Nematodes: Good or Bad in the Garden?

by Erin Marissa Russell

Nematodes in the garden can be beneficial, helping gardeners to aerate soil and produce a bumper crop, or they can be parasitic to plants. This article will help gardeners learn the difference between helpful and harmful nematodes as well as what to do to rid the garden of the nematodes that can spell disaster.

What Are Nematodes?

You may have heard that nematodes are worms, but that’s not the whole story. Specifically, nematodes are unsegmented roundworms. They are not the same creatures as earthworms, segmented worms called annelids, or flat and slimy worms called flatworms.

How Can Nematodes Benefit Gardeners?

Most of the nematodes in the garden are beneficial to soil and plants. They feed on the organisms that can harm crops, such as bacteria, fungi, and other microscopic organisms. Some gardeners may even use nematodes to help control the population of insects that are parasitic to plants.

Entomopathogenic nematodes, also known as beneficial nematodes, include colorless roundworms from the families Steinernematidae and Heterorhabditidae. These worms are usually microscopic, with non-segmented bodies that have an elongated shape. They live in soil, so they can be put to work defending the garden against insects that come from the soil, but they are unfortunately useless against pests that live in the leaf canopy on on the plants themselves.

Nematodes can help gardeners defend against beetles, caterpillars, cutworms, crown borers, corn rootworms, crane flies, fungus gnats, grubs, and thrips. They will not have an effect on beneficial organisms such as earthworms, plants, animals, or humans, so they are a natural way to defend against pests that’s good for the environment.

There are more than 30 species of beneficial nematodes out there, and each species targets a specific host organism. That means the type of nematode a gardener should deploy depends on which pest they’re fighting against. Nematodes come into contact with pests during the fourth part of their five-part life cycle, which consists of egg, four larval stages, then an adult stage. During the third larval stage, beneficial nematodes seek out their pest counterpart, usually a larval insect, and enter its body, transferring Xenorhabdus sp. bacteria that will lead to the insect’s death in just a day or two. The nematodes will then consume the host’s body, eventually leaving it behind in their third juvenile phase.

There is no immunity to the bacteria nematodes use on their insect hosts. However, beneficial insects are often more active than parasitic species, and therefore evade the nematodes and are unaffected. Beneficial nematodes tolerate the tools gardeners use, such as insecticides, herbicides, and fertilizers, well. They can even survive for a while without nutrition as they search for an appropriate host.

Beneficial nematodes can be purchased for the garden in the form of a spray or soil drench. It is imperative that gardeners apply the nematodes when conditions are in line with their survival—when it is warm and moist. Be sure to irrigate the application site before and after introducing nematodes, and only use nematodes when the soil temperature is between 55 and 90 degrees Fahrenheit in filtered sun. Remember that sprays and soil drenches with nematodes contain living creatures, so they must be used within the year and must not be stored in locations with high temperatures.

How Do Other Nematodes Hurt Gardens?

Nematodes that are parasitic to the plants in a garden tend to be very small and can often only be seen with a microscope. The shape of a plant-parasitic nematode’s mouth, or stylet, is like a hypodermic needle that they use to puncture a plant’s cells, injecting their own digestive juices and draining the plant fluids into their mouths.

Many of these plant-parasitic nematodes target the roots of plants. Those called ectoparasites live their entire lives in the soil, using their stylets to drain nutrition from the roots of plants. Ones called endoparasites insert their bodies, in whole or in part, into plants, such as the migratory endoparasites that do this in the root area. Some, called sedentary endoparasites, set up shop in one location, keeping a feeding spot at the root of plants that they return to over and over. Sedentary endoparasites change in shape as they grow older, with females becoming swollen over time.

In addition to plants displaying damage where parasitic nematodes have fed on them, they can sustain other problems as a result of nematode activity as well. The place where a nematode inserts its needle-like mouth can be a handy access point for bacteria or fungi to plague plants. Nematodes can also be carriers of bacterial or fungal diseases, which they can pass along to plants while feeding.

What Does Nematode Damage Look Like?

As nematodes damage the root systems of plants in a garden, the plant’s natural ability to derive water and nutrients from the soil is compromised. Eventually, symptoms of the problem may become visible above ground as the nematode population grows or the problem continues for a long while.

A garden that’s been impacted by plant-parasitic nematodes looks a lot like one afflicted by drought or nutrient deficiencies. The damage will become evident in patches. Gardeners struggling with plant-parasitic nematodes may see yellowing, wilting, or stunting of plants.

If plants are transferred into beds where nematodes already proliferate, stunting is likely to occur. There may be no growth at all after transplanting into an area with plant-parasitic nematodes. Plants with edible parts that grow below ground, such as radishes or potatoes, may be damaged in the areas humans intend to eat. The areas with nematode damage usually spread slowly as time goes on.

Other plant-parasitic nematodes can cause root knots or galls, leaf galls, injured root tips or root branching, or tissue problems such as lesions, patches of dying tissue, and twisted or distorted leaves. Plants that may see root damage from plant-parasitic nematodes include carrots, cherry tomatoes, corn, lettuce, potatoes, and peppers. Crops that may show damage on their leaves or stems include alfalfa, chrysanthemums, onions, and rye. If nematode damage is suspected below ground, gardeners can check by gently uprooting a plant from the soil, washing clinging dirt from its roots, and looking for galls, lesions, branching, injured root tips, or rot.

How to Control Plant-Parasitic Nematodes

Plant-parasitic nematodes don’t have to be a plague on the garden. First of all, soil should be kept well draining. Moisture in soil helps these parasitic nematodes to move around, so by keeping it drained well, you’re impeding their progress. Crop rotation is another tried-and-true way to reduce nematode damage by moving susceptible crops from plot to plot. Moving a crop just a few feet from its previous year’s location can prevent nematode damage, especially if the new spot has been growing grass for a few years.

There are also resistant varieties on the market of many vegetables that are common victims of plant-parasitic nematodes. Resistant varieties can produce good crops even when nematodes are present in the soil and doing their worst to the plants. If resistant varieties are used and crop rotation is in effect, the nematode population of a garden can actually be shrunk as years go on, and the vegetables the garden produces won’t be as affected by nematodes as they would be otherwise.

Cultural methods for fighting plant-parasitic nematodes can involve more work for the gardener, but they are overall effective. These methods include removing the roots of a crop after harvest, then tilling soil two or three times in succession. In the fall, gardeners can till up the entire garden two or three times, then plant a winter cover crop like annual rye grass, rye, or wheat.

Simply caring for soil in the best possible way, keeping its pH level, fertility, and moisture at optimum levels for your crops, will go a long way toward fending off plant-parasitic nematodes. Plants in these conditions can tolerate nematodes at low to moderate levels, and they’ll have the healthiest output they can at any rate, as they’ll be less likely to fall victim to other garden problems.

Amending soil with plenty of organic matter (400 to 500 pounds of material per 100 square feet) will not only benefit the soil, it will also help curb the parasitic nematode population. Be aware that adding this much organic material may mean you also need to add nitrogen.

Although a nematode problem can be frustrating to deal with, it’s a problem that can be overcome by following the practices outlined above. Each of the approaches that can help fight nematodes is also good gardening advice for other reasons, so by putting these strategies into practice, you know you’re making your garden the best it can be.

Want to learn more about helpful and harmful nematodes?

NC State Extension covers Control of Root-Knot Nematodes in the Home Vegetable Garden

University of Florida Extension covers Nematode Management in the Vegetable Garden

Gardening Know How covers Nematodes as Pest Control

Good Housekeeping covers What are Nematodes?