Companion plants complement each other in the garden and may even help each by repelling bugs, enriching the soil, or providing shade. Dahlias repel nematodes, which are microscopic worms that live in the soil and attack certain species of flowers. They make good companion plants for flowers, like roses, that are susceptible to nematode attacks. Slugs, snails, Japanese beetles and aphids attack dahlias. Effective companion plants are plants that can repel these pests.
Plants in the Artemisia genus have a strong scent that deters slugs. Many of these plants have feathery leaves that are grey or green and they make good border plants. Wormwood, tarragon, Ghostplant and Sage Bush are all good choices from this group of plants. Creeping comfrey has several benefits as a companion plant for dahlias. In addition to deterring slugs, it chokes out weeds. It's low-growing and it tolerates shade, making it a good choice to plant as groundcover. Red comfrey is another appropriate choice as it, too, deters slugs. The flowers on the red comfrey are red and creeping comfrey has light yellow flowers and crinkled leaves.
Nasturtiums are low-growing flowers that grow well in partial shade, which makes them an ideal flower to plant underneath tall dahlias. The red, orange, and yellow flowers repel aphids, and they're edible, too. White geraniums and four o' clocks attract Japanese beetles away from dahlias and four o' clocks are poisonous to any aphids that eat them, making them both good choices for companion plants.
Planting flowering sage as a companion plant for dahlias will draw hummingbirds and butterflies to your garden. These, in turn, will eat aphids and beetles that are attacking your dahlias. The purple flower spikes look attractive with dahlias, but don't plant them too close to your dahlias since they need full sun. Anise is a low-growing plant with feathery upper leaves and small white or yellowish white flowers. It attracts predatory wasps that kill aphids, making it a useful choice for planting with your dahlias. In addition, anise deters aphids.
Dahlias can grow from 1 to 6 feet tall, with flowers that range from 2 to 12 inches across. By selecting different heights and colours, you can create some beautiful effects by planting dahlias with Oriental lilies, ornamental goldenrod and fall-blooming asters. Oriental lilies grow to be 3 to 6 feet tall, and their flamboyant white, pink, and crimson flowers look striking next to the vivid orange, purple, red, yellow or pink dahlias. Asters are daisy-like flowers that grow from 1 to 4 feet tall. They come in a variety of colours, too, including purple, blue, pink, and white. Ornamental goldenrod has tiny clusters of yellow flowers that grow on stalks 2 to 4 feet high.