A Guide to Nematodes For Polytunnel Gardeners

If you are a keen gardener, you may have heard talk of nematodes. But many gardeners profess confusion about these soil dwelling organisms. Understanding these organisms could be important for those who are keen to understand and protect the soil ecosystem. Learning more about nematodes could be a good step for anyone who wants to garden organically in a successful way.

In this guide, we will begin by exploring what exactly nematodes are and where they are to be found. We will go on to discuss how nematodes can affect your garden, in both positive and negative ways. Finally, we will look at why some gardeners choose to introduce nematodes to their gardens, and whether or not this is a good idea.

What Are Nematodes?

Nematodes are tiny organisms, roundworms, which constitute the phylum Nematoda. The word nematode comes from the Modern Latin ‘nemat’ (thread) (from Greek ‘nema’) and ‘odes’ (like, of the nature of). There are a huge number of different species of these small, thread-like creatures- though there is some disagreement about exactly how many. The figure is believed to be over 25,000, and could be much higher. Fact based estimates of the number of nematode species put the figure at somewhere around 40,000 species worldwide.

Nematodes are typically around 5-100 micro-metres thick, and 0.1-2.5 mm long, though the smallest can be microscopic, and some species can grow to much larger size.

In a garden, it can be helpful to understand that nematodes can be ‘free living’ or parasitic. Both free living and parasitic roundworms can be beneficial in a garden. But other species can also be pests and could hinder your growing efforts.

Where are They Found?

Nematodes are found in almost every ecosystem on earth. They are found in water (both fresh and salt), in soils in our gardens and around the globe, from polar regions to the tropics, at all elevations, from the highest mountain peak to the deepest oceanic trench, and deep levels within the earth’s lithosphere. Astoundingly, more than a million nematodes can be found within just 1 sq m, and they account for about 80% of all individual animals on earth.

In our gardens, nematodes in the soil ecosystem are the most pertinent types. Around 90% of nematodes live in the top 15cm of soil. There are a huge range of different nematodes found in garden top soil. These can have important and profound effects on your garden, and your plants.

How Nematodes Can Affect Your Garden

Nematodes can affect your garden in both negative and positive ways. Some nematodes are can cause problems for your plants, while others can be beneficial for gardeners, soil and the things you grow.

Negative Nematodes

Nematodes can be detrimental in a garden in certain ways. For example, there is one nematode called the potato cyst, which can decimate your potato crop. There are a number of different roundworms which will burrow into plant roots and make their way up the stems and into the leaves. The damage they do can make it difficult for plants to absorb the nutrients they need, which can result in small, weak plants – or even kill plants outright.

It is a challenge to tackle negative nematodes, since they can be so abundant in the soil. In an organic garden, chemical solutions are never the answer. In any case, there are no chemical controls for nematodes that are approved for use by home gardeners.

Successful implementation of a crop rotation system, careful choices with regard to which crops to grow in a given area, and companion planting are generally the best ways to deal with a nematode problem.

– Marigolds:

Marigolds have a natural nematicide. This means that when they are infested with nematodes, they kill them off in that particular spot and reduce their numbers. Marigolds may also excrete a chemical which can repel nematodes in the surrounding area (though scientists disagree over whether or not this is the case).

Companion planting with marigolds may help nearby plants, though the science is currently inconclusive. Planting marigolds in a given,exact location in your vegetable beds before the vegetables can, however, definitely reduce nematodes there and make it less likely that your vegetables will be struck.

Since there is no way of knowing exactly which particular roundworms you are dealing with in your soil, however, there is no way to develop a comprehensive and definitive solution to the problem,not to know which type and variety of marigold would be the best one to plant. Still, growing marigolds in your vegetable patch could be better than taking no measures at all – even with the uncertainty involved.

– Other Methods of Control:

While it is unlikely that you would ever be able to completely eradicate certain species of negative nematode from your garden,there are certain things that you can do to control their spread, limit their numbers, and protect your plants. For example, you can:

• Amend soil with plenty of organic matter prior to planting. The organic matter can help to suppress certain nematodes and keep them from causing quite as much damage.
• Make sure plants are healthy and well-watered. Plants suffering from lack of water will be stressed and so less able to withstand nematode attack.
• Wash your hands, pots and containers and garden tools well and thoroughly to prevent contamination from one area of your garden to another.
• Remove plants and all roots from a contaminated area at the end of the growing season and carefully dispose of the plant matter.
• Introduce natural antagonists of the nematodes involved to your garden. Examples include the fungus Gliocladium roseum.
• Use Chitosan, a natural biocontrol, which causes plant defence responses to destroy parasitic cyst nematodes on crop roots without harming beneficial ones in the soil. It is made by treating the chitin shells of shrimp and other crustaceans with an alkaline like sodium hydroxide.

Beneficial Nematodes

As mentioned above, however, not all nematodes that are in the soil in your garden are bad for your plants, or for you as the gardener. Nematodes serve important ecological purposes in the soil ecosystem. They can help to effectively regulate the bacterial population of the soil – some eat up to 5,000 bacteria per minute. Some also play a vital role in the nitrogen cycle by way of nitrogen mineralisation.

Biological Pest Control

Nematodes are also frequently uses as a biological pest control in organic gardens. In such cases, beneficial ones are deliberately introduced to the soil ecosystem. Pathogenic ones are frequently introduced to deal with pest problems, such as issues with slugs, chafer grubs, sawfly and vine weevil.

These worms, used to control pests, enter the pests’ bodies and releasing bacteria. This results in infections which kills the pests, after which the nematodes feed and multiply on the decomposing body.

Should You Use Nematodes to Kill Pests?

While this can be used as a solution in the case of extreme pest infestation, it is generally better to consider other organic methods of pest control. If you introduce nematodes, for example, to kill off all the slugs in your garden, you can inadvertently make the problem worse, since, by getting rid of the slugs, you can upset the natural balance in your garden.

Without slugs to eat, other natural slug predators, such as certain birds, may leave your garden. This means that, should the biological control population fail to thrive, you might develop an even worse slug problem in future.

Rather than trying to eradicate a pest species through measures such as direct biological control, it is best to use indirect methods of biological control – encouraging natural predators through planting, habitat creation and other organic gardening practices. The goal is not to get rid of the pest species entirely, but to create a natural, balanced system where no one species is able to get out of control.

Nematodes are fascinating organisms that most of us know far too little about. Taking some time to understand these remarkable creatures can be a great step on the road to becoming a better gardener. It could help you to understand how the soil ecosystem works, how to protect it, and how to make your organic garden a success.