Are Marigolds Dangerous to Earthworms?

Marigolds in the Garden

• Marigolds thrive in hot, sunny conditions, which makes them ideal for planting around the vegetable bed. The flowers bloom from late spring often into early fall and are easy to start from seed or nursery seedlings. The best soil for marigolds is an organic rich, well-draining and mildly acidic. Marigolds have few pest or disease problems which leads some to believe that the plants provide some sort of insect-repellent action. The plants tend to grow in low bushes but there are also tall African marigolds. Flower size, style and colors are diverse.

Marigolds Versus Nematodes

• Nematodes are nearly microscopic roundworms that live in the soil. They are not the same organism as earthworms. There are beneficial nematodes which prey on many insect pests and can be an important part of an integrated pest management program. There are also bad nematodes which feast or live on plant roots and can kill a plant. They will overwinter in even dead root material in the soil and re-infest new plants the following spring. Marigolds have been found to contain compounds which are toxic to the root knot nematode, a deadly pest of many types of plants.

Marigold Pesticide Action

• Earthworms have nothing to fear. While marigolds are often mentioned as having pesticide qualities, there is no evidence to prove this. In fact, the Alabama University Cooperative Extension reports marigolds may actually attract harmful pests such as spider mites. Marigolds do attract beneficial insects that will prey upon pest species. Syrphid flies feed on their nectar, as do lacewings and lady beetles. All these species feed on aphids and other sucking insects which will help keep the pest population down to less-destructive levels.

How to Use Marigolds

• The most beneficial variety is the French marigold. This variety has been shown to have the most effect against the root nematodes, but you have to use the plant as a cover crop and it can take up to four months to see effects. Another beneficial aspect of marigolds is their ability to repel cabbage worms. Again,you need high numbers of the plant and the minor repellent property may not be worth the amount of resources the flowers steal from the cabbages. African marigolds kill annoying weeds like ground ivy and bindweed. The effect is likely due to the height of the plants and subsequent blocking of light and use of local nutrients. Marigolds are easy to care for and their effects, if any, are not dangerous to earthworms or other beneficial organisms.