🪱Parasitic and Aquatic Weeds

When Weeds Feed on Crops

In the previous lesson, we explored the 12 classification systems of weeds, including a brief mention of parasitic and aquatic weeds under “specificity.” Now we dive deep into these two special categories that behave very differently from ordinary weeds — they do not merely compete, they directly parasitise host plants or infest water bodies.

In the tobacco fields of Andhra Pradesh, farmers sometimes notice stunted, yellowing plants long before any above-ground intruder is visible. Underground, Orobanche (broomrape) has already latched onto the tobacco roots, silently draining water and nutrients. By the time its fleshy purple flower stalk breaks through the soil, the damage is done.

This lesson covers:

1. Four types of parasitic weeds — total/partial, root/stem combinations
2. Aquatic weed categories — submerged, emerged, marginal, floating
3. Control methods for parasitic and aquatic weeds
4. Taxonomic families of important weeds

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Parasitic Weeds — Overview

Parasitic weeds are classified on two axes:

1. Degree of dependence: Total (holoparasite — no chlorophyll) vs Partial (hemiparasite — develops chlorophyll later)
2. Site of attachment: Root parasite vs Stem parasite

This creates four distinct types:

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Type 1: Total Root Parasite (Holoparasite)

Takes all nutrition from host roots. Completely lacks chlorophyll — appears as non-green, fleshy structures emerging from soil near the host.

• Orobanche sp. (Broomrape) parasitises tobacco, tomato, fababean, chickpea, mustard, chilli, brinjal, potato — mostly Solanaceous crops
• Attaches to host roots underground; difficult to detect until it emerges to flower
• Control: Long crop rotation with tobacco once in 3 years, preceded by chilli as a trap crop (stimulates Orobanche germination but chilli is a poor host, so the parasite dies without reproducing)

Type 2: Partial Root Parasite (Hemiparasite)

Depends on host roots during the underground stage, but after emergence becomes green and photosynthetic — a “semi-independent” parasite.

• Striga hermonthica / asiatica (= lutea) (Witch weed) attacks sorghum, maize, sugarcane, sunflower
• Called “witch weed” because it causes severe crop damage before it even emerges — the crop appears bewitched
• Striga gesnerioides specifically parasitises cowpea

Type 3: Total Stem Parasite (Holoparasite)

Takes all nutrition from the host stem through specialised haustoria. No chlorophyll — appears as yellow or orange thread-like vines wrapping around the host.

• Cuscuta campestris / chinensis / epilinum (Dodder) parasitises alfalfa (lucerne), niger, linseed respectively
• Earlier placed in family Convolvulaceae; now in separate family Cuscutaceae
• Control: Float fenugreek and lucerne seeds in 20% salt solution before sowing — lighter Cuscuta seeds float to the surface for easy removal

Type 4: Partial Stem Parasite (Hemiparasite)

Initially depends on host stem, but later becomes green and can photosynthesise. Typically attacks perennial woody plants.

• Loranthus longiflorus (Birdvine) — found on mango and other fruit trees; birds disperse its sticky seeds
• Cassytha filiformis on orange, eucalyptus — twines anti-clockwise like Cuscuta but is greener (partial photosynthesis)
• Control: Spray of Glyphosate + 0.2% on infected branches

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Comparison Table: Four Types of Parasitic Weeds

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Aquatic Weeds

While parasitic weeds attack crops directly by feeding on them, aquatic weeds create a different set of problems entirely. They grow in water and complete at least part of their life cycle in water, choking waterways, impeding irrigation, reducing dissolved oxygen, and lowering fish production. Unlike terrestrial weeds that compete in fields, aquatic weeds disrupt water infrastructure critical for agriculture. They are classified by their position relative to the water surface.

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1. Submerged Weeds

Produce most vegetative growth beneath the water surface. Leaves are thin and finely divided to maximise surface area for absorbing dissolved gases.

• Examples: Utricularia stellaris, Ceratophyllum demersum, Hydrilla

2. Emerged (Emersed) Weeds

Rooted in bottom mud, with aerial stems and leaves at or above the water surface. They do not rise or fall with changing water levels — rigid stems hold them in place.

• Examples: Nelumbium speciosum (Lotus), Jussiaea repens

3. Marginal Weeds

Emergent rooted weeds found in moist shoreline areas with water depth up to 0.5 metres (60-90 cm). They form dense stands along pond, lake and canal edges.

• Examples: Typha sp. (Cattail), Polygonum sp., Cephalanthus, Scirpus

4. Floating Weeds

Leaves float on the water surface. Some are free-floating (not attached to the bottom), others are rooted with floating leaves. Free-floating types spread most rapidly.

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Comparison of Aquatic Weed Types

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Obsolete Mechanical Controls for Aquatic Weeds

Both methods are rarely used today due to high cost and environmental disturbance.

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Families of Important Weeds

Beyond the parasitic/aquatic distinction, knowing the taxonomic family of a weed is essential because herbicide selectivity often works at the family level. A herbicide effective against Poaceae (grasses) may be useless against Cyperaceae (sedges) or Asteraceae (composites).

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Summary Cheat Sheet