Plants that assist each other to grow well, repel insects or even other plants when grown next to each other is called companion planting and can be a sustainable and eco-friendly way to improve and protect your garden against unwanted pests and disease.
Chromatography has been used to explain why some plants like or dislike being planted with other plants. It is possible to make a specific chromatographic test to find out why, or if at all, a plant is helping or hindering its neighbors. Chromatography has also been used to prove that plants do significantly better with compost than without.
Chives are good companion plants for carrots and will improve their growth and flavor. Chives are also beneficial if planted around the base of fruit trees in an orchard to deter mice and to prevent apple scab.
Make a tea of chives to spray on cucumbers, apples and gooseberry against powdery mildew.