# Parasitic plant
A parasitic plant is a flowering plant that obtains water, mineral nutrients, and photosynthates directly from another living plant, its host, through a specialized organ called the haustorium. About 1% of flowering plant species, roughly 4,000 to 4,750 species, have this growth habit[1](https://www.nhm.ac.uk/discover/parasitic-plants.html) • [2](https://doi.org/10.1146/annurev-ecolsys-102320-115331). The group includes the world's largest flowers, economically important crop pests, and plants that shape the composition of natural communities[3](https://www.obga.ox.ac.uk/evolution-parasitic-plants).
| Key fact | Detail |
|---|---|
| Species count | Approximately 4,750 described flowering plant species have a parasitic growth habit, about 1% of the more than 374,000 described flowering plants[2](https://doi.org/10.1146/annurev-ecolsys-102320-115331) • [1](https://www.nhm.ac.uk/discover/parasitic-plants.html) |
| Defining organ | The haustorium attaches to the host, penetrates its tissues, and fuses with the host's xylem and phloem to extract water and nutrients[4](https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-01-18-0020-FE) |
| Photosynthetic types | Hemiparasites retain chlorophyll and photosynthesize; holoparasites lack chlorophyll and draw all water, minerals, and photoassimilates from the host[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8418411/) |
| Attachment site | Parasites are classified as root or stem parasites according to the host organ they connect to[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8418411/) |
| Evolutionary origins | Parasitism has been acquired independently 12 times in extant flowering plants; all but three of the resulting lineages contain one to three genera and fewer than 40 species[2](https://doi.org/10.1146/annurev-ecolsys-102320-115331) |
| Host detection | Obligate root parasites such as *Striga* and *Orobanche* require host-derived strigolactones to germinate, and strigolactones also guide directional growth toward the host root[6](https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202519030) |
| Agricultural impact | Parasitic weeds in the genera *Striga*, *Orobanche*, and *Phelipanche* attack cereals, legumes, and oilseed crops and can destroy yields[1](https://www.nhm.ac.uk/discover/parasitic-plants.html) |
## Classification
Two traits dominate the classification of parasitic plants. The first is photosynthetic modality: hemiparasites possess chlorophyll and can photosynthesize but still draw water and nutrients from hosts, while holoparasites lack chlorophyll and depend entirely on hosts for water, minerals, and photoassimilates[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8418411/). The second is the site of attachment, which yields four commonly recognized types: root hemiparasites, stem hemiparasites, root holoparasites, and stem holoparasites[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8418411/). A further distinction separates obligate parasites, which cannot survive without a host, from facultative parasites, which can live and reproduce without one[1](https://www.nhm.ac.uk/discover/parasitic-plants.html).
A small group of endoparasites takes the habit to an extreme. Species such as *Rafflesia*, *Pilostyles*, *Cytinus*, and *Mitrastemon* spend their entire lives inside host tissues as filaments or cell masses, emerging only to flower and set seed[3](https://www.obga.ox.ac.uk/evolution-parasitic-plants). Functional classification based on photosynthetic capacity remains overly simplistic, and recent work proposes frameworks built instead on life-history stages[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8418411/).
## The haustorium
The haustorium is the organ that defines parasitism: if a haustorium is present, the plant is a parasite[7](https://s3.amazonaws.com/bi-dam-prod/6439851bd4d392879a059073/3831201a0a51e0a0d61cf9c68b269315a563bfa1/9781789247633_23_898a3417526120503fe2e7274ac4f6bd.pdf). It attaches to the host, invades its tissues, and fuses into the xylem and phloem conductive systems, enabling the parasite to extract water and nutrients[4](https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-01-18-0020-FE). Haustoria are multicellular invasive organs that can sometimes overcome tissue incompatibilities, allowing many parasitic plants to attack taxonomically distant host species[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC11207070/).
The organ can be mechanically remarkable. The Australian Christmas tree, *Nuytsia floribunda*, connects to the roots of neighboring plants and severs their xylem vessels with a guillotine-like structure in its haustoria; this cutter is strong enough to chop through underground fiber optic and power cables[1](https://www.nhm.ac.uk/discover/parasitic-plants.html).
## Finding and invading a host
Parasitic plant seeds carry limited nutrient reserves, so parasites must detect and locate a host quickly. Root parasites in *Orobanche* and *Striga* germinate only in the presence of a suitable host, responding to strigolactones exuded from host roots; these compounds act both as germination stimulants and as chemoattractants directing growth toward the host[6](https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202519030) • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC11207070/). The stem parasite *[Cuscuta](https://www.edgepedia.org/cuscuta)* (dodder) has no roots or leaves; its seedlings must find a host within 5 to 10 days before their energy runs out, and they can sense which nearby plants would make the best hosts and grow toward them[1](https://www.nhm.ac.uk/discover/parasitic-plants.html).
Once contact is made, host-derived compounds induce haustorium formation in orobanchaceous parasites. These haustorium-inducing factors include 2,6-dimethoxy-1,4-benzoquinone (DMBQ), flavonoids, and phenolic compounds, which share a redox-active methoxyphenol moiety and likely originate from host cell wall lignin[6](https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202519030). [Reactive oxygen species](https://www.edgepedia.org/reactive-oxygen-species), particularly hydrogen peroxide, are essential for haustorium induction, and the receptor CARD1 has been identified as the receptor for DMBQ governing haustorium formation in orobanchaceous parasites[6](https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202519030). In *Cuscuta*, by contrast, haustorium formation does not require host-derived compound induction[6](https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202519030).
## Evolution
The shift to parasitism has occurred repeatedly and independently. Twelve independent acquisitions of parasitism are recognized among extant flowering plants, and the resulting lineages are mostly small: all but three are represented by one to three genera and fewer than 40 species[2](https://doi.org/10.1146/annurev-ecolsys-102320-115331). The family [Orobanchaceae](https://www.edgepedia.org/orobanchaceae) is unique among parasitic plants in that its extant species span the full range of host dependence, from facultative to obligate parasites[9](https://www.sciencedirect.com/science/article/pii/S1360138510000075).
Parasitism long obscured evolutionary relationships because it is associated with the degeneration of features traditionally used in plant classification, such as well-developed leaves[3](https://www.obga.ox.ac.uk/evolution-parasitic-plants). Molecular phylogenetics has since overturned older groupings: species once placed in the Rafflesiaceae belong to at least four distinct clades with independent evolutionary origins, and the stem-parasitic growth form, once gathered in a single large family, is now known to have evolved five times within three families[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8418411/).
The intimate connection with hosts leaves genetic traces. [Horizontal gene transfer](https://www.edgepedia.org/horizontal-gene-transfer), the rare exchange of genetic material between non-mating organisms, is unusually frequent in parasitic plants; plants in the Rafflesiaceae contain genes from cucumber and carrot relatives, presumably reflecting past host associations, and these transfers can serve as DNA fossils for reconstructing ancient relationships[3](https://www.obga.ox.ac.uk/evolution-parasitic-plants). Host shifts may also drive speciation, a process under study in genera such as *Hydnora*[3](https://www.obga.ox.ac.uk/evolution-parasitic-plants).
## Notable examples
**Rafflesia arnoldii**, the giant padma, produces the world's largest flower, up to one meter across and weighing up to 11 kilograms. It cannot photosynthesize and steals nutrients from the roots of nearby *Tetrastigma* vines; its smell attracts flies for pollination[1](https://www.nhm.ac.uk/discover/parasitic-plants.html).
**Cuscuta** (dodder) is an aggressive parasite with yellow or orange stringy stems and no roots or leaves. It drains its host's nutrients until it has killed the plant[1](https://www.nhm.ac.uk/discover/parasitic-plants.html).
**Mistletoes** in the order [Santalales](https://www.edgepedia.org/santalales) are stem hemiparasites; the dwarf mistletoe *Arceuthobium americanum* spreads by rupturing its fruits and shooting single seeds at speeds of up to 90 kilometers per hour from treetop to treetop[1](https://www.nhm.ac.uk/discover/parasitic-plants.html).
**Striga** (witchweed) and the broomrapes **Orobanche** and **Phelipanche** are root parasites of crops. *Striga* targets cereal, maize, millet, and rice crops in Africa, Asia, and Australia and is entirely dependent on its host for the first four to six weeks of life; broomrapes attack legumes, oilseed crops such as canola and sunflower, and solanaceous crops such as tomato and potato[1](https://www.nhm.ac.uk/discover/parasitic-plants.html).
**Rhinanthus minor** (yellow rattle) is a root hemiparasite of grasses that slows grass growth, giving other plants a chance to thrive, which makes it an important element of meadow biodiversity[1](https://www.nhm.ac.uk/discover/parasitic-plants.html).
## Host defenses and agriculture
Host plants perceive and resist parasites both before and after attachment. Pre-attachment resistance includes producing no or reduced germination stimulants, producing germination inhibitors, delaying or inhibiting haustorium formation, and developing preformed mechanical or structural barriers on the host surface[10](https://www.annualreviews.org/doi/pdf/10.1146/annurev-phyto-082718-100043). Most current research overlooks organ specificity in these responses, yet cross-organ parasitism occurs, for example when stem parasites form haustoria on host roots; understanding how different tissue types respond could support a universal resistance mechanism in crops against both root and stem parasites[11](https://www.annualreviews.org/content/journals/10.1146/annurev-arplant-102820-100635).
Damage can be rapid and severe. *Striga* infection produces a striking early effect on hosts, wilting and chlorosis within the very first stages of infection[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC11207070/). The haustorial connection also facilitates the movement of viruses, proteins, and RNA between parasite and host, so parasitic plants can act as vectors of phytopathogens[3](https://www.obga.ox.ac.uk/evolution-parasitic-plants) • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC11207070/).
## References
1. [Unwanted guests: The weird world of parasitic plants, Natural History Museum, London](https://www.nhm.ac.uk/discover/parasitic-plants.html)
2. [Functional Roles of Parasitic Plants in a Warming World, Annual Review of Ecology, Evolution, and Systematics](https://doi.org/10.1146/annurev-ecolsys-102320-115331)
3. [The evolution of parasitic plants, Oxford Botanic Garden and Arboretum](https://www.obga.ox.ac.uk/evolution-parasitic-plants)
4. [Parasitic weeds and their hosts (Plant Disease, APS)](https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-01-18-0020-FE)
5. [Life history, diversity, and distribution in parasitic flowering plants, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8418411/)
6. [Parasitic Plant-Host Interactions: Molecular Mechanisms and Agricultural Resistance Strategies, Advanced Science](https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202519030)
7. [Parasitic plants chapter, book PDF (Chapter 1)](https://s3.amazonaws.com/bi-dam-prod/6439851bd4d392879a059073/3831201a0a51e0a0d61cf9c68b269315a563bfa1/9781789247633_23_898a3417526120503fe2e7274ac4f6bd.pdf)
8. [Parasitic Plants: Potential Vectors of Phytopathogens, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11207070/)
9. [The evolution of parasitism in plants, Trends in Plant Science](https://www.sciencedirect.com/science/article/pii/S1360138510000075)
10. [Molecular Dialog Between Parasitic Plants and Their Hosts, Annual Review of Phytopathology](https://www.annualreviews.org/doi/pdf/10.1146/annurev-phyto-082718-100043)
11. [Parasitic Plants: An Overview of Mechanisms by Which Plants Perceive and Respond to Parasites, Annual Review of Plant Biology](https://www.annualreviews.org/content/journals/10.1146/annurev-arplant-102820-100635)
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*Topic: Encyclopedia › Life and health › Plants and algae › Seed plants › Other flowering plants › Asterids › Apiaceae: carrot and parsley family*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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