Nematodes, commonly known as roundworms, are some of the most common and widespread animals in the world. Found in virtually all environments, from mountainous biomes to the ocean floor, these slender worms may be free-living or wholly parasitic. Measuring anywhere from 0.1 mm to a meter long, they come in approximately 30,000 – 1 million species. It’s truly no surprise that many of them are naturally present in the home garden.
Roundworms are often perceived as pests because their colonies can significantly damage landscape plants, patches of ornamentals, and entire vegetable beds. Root-knot nematodes (Meloidogyne spp.), for example, can quickly wipe out a wide range of crops. In the process, they rapidly spread throughout substrates via infested plants and transported soils. Other common species include the ring nematode (Criconemoides xenoplax), citrus nematode (Tylenchulus semipenetrans), and root lesion nematodes (Pratylenchus spp.).
Garden nematodes tend to be present in multiple life stages. In warm regions, they can rapidly move through their life cycle, becoming egg-laying adults in less than a month’s time. To prevent them from laying waste to your plots, it would be best to grow or rotate between rootstocks that are naturally resistant or repellent to nematodes. Nematode-suppressive plants, which may emit nematicidal chemicals in surrounding soils, should help protect vulnerable crops.
1) French marigold (Tagetes patula)
French marigold is an eye-catching, annual herb that provides a wealth of ecological services to both home gardens and wider landscapes. This highland plant comes in hundreds of lovely varieties, most of which produce some of the most vivid yellow and orange hues in nature. The floral heads are irresistible to pollinators and beneficial insects, including tachinid flies. These produce larvae with potential for use in pest control.
The main mechanisms through which marigolds repel pests are facilitated by their leaves and root systems. The leaves are packed with a pungent essential oil containing a variety of pesticidal phytochemicals. The roots are particularly useful in suppressing the spread and growth of root-knot nematodes. These emit a compound called alpha-terthienyl, which actively inhibits the development of nematode eggs.
Not all French marigold varieties are able to suppress nematodes to the same extent. Moreover, while a nuanced selection of varieties may repel certain species of worms, it might attract others. The physical and chemical parameters of differing substrates may also influence this plant’s effectiveness. In general, however, the best varieties for combatting plant-parasitic nematodes include ‘Tangerine’, ‘Nemagold’, ‘Petite Blanc’, and ‘Happy Days’.
2) Castor bean (Ricinus communis)
Castor bean is occasionally cultivated as a ‘cover crop’ in agricultural sites, fields, and orchards used for growing vulnerable cash crops. Essentially, cover crops aid in improving the quality of substrates, reducing erosion rates, suppressing weed growth, and repelling various pests. When these non-host plants cover a substantial surface area in between crops, nematodes may struggle to colonize the landscape. They are more likely to die of starvation as they can seldom migrate over large distances.
A member of the spurge or Euphorbiaceae family, castor bean is a flowering perennial. The sole member of its genus, it naturally occurs as a suckering shrub or small tree in optimal conditions. It is distinguished by its fairly large, palmate leaves, each of which may have a spread of 6 – 18 inches (15 – 46 cm)! This species’ flowers and spiky fruit capsules strongly contrast the appearance of the glabrous leaves.
While castor bean roots are able to repel pests by producing a host of pesticidal compounds, the plant is especially effective at killing nematodes when it is used as ‘green manure’. Keep in mind that it may display invasive signs of spread in wastelands and warm regions. Moreover, as it possesses extremely allergenic properties, its stands should not be maintained with bare hands.
3) Cowpea (Vigna unguiculata)
The humble cowpea, also commonly known as the black-eyed pea, is a suitable legume for cultivation in tropical regions. Equipped with specialized root nodules, it is often used in intercropping systems to increase the availability of nitrogen for plants in resource-poor areas. This efficient nitrogen fixer can also be used as a forage crop for many commercially important animals, including cattle.
Unsurprisingly, cowpea is likewise useful in nematode management. As a cover crop, however, its effectiveness is largely dependent on the specific cultivar used and the type of plant-parasitic nematode present. The infamous southern root-knot nematode can be suppressed by two cowpea cultivars – ‘Iron Clay’ and ‘Mississippi Silver’. These may reduce reproduction rates, to an extent, but their resistance may also be inconsistent when faced with isolated nematode populations from different regions.
Crop farmers should avoid cultivating cowpea cultivars that may serve as good hosts to plant-parasitic nematodes, especially if they intend to incorporate them into a crop rotation system. By using cultivars that are “poor hosts” in between more valuable crops, nematode isolates are less likely to spread. As a rule, aim to test the effectiveness of cultivars in a small area prior to planting them throughout the landscape.
4) Common vetch (Vicia sativa)
A member of the Fabaceae family of pea plants, the common vetch often persists as a weed in grainfields. This annual herb is also intentionally cultivated as a form of fodder for livestock or as green manure. It is distinguished by its lance-shaped leaves and its slender shoots, which can measure up to 6.5 feet (2 meters) long. Consisting of wedge-shaped leaflets, each leaf tapers into a branching set of tendrils. As is typical of pea plants, its seeds are found in leguminous pods.
In fields with pest problems, the common vetch may occasionally be grown as a rotation crop. Alternating it with more high-value and susceptible crops should help arrest the continuity of the nematode life cycle. In particular, the ‘Cahaba’ variety of this vetch species has been shown to have potential in the management of the southern root-knot nematode (M. incognita).
Considered a winter legume, the common vetch has also shown promise against the root-knot nematode (Meloidogyne hapla). Both the seeds and vegetative tissues contain cyanogenic compounds. When these compounds undergo hydrolysis, they subsequently release hydrogen cyanide. Over time, this was likely adopted as one of the plant’s chief defensive mechanisms against grazers.
5) Rapeseed (Brassica napus)
Set apart by the high volume of oils in its seeds, rapeseed is now heavily cultivated as one of the globe’s primary sources of vegetable oil. When this cruciferous plant is grown in extensive colonies, it can brighten up a landscape and spread to regions where it may persist as a non-native plant. Though it does have its fair share of pests, it is often used in rotations as a break crop. When alternately grown with susceptible crops like wheat and barley, it can impede the survival of some types of nematodes.
When incorporated into substrates as green manure, rapeseed may aid in the control of root-knot nematode populations. All parts of the plant contain a complex mixture of glucosinolates and sulfur-containing compounds. Though these are not necessarily harmful on their own, enzymatic reactions can convert them into byproducts with fungicidal and bactericidal properties. Moreover, they can enhance the bitter taste profile of the plant.
While rapeseed may not be the most ideal cover crop in small to medium-sized gardens or in plots with a more diverse array of vegetables, its capacity for rapid growth and spread makes it useful in intensive cultivation. As an alternative to nematicides, its facilitated growth would be more ecologically friendly and should help improve the quality of the soil.
6) Florida velvet bean (Mucuna pruriens)
Another member of the Fabaceae or pea family, the Florida velvet bean is a notably useful annual. In some parts of its native range, its beans are eaten or further processed to create a meat substitute that is similar to tempeh (made of soybeans). In tropical regions, it is often used as a fodder plant and as an ingredient in herbal remedies. Its eye-catching inflorescences provide it with ornamental value.
As suggested by its common name, this species produces seed pods that are notably fuzzy. The ripening fruit is densely covered in short, loose, and orange hairs. Capable of causing a severe itch, the hairs contain a protein called mucunain. Though the seeds are safe to eat in their cooked form, they can cause unpleasant symptoms if they are ingested raw.
Capable of producing dense vines, Florida velvet bean may be cultivated as a cover crop to discourage the spread of nematodes. It may effectively suppress the growth of southern, peanut, and Javanese root-knot nematode species, compromising their capacity to spread between plots of vulnerable plants. Take note that this species may become invasive, however, in disturbed lands and warm broadleaf forests.
7) Showy rattlepod (Crotalaria spectabilis)
This flowering plant is yet another member of the pea family. Unlike its cousins listed above, it is known for being toxic to livestock and for being a potentially problematic plant in agricultural landscapes. Nonetheless, it can be utilized to aid in the control of various lesion and root-knot nematodes, ideally in farms or fields where it is unlikely to be mistaken as fodder.
The showy rattlepod is particularly inhospitable to herbivorous pests because it contains monerotatum. This toxic ester functions as a nematicide towards a handful of plant-parasitic nematodes, highlighting the plant’s potential as a biopesticide. Its close cousin, Crotalaria juncea, likewise possesses suppressive properties, albeit to a milder effect. Its tissues contain monocrotaline, a compound that is toxic to phytoparasitic roundworms.
Toxic to large mammals and birds, Crotalaria or “sunn hemp” species should be cultivated with caution. Take note that showy rattlepod is now considered an invasive plant in the US. The allergic reactions caused by the ingestion of its leaves, roots, seeds, and stems likely help maintain the wild stands of these highly evolved pea plants.
8) Blue lupin (Lupinus angustifolius)
The blue lupin has a rich history as it has been cultivated for thousands of years. Also known as narrowleaf lupin, its palmate leaves consist of markedly narrow leaflets that seldom measure more than 1.5 inches (4 cm). Spritely and deep green, they arise on occasionally branching and erect shoots. In spring to summer, spike inflorescences with white, pink, blue, or purple blooms appear on the tips of mature stems.
Hardy to USDA zones 3 – 8, blue lupin is quite sensitive to high temperatures. In optimal conditions, its tissues possess high concentrations of lupinin, which is a toxic alkaloid. Nowadays, cultivars with low alkaloid concentrations tend to be available. Nonetheless, it’s likely that these compounds help protect wild colonies from nematode infestations, with a higher alkaloid content aiding more significantly in pest suppression.
As sources of anthelmintic alkaloids, members of the Lupinus genus can be grown as cover crops or rotation crops. Take note that their continuous cultivation may attract their own hosts of nematodes, so a system utilizing rotating plants should be more desirable for keeping pests at bay.
9) Red clover (Trifolium pratense)
Red clover is a stellar herb with well-known nitrogen-fixing capabilities. In some agricultural landscapes throughout its native range, this short-lived perennial is cultivated to both scavenge mineralized nitrogen and aid in the conversion of atmospheric nitrogen to plant material. Its symbiotic relationship with Rhizobium bacteria plays a crucial role in the process as it supplies the latter with the necessary growth media for cycling nitrogen.
Apart from its role in the carbon and nitrogen cycles, red clover is useful as a cover crop through winter. Its ‘Cherokee’ variety has potential as a biocontrol agent against several root-knot nematodes. Researchers have proposed its use in a corn-soy-wheat rotation. When it is used as a living mulch in the corn phase of this rotation system, it may aid in interrupting the life cycle of the soybean cyst nematode.
Red clover is a great addition to ornamental gardens because it is easy to maintain and can be relied upon to thrive in a wide range of environments. Its pink blooms unfailingly attract a wealth of pollinators and beneficial insects, making them wonderful components of natural corridors in increasingly fragmented ecosystems.
10) Chrysanthemum ‘Escapade’ (Chrysanthemum sp.)
Set apart by their solitary and stunning flower heads, chrysanthemums are some of the most striking additions to ornamental gardens. Easy to grow, they can effectively brighten up a spring-to-summer arrangement while attracting dozens of beneficial insects and lively pollinators. As companion plants, they can help your crops flourish by repelling many common pests.
The southern root-knot nematode (Meloidogyne incognita) may be effectively suppressed by dense stands of ‘Escapade’. This variety is notable for the delicately pink petals of its flowerheads. The roots produce botanical toxins that should help control populations of pesky nematodes in the soil. Moreover, any fallen blooms should release pyrethrins, which function as pest-repellent proteins, as they disintegrate.
