Soil chemistry and soil biology are big, complicated subjects. They’re also important to most gardens; unless we’re cultivating air-feeding plants our gardens most likely contain some form of soil. We’ve written before about how the soil biome affects plant health and how soil amendments and fertilizers change soil chemistry. But there’s always more to say on the subject of dirt, and for this article we’ll be tackling a big and complex component of soil health: nematodes. Most of us have heard the word “nematode” and many of us may know that they’re a tiny organism living in the soil. But what are they, exactly, and what do nematodes do in the soil? Join us as we explore the good and bad of nematodes, what nematodes do in the soil, and how they affect the health of our gardens. Let’s dig in!
The Biology of Nematodes
The biology of nematodes is interesting in and of itself and it’s a great place to start when exploring the role of nematodes in soil biology and garden health. Nematodes are also referred to as roundworms or eelworms, and they’re related to the flatworms of the phylum Platyhelminthes. They differ, however, in that nematodes have a tubular digestive system, so that food comes in one end and waste goes out the other; flatworms only have one opening to their digestive systems and thus cannot process food continually.
Contrary to some popular nomenclature, nematodes are not generally microscopic—at least not in their mature state. While most species of soil nematode are between 1mm and 3mm in length, some can be up to 7mm long when fully grown. Aquatic and parasitic nematodes can get even longer, up to 50cm in length!
Nematodes are widespread, and members of the phylum Nematoda are found around the world in deserts, subarctic regions, temperate zones, and even in the depths of the ocean. With so much diversity and such a wide range of habitats, interactions between humans, agriculture, and nematodes are complex. Let’s explore that further.
Nematodes, Plants, and Humans
Nematodes are everywhere, which means they interact with human beings and human-growing plants regularly. With so many kinds of nematodes over such a wide range of environments, those interactions are complex. Some are beneficial to human agriculture and gardening, some are detrimental, and some are just benign.
Helpful nematodes are pretty great. Some nematodes of the family Rhabditidae are parasitic and target slugs and snails that eat garden plants. Even more useful to gardeners and farmers, nematodes from the genus Steinernema target a wide swath of garden pest species including cutworms, army worms, weevils, wood-borers, and plant-eating moths. For those of us interested in organic pest control, some companies do sell nematodes or blends of nematodes designed to limit the spread of pests in our gardens. We’ve never used it ourselves but we’re interested now and we’d love to hear from any readers who have.
While some nematodes are useful to gardeners, some are harmful. Here at the Gardzen Test Gardens in southeastern North Carolina, USA, we generally grow tomatoes in containers and raised beds to avoid contact with harmful ground nematodes that limit tomato growth. Globally, the root-knot nematode (Meloidogyne spp.) is a perennial issue for anyone growing plants in hot climates with short (or no) winter. Root-knot nematodes take their name from the hard, round galls that form on the root of a plant in which they’ve taken up residence. These galls are the problem; they drain energy from the plant and prevent the roots from gathering nutrients and water. As a result, the plants are weaker, less productive, and may eventually die. Globally root-knot nematodes account for at least 5% of crop losses and cost our collective economy billions of dollars in lost food.
Controlling Nematodes in Home Gardens
If you suspect that your garden, your crops, or your landscaping plants are suffering from harmful nematodes, the first step is getting the soil tested. Early to mid-autumn is generally the best time to test, but we strongly recommend reaching out to your state/provincial department of agriculture or your local cooperative extension for localized guidance.
Once the presence of nematodes has been established and the type of nematode is known, remediation and control can begin. Again, listen to your local experts and follow their advice but controlling nematodes in home gardens may involve a number of different approaches. Chemical control of nematodes is not recommended for home gardens. The relevant pesticides are expensive, toxic to work with, and generally only available in large quantities. Home gardeners are generally better off focusing on other modes of nematode control:
· Sanitation is a big step in nematode control. Infected plants should be removed quickly and disposed of in the trash, not the compost heap. Tools—including hands/gloves and the shoes we wear—should be cleaned after working with nematode-infested soil. Above all avoid spreading soil that has nematodes in it; moving infected soil around is a fast way to ensure the problem only gets worse!
· Grow resistant plant varietals in order to slow and eventually even eliminate nematode infestations. Some resistant plants might surprise you: brassica including broccoli can help limit the spread and propagation fo nematodes, as can marigolds. These plants aren’t suitable for nematode reproduction and thus the population gradually dies off. There are resistant varieties of tomatoes, beans, peas, and other popular garden plants, so give them a try.
· Solarization is a very cool word and an even cooler way to eliminate nematodes. Till and clear the soil, water deeply, and then spread a 2- to 4-mil sheet of clear plastic over the affected area. Weigh down the edges with loose soil to form a “good enough” seal and leave in place for six to eight weeks. The heat and humidity will build up in the soil and eventually kill off nematodes at all stages of their life cycle. When done, add additional clean organic material, re-till, and start again.
As with all things in the garden, prevention is better than cure. By being careful with the plants, soil, and soil amendments we introduce to our gardens we can help prevent the spread of many garden pests, including nematodes. Avoidance is always easier than solving the problem after it has established itself, so careful screening and sanitation to prevent nematode spread is a smart move.
Understanding Nematodes
Nematodes are neither good nor bad. They’re just another organism living life as it evolved to do, and it’s our job to understand them and work with and around them as needed. With a better understanding of both the beneficial and harmful roles nematodes can play in our gardens we can work with both of them to ensure our plants stay healthy. We can also learn a bit more about soil, soil biology, and how various organisms interact with plants. We hope this guide helps you understand what nematodes are and how they act in our gardens. If you have tips for dealing with nematodes in the garden (especially if you’re from eastern North Carolina) we’d love to hear from you. Gardzen is all about community so please reach out via email or in the comments!
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