What never to grow with dahlias – 5 types of plants that can hinder their performance if kept too close

With their dazzling show that lasts well into the fall, dahlias are some of the best summer flowers for filling out borders. Growing them alongside all your other seasonal favorites, or even next to some veggies, can be tempting. However, not every type of plant will make a good neighbor.

While you don't need to be a super experienced gardener to grow dahlias successfully, there are a few things to consider to help them thrive. Deborah Stone of Stone Hollow Farmstead says, 'Dahlias are heavy feeders with delicate needs, especially when it comes to pests, airflow, and root competition. While many plants make wonderful companions, there are a few that can hinder their growth or invite trouble.'

Below, you'll find five types of plants to avoid growing next to your dahlias, and reasons why. If you're hoping to achieve your best dahlia display yet, be sure to keep these in mind.

1. Water- and drought-loving plants

When choosing plant companions, it's crucial to consider whether they like the same growing conditions. Some plants do best with plenty of water, for instance. It's no use putting these alongside those that prefer a drier environment – you'll only make maintenance tough.

When it comes to picking neighbors for dahlias, Peggy Anne Montgomery of Garden Media Group underlines this point. 'Water-loving plants like flag iris and marsh marigolds that thrive in soggy soils are poor companions, because dahlia tubers will rot in wet soil,' she says. 'Keep water-loving plants in rain gardens and wet areas.

'Drought-loving plants and desert natives are also not a good combination because dahlias need moist, well-drained soil,' she adds.

Peggy Anne Montgomery

Peggy Anne Montgomery is a horticulturist with more than 35 years of experience in the industry in the Netherlands and the U.S. Her home garden has appeared on television, in several gardening books, and in many magazines. Today, she is an executive account manager at the Garden Media Group, where she represents Dutch Royal Anthos, the largest consortium of Dutch bulb growers and exporters in the U.S. and Canada.

2. Certain types of veggies

According to Peggy Anne, plants in the cabbage family (such as broccoli and kale) should not be grown near dahlias. 'They can attract aphids and caterpillars, which can spread and harm dahlias and cause damage to flowers and leaves,' she says.

While it is possible to get rid of such pests, it's best to keep these leafy greens in a vegetable garden, safely away from your vulnerable ornamentals. Certain companion plants, such as marigolds, can help keep the pests away from your crop.

Potatoes are another veggie to keep away from your dahlias. Peggy Anne warns that they share similar disease susceptibilities, making them a risky combination. 'Potatoes can carry viruses like potato virus Y and potato spindle tuber viroid, which can spread to dahlias, causing them to become diseased,' she says.

Plant tomatoes, peppers, and eggplants away from dahlias, too. Deborah explains they are notorious for attracting spider mites, especially in warm climates. 'Since spider mites are one of the most damaging pests for dahlias, planting these crops nearby increases the risk of infestation.'

Deborah Stone

In 1999, after completing over 30 years of work within the aesthetic, cosmetic, and Medi-spa industries, Deborah Stone founded Stone Hollow Farmstead. The farm began with 80 raised beds of organic herbs and a small lab where Deborah produced her small batch skincare, Botaniko Skin, made from the herbs grown and distilled at the farm. Today, Stone Hollow produces award-winning culinary goods, herbal wellness products, and botanical skincare, and is renowned for its vibrant dahlia flowers.

3. Large perennials and shrubs

One place you should never plant dahlias is somewhere with limited sunlight, as they need plenty of it to perform well. With this in mind, be sure to consider the mature height of potential companion plants.

Hydrangeas, large shrubs, and trees are poor companions as they can overgrow dahlias, says Peggy Anne. This can deprive them of sunlight, as well as water and nutrients.

'Dahlias have a shallow root system, and these larger plants will easily out-compete them. The same goes for large perennials like Joe Pye weed and tall rudbeckia,' she adds.

4. Zinnias

Like dahlias, zinnias make excellent cut flowers, however Deborah advises against planting them next to one another.

This is because zinnias are highly susceptible to powdery mildew – 'a fungal disease that spreads easily in warm, humid conditions. When planted near dahlias, they can reduce airflow and contribute to mildew problems that quickly affect both plants.'

Instead, she recommends growing zinnias in a separate cutting garden or in open, breezy locations where good air circulation can help keep mildew at bay.

5. Sunflowers

Sunflowers are another popular summer annual that should be planted away from dahlias.

Deborah explains that they are tall, nutrient-hungry plants that can compete aggressively with dahlias for water and minerals.

'Position sunflowers in their own sunny corner well away from dahlia beds, or ensure generous spacing and supplemental feeding to prevent nutrient competition,' she says. You can also grow sunflowers in pots – try the 'Tiger Eye Hybrid' variety from Burpee.

FAQs

What kills dahlia tubers?

Wet and cold conditions can cause dahlia tubers to rot, so be sure to choose the right planting spot for them and to overwinter them correctly. Many gardeners dig theirs up in the fall and keep them safely in storage before replanting in the spring.

How far should you plant dahlias from other plants?

Some plants do make good companions to dahlias, but don't plant them too close. Dahlias need space to grow well and get the nutrients they need. The exact spacing required will depend on the variety, as some dahlias grow much larger than others, but a minimum of 12 inches apart is generally recommended. Many dahlia varieties come with specific spacing instructions on the label.

'Keeping incompatible plants at a distance allows your dahlias to flourish – free from competition, pests, and disease,' says Deborah. 'A little thoughtful spacing can go a long way toward healthy, abundant blooms.'

There are other ways to get the most from these beauties, too. Our guides on pinching out and deadheading dahlias will come in useful.