Control in Cassava Farms

6

What are the common weeds incassava farms?

Weeds occurring in cassava farms can be putinto three main groups, namely, grasses, sedges,and broadleaf weeds. Occasionally, cassava isalso damaged by certain parasitic plants. Nu-merous kinds of weeds occur in particularplaces, the following is simply a selection ofthe most widespread species.

Grasses: Grasses are usually slender, erect, orcreeping plants. Their stems can be oval orcylindrical in shape. The leaves of grasses aremuch longer than they are broad and arenever subdivided into little leaves (leaflets).Grass weeds which are commonly found incassava farms include spear grass, Imperatacylindrica (Figure 1); bermuda grass, Cynodondactylon (Figure 4); guinea grass Panicum maxi-mum (Figure 5); and the feathery pennisetum,Pennisetum polystachion (Figure 6).

Segdes: Sedges resemble grasses but are al-ways erect and usually have solid and triangu-lar shaped stems. The common sedges whichcause problems in cassava farms are Mariscusalternifolius (Figure 2), and purple nutsedge,Cyperus rotundus (Figure 7).

Figure 4: Bermuda grass, Cynodondactylon

Figure 6: Feathery pennisetum,Pennisetum polystachion

Figure 5: Guinea grass, Panicummaximum

Figure 7: Purple nutsedge, Cyperusrotundus

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IPM Field GuideWeed Control in Cassava Farms

8

Broadleaf weeds: Broadleaf weeds areherbs, creepers, climbers, and shrubs whosestems are solid and irregular in shape. Theirleaves are broad, expanded, and single orsubdivided into leaflets to form compoundleaves. Broadleaf weeds which are commonlyfound in cassava farms include Siam weed,Chromolaena odorata (Figure 3); wild poinset-tia, Euphorbia heterophylla (Figure 8); giant sen-sitive weed, Mimosa invisa (Figure 9); tridax,Tridax procumbens (Figure 10); goat weed, Ag-eratum conyzoides (Figure 11); waterleaf,Talinum triangulare (Figure 12); and tropicalspiderwort, Commelina benghalensis (Figure13).

Figure 10: Tridax, Tridax procumbens

Figure 12: Waterleaf, Talinum triangulare Figure 13: Tropical spiderwort, Commelinabenghalensis

Figure 9: Giant sensitive weed, Mimosa invisaFigure 8: Wild poinsettia, Euphorbiaheterophylla

Figure 11: Goat weed, Ageratum conyzoides

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IPM Field GuideWeed Control in Cassava Farms

10

How do weeds spread incassava farms?

Whenever an area is cleared for farming, weedsare among the first plants you see growing onthe land. Because they grow and reproducevery quickly, they are able to establish them-selves on farms within a short time. Weeds re-produce and spread through seeds and vegeta-tive structures, for example, rhizomes, stolons,tubers, and stem/root basal stocks. Table 1 liststhe structures by which the common weeds incassava farms reproduce and spread.

Spread by seeds: A number of weeds whichlive for only one year (annual weeds) repro-duce and spread mainly by seeds. Examples ofsuch weeds are Siam weed, Chromolaenaodorata (Figure 3); wild poinsettia, Euphorbiaheterophylla (Figure 8); tridax, Tridax procumbens(Figure 10); and goat weed, Ageratum conyzoides(Figure 11); These weeds shed their seedsmostly in the dry season and die soon after-wards. The seeds survive in the soil and germi-nate the following season, or can remain in thesoil for many years until disturbed when thesoil is tilled for planting. In order to effectivelycontrol annual weeds, you need to removethem early before they start flowering. Thisprevents the weeds from seeding and increas-ing the quantity of seeds in the soil.

Spread by rhizomes: Rhizomes are under-ground stems running horizontal to the soilsurface. They have thin brown papery leaveswrapped around the stem. The rhizomes haveroots. Spear grass, Imperata cylindrica, repro-duces mainly by rhizomes (Figure 14). If rhi-zomes are cut up into pieces, each piece iscapable of growing into a new plant. You are

therefore very likely to spread spear grass inthis way during land and seedbed preparationfor planting cassava.

Spread by tubers: Tubers are undergroundstems without leaves but with buds (“eyes”).The stems are swollen with stored food.Cyperus and Mariscus species spread by tubers.The tubers of Cyperus species occur as large“beads” on underground slender stems whichjoin the individual plants together (Figure 15).If cut up into pieces each piece of the tuberwith a bud (“eye”) is capable of growing into anew Cyperus plant.

Spread by stolons: A stolon is a slenderstem that runs on the surface of the soil. Sto-lons have normal leaves and roots. Thebermuda grass, Cynodon dactylon, reproducesby stolons which are very long (Figure 16). Ifstolons are cut up into pieces, each piece iscapable of growing into a new plant. You aretherefore very likely to spread bermuda grassin this way during land and seedbed prepara-tion for planting cassava.

Other vegetative structures: In additionto rhizomes, stolons, and tubers, some weedsspread through other vegetative parts such asstem cuttings and basal shoot stocks. The wa-terleaf, Talinum triangulare (Figure 11), andtropical spiderwort, Commelina benghalensis(Figure 13), are propagated by both seeds andstem cuttings. In addition to the production ofseeds, Chromolaena odorata (Figure 3) is ca-pable of sprouting into whole plants from cutbasal shoots.

Figure 15: Tubers (arrows) of purple nutsedge,Cyperus rotundus

Figure 14: Rhizomes (arrows) of spear grass,Imperata cylindrica

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IPM Field GuideWeed Control in Cassava Farms

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Table: Weed propagating structures and life cycles

Problem weed Propagating structure Life cycleSeed Rhizome Stolon Tuber Cutting

GrassesSpear grass • • PerennialImperata cylindricaBermuda grass • • PerennialCynodon dactylonGuinea grass • PerennialPanicum maximumFeathery pennisetum • • AnnualPennisetum polystachion or perennial

SedgesCyperus rotundus • • PerennialMariscus alternifolius • • Perennial

Broadleaf weedsSiam weed • PerennialChromolaena odorataGiant sensitive weed • PerennialMimosa invisaTropical spiderwort • • PerennialCommelina benghalensisWild poinsettia • AnnualEuphorbia heterophyllaTridax • AnnualTridax procumbensGoat weed • AnnualAgeratum conyzoidesWaterleaf • • AnnualTalinum triangulare

Parasitic weedsDodder • • AnnualCuscuta australis

Perennial = persists for several years and present at all seasons of the year

Figure 16: Farmer showing stolon lengthof bermuda grass, Cynodon dactylon

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IPM Field GuideWeed Control in Cassava Farms

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How do weeds become aproblem in cassava farms?

Cassava grows poorly in weedy farms andproduces fewer and smaller storage roots.However, the presence of weed plants in yourfarm does not always mean that they willcause severe losses in food and income fromthe farm. Weeds become a problem in growingcassava because of a number of reasons.

Weeds grow abundantly: Weeds harm cas-sava mainly when they are abundant in thefarm. They will cover the ground almost com-pletely and increase the time you spend onweeding. When they are abundant, they willalso use up lots of nutrients and water fromthe soil, making these materials unavailable forcassava plant growth.

Weeds grow vigorously: Weeds grow morerapidly than cassava and this enables them to“choke” cassava farms very quickly. For ex-ample the giant sensitive weed, Mimosa invisa,rapidly occupies the spaces between cassavaplants (Figure 17); Siam weed, Chromolaenaodorata (Figure 3), chokes cassava farms in asimilar manner. By their rapid growth, weedsalso shade cassava plants from sunlight.

Weeds are difficult to remove: Weeds aredifficult to remove for a number of reasons.For example, if the thorns of the giant sensi-tive weed, Mimosa invisa, hook into young cas-sava stems, it will be difficult to remove theweed without breaking the stems. Similarly,the parasitic weed dodder, Cuscuta australis(Figure 18), which climbs and wraps tightly oncassava plants, causes breakage of cassavashoot tips during hand weeding. Weeds canalso be difficult to remove if they injure

people. For example, it is difficult to removeMimosa invisa (Figures 9 and 17) by hand-weeding or hoeing because its thorns scratchand cause wounds to people. Similarly, thehairy pods of Mucuna pruriens var. pruriens(Figure 18), and several hairy weeds cause in-tense itching.

Weeds which reproduce by rhizomes (Figure14), stolons (Figure 15), and tubers (Figure 16)are difficult to remove from the soil. Theybreak easily into pieces during hand or hoeweeding. The pieces remain in the soil andsprout later. Weeds with these structures willtherefore be “stubborn” in the farm. Examplesof such stubborn weeds are spear grass,Imperata cylindrica (Figure 1), bermuda grass,Cynodon dactylon (Figure 4), and the purplenutsedge, Cyperus rotundus (Figure 7).

Some weeds grow on cassava: Certainweeds grow and feed directly on cassavastems. For example dodder, Cuscuta australis(Figure 19), attaches itself tightly to cassavastems and sucks water and nutrients from theplant.

Weeds harbor pests: Cassava pests anddiseases can multiply on weeds and latermove on to cassava. For example, immaturestages of the variegated grasshopper,Zonocerus variegatus, gather on bushes ofChromolaena odorata (Figure 20) and thenmove on to cassava plants as they becomemature grasshoppers.

Weeds injure cassava roots: The emergingshoots of spear grass, Imperata cylindrica,sometimes pierce and destroy cassava stor-age roots.

Figure 17: Thick growth of giant sensitiveweed, Mimosa invisa, in a cassava farm

Figure 19: The stem parasitic plantdodder, Cuscuta australis, covering acassava plant

Figure 20: Nymphs of variegatedgrasshopper on Siam weed,Chromolaena odorata

Figure 18: Pods of Mucuna pruriens var.pruriens

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IPM Field GuideWeed Control in Cassava Farms

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How can I best control weeds incassava farms?

Under traditional farming conditions and onsmall cassava farms, weeds are commonly con-trolled by slashing, or hand or hoe weed-ing. This can be very labor intensive. The bestway to control weeds in a cassava farm is tocombine different cultural practices, especiallyat land clearing, seedbed preparation, plantingand post-planting stages of growing the crop. Ifherbicides are to be used, you should consultthe label for guidelines on their applicationmethods and how to avoid personal and envi-ronmental hazards associated with their use.

IPM practices at planting

Prepare your land properly: When clear-ing land for planting cassava in wet areas, youcan slash the bush before the rains start.Weeds will emerge on the cleared land soonafter the rains. You can then remove theseweeds as you plant cassava stem cuttings. Thiswill greatly reduce the frequency and labor ofweeding in the crop. When preparing seed-beds for planting cassava, remove weed rhi-zomes, stolons, tubers, and other structuresthat germinate and spread the weed in thefarm. Farm tillage tools should also bechecked and cleaned from time to time tomake sure they are free of these weed struc-tures, otherwise you will spread the weedsthrough contaminated tools.

Grow suitable cassava varieties: Some cas-sava varieties have an early, low, and muchbranching habit (Figure 21). This habit allowsthese varieties to suppress weeds because theydevelop a lot of branches and leaves which

form a thick canopy that shades weeds andprevents them from growing properly. Cassavavarieties with a late, high and less branchinghabit (Figure 22) are not very good at sup-pressing weeds. In selecting suitable cassava va-rieties against weeds, you should find out if theselected variety also has other features youmay want. Table 2 indicates the weed suppres-sion ability and other features of some cassavavarieties available in West and Central Africa.

Mulch cassava seedbeds with dead plantfoliage: Mulching involves covering the soilsurface with very large amounts of plant foli-age. The companion IPM guide, “Starting a Cas-sava Farm”, presents mulching as a soil im-provement practice. In addition to improvingsoil properties, mulching reduces weed prob-lems. Good sources of mulch materials arefoliage from alley crops, leguminous plants,rice husk, coffee hull, and general crop andweed residues. Straws of maize and guineagrass (Figure 5) are bad mulch materials be-cause they take too long to rot. Avoid usingweed residues containing weed seeds, rhi-zomes, stolons or tubers as mulch becausethese can cause weed problems on your farm.

Use cover crops as live mulch on seed-beds: You can use food crops such as egusimelon as cover crops, and/or plant intercrops(such as maize) to reduce weed infestation incassava farms. Egusi melon is a good “livemulch” in cassava farms (Figure 23). If you de-cide to use egusi melon as live mulch youshould plant it before planting the cassava. Theegusi melon should be planted at very closespacing on the seed beds to enable it tospread and cover the soil very quickly.

Figure 22: Cassava variety with late,high, and less branching habit

Figure 21: Cassava variety with early,low, and much branching habit

Figure 23: Cassava in a live mulch of egusimelon intercropped with maize

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IPM Field GuideWeed Control in Cassava Farms

18

In this way it suppresses many types of weeds,especially annual weeds. However, egusi melonis not very effective against stubborn grasseslike spear grass, Imperata cylindrica.

Plant cassava in association with othercrops: You can also reduce weed problems incassava farms by intercropping (mixing) cas-sava with other crops at planting. Crops thatare commonly intercropped with cassava aremaize (Figure 23), rice, grain legumes, and veg-etables. It is helpful to intercrop cassava withgrain legumes such as cowpea and ground-nuts, which manufacture and release nutrientsinto the soil.

IPM practices after planting

It is important that you weed at the right timeto prevent weeds from competing with cas-sava storage root formation and bulking (thefilling of storage roots with food/starch) andreduce other damage the weeds may cause tocassava. When cassava is planted alone (as asole crop), it is best to weed three times be-fore harvest: at 3–4 weeks after planting, 7–8weeks after planting, and 12 weeks after plant-ing. After this you can slash any weed re-growth to make it easy to work between theplants and also to discourage rodents.

The frequency and labor required to handweed will be greatly reduced if you:

• cleaned cassava seedbeds of rhizomes,stolons, and tubers,

• planted cassava varieties that are capable ofsuppressing weeds,

• adopted suitable intercrops, and• mulched where necessary.

If herbicides are required to kill weeds, youshould consult the label for guidelines on theirapplication methods and how to avoid per-sonal and environmental hazards associatedwith their use.

The companion field guide “Pest Control inCassava Farms” explains that certain weedsprovide shelter for natural enemies of pests,and thereby play useful roles in pest control.Natural enemies are commonly insects andmites which feed on pests. For example,certain natural enemies of the cassava greenmite (Figure 24) live on the weeds Euphorbiaheterophylla (Figure 8) and Mallotusoppositifolius (Figure 25). Such weeds aretherefore useful in the control of the pest.During weeding you can leave the weeds togrow along the margins or in other parts ofcassava farms to attract the natural enemies.This cultural practice will be especially usefulat sites where cassava is grown continuouslywith little or no fallow.

Figure 24: Cassava green mites (as seenenlarged under the microscope)

Figure 25: Shoot of Mallotus oppositifolius

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IPM Field GuideWeed Control in Cassava Farms

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Improved fallow practices

Non-food crops can be used as “live mulches”,but these are normally grown as improved fal-low plants. For example, during fallow periodsyou can grow Mucuna pruriens var. utilis (Figure26) on land you select for growing cassava inthe following season. The growth of Mucunawill completely cover the land and greatly re-duce the population of weeds, the frequencyof weeding and the labor required to weed inthe following season. At cassava planting timeyou can plant the cassava stem cuttings eitheron mounds or ridges, or directly into themulch covering with little or no land clearing.Mucuna pruriens var. utilis is effective againstspear grass, Imperata cylindrica, and manyother stubborn weeds. It produces lots ofleaves which stay green and cover weeds forup to 9 months after it has been planted. Bythe time it produces seeds and dies it will havecontrolled most of the weeds on the land. Im-proved fallow with Mucuna pruriens var. utilis istherefore good for long-term weed controland soil improvement. Mucuna pruriens var.utilis is, however, a fire hazard in the dry seasonwhen its foliage dries.

Canavalia ensiformis (Figure 27) is another le-guminous fallow plant that is effective againstspear grass and other stubborn weeds.Canavalia ensiformis should be planted at veryclose spacing in order for it to be effective.However, the leaves of Canavalia ensiformisdrop early, usually at 4–5 months after plant-ing. After this period, if weeds receive ad-equate sunlight and water they will growabundantly and “undo” the benefits gainedfrom the improved fallow plant. Therefore, inzones with two wet seasons a year it will be

Figure 27: Farmer in his live mulch ofCanavalia ensiformis

Figure 26: Live mulch of Mucuna onfallow land

necessary to plant Canavalia ensiformis twice ayear, early in the first and second wet seasons.Canavalia ensiformis is therefore good as ashort duration fallow crop and largely usefulin areas where farmers cannot afford to leave

the land to fallow plants for long periods. Theseeds of Canavalia ensiformis can be used asfood and/or animal feed.

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IPM Field GuideWeed Control in Cassava Farms

22

Summary

• Remove weed rhizomes, stolons, and tubers from cassava seed beds to re-duce weed problems.

• Mulch cassava seed beds with live or dead mulch materials to reduce weedproblems and improve soils.

• Grow cassava varieties with early, low, and much branching habit; these willsuppress weed growth better than varieties with late, high, or no branchinghabits.

• Intercrop cassava with appropriate crops to reduce weed problems and im-prove soils.

• Use improved fallow plant as “live mulch” on land for planting cassava.

• Hand weed three times within 3 months after planting cassava; this will re-duce weed competition with cassava for nutrients.

• Combine the most appropriate weed control practices for more effectivecontrol of the weeds.

Acknowledgements

Special thanks to the United Nations Development Programme and the Austrian governmentwhich provided funds, and to the following institutions which provided materials, informationand services for the production of the set of cassava IPM field guides:

• Agricultural Development Programmes (ADPs) in Abia, Akwa Ibom, Anambra,Benue, Cross Rivers, Rivers, Delta, Edo, Enugu, Imo, Kogi, Kwara, Ogun, Ondo, Osun,Oyo, and Plateau State Governments, Nigeria

• Centre d’Action Régionale pour le Développement Rural (CARDER), Bénin

• Centro International de Agricultura Tropical (CIAT), Cali, Colombia

• Crop Services Department (CSD), Department of Agricultural Extension Services(DAES), and Plant Protection and Regulatory Services Department (PPRSD), Minis-try of Food and Agriculture, Ghana

• Federal University of Technology, Owerri, Nigeria

• Fourah Bay College, University of Sierra Leone, Freetown, Sierra Leone

• IITA Eastern and Southern African Regional Centre (ESARC), Uganda

• Institut de Recherche Agronomique et du Développement (IRAD), Cameroon

• Institut National de Recherche Agronomique du Bénin (INRAB), Bénin

• National Root Crops Research Institute (NRCRI), Umudike, Nigeria,

• Rural Training Centre (RTC, Presbyterian Church) in Fonta and Kumba, Cameroon

• Sasakawa Global 2000, Bénin

• Service de Protection des Végétaux et du Contrôle Phytosanitaire (SPVC), Bénin

• Southern African Root Crops Research Network (SARRNET), Malawi

• University of Agriculture, Abeokuta, Nigeria

• University of Buea, Buea, Cameroon

• University of Cape Coast, Cape Coast, Ghana