The Mistake That Comes With Planting Cosmos And Black-Eyed Susan Too Close In The Garden

The Mistake That Comes With Planting Cosmos And Black-Eyed Susan Too Close In The Garden

Certain plants may not thrive well together due to compatibility issues caused by differences in their requirements. For instance, plants that need moist soil may not grow well near species that require dry soil. In some cases, certain plants may release chemicals into the soil that are harmful to their surrounding plants. Black walnut trees and sunflowers are examples of plants that release such chemicals. When planting cosmos (Cosmos bipinnatus) and black-eyed Susan (Rudbeckia hirta) in the same space, it is important to note that the height difference between the two plants can cause problems.

Cosmos plants can be grown in both flower and edible gardens. They have thin stems, showcase elegant blooms, and attract many pollinators. These characteristics make them stand out in a garden. However, if you plant cosmos next to short and sun-loving plants like black-eyed Susan, they may block out the sunlight and deprive their neighbors of it. To avoid this, it is important to be strategic with the layout when planting both of these flowers in the same garden bed to ensure that they all thrive.

Why cosmos and black-eyed Susan aren’t good neighbors

tall pink cosmos blooms

Ishihama Osamu/Shutterstock

Cosmos and black-eyed Susan are wildflowers you’ll often find in the same seed mixes. They both love the sun and tolerate drought conditions, so it’s no wonder they’re often put in the same bag. However, cosmos can easily shade out black-eyed Susan since the stems can reach up to 7 feet tall, making them unfortunate neighbors. Black-eyed Susan flowers barely reach half of that, often only growing to be about 2 feet tall. If these blooms are surrounded by towering cosmos, they’re likely to not survive.

Another reason why these plants may not be good neighbors is because black-eyed Susan can’t provide cosmos with the support it needs. Cosmos plants do best when they have some kind of support for their tall, lanky stems. A harsh gust of wind can bend the flowers over so much that their stems break. You could take the time to support them with stakes or you could use other tall flowers as a natural and convenient way to hold them up. Black-eyed Susan blooms aren’t a good support system since they’re much shorter. Spider flowers and ornamental grasses are a couple examples of the plants you should put next to your cosmos, as these are both tall and supportive.

How to use cosmos and black-eyed Susans in the same space

black-eyed Susans in garden

Alex Manders/Shutterstock

While black-eyed Susan and cosmos flowers may not be ideal for planting right next to one another, that doesn’t mean they can’t be placed in the same garden bed. In fact, placing them near each other could actually be beneficial, since they have most of the same care requirements. If you do decide to plant cosmos and black-eyed Susan in the same garden bed, try to plant them so that the short flowers will still receive ample sunlight. Though black-eyed Susan flowers enjoy a lot of sunlight, they can benefit from afternoon shade in the heat of summer. Plant them to the east of your cosmos so they can benefit from sunlight until the sun is in the west. Cosmos will also benefit from this setup since they like as much sunlight as they can get.

At the same time, black-eyed Susan can become somewhat weedy and may be invasive if it doesn’t have flowers to compete with. The flowers will quickly fill up a flower bed and take over smaller plants. If your Susans are overstaying their welcome, add some cosmos into the mix. They’ll shade out some of the flowers and will help you get your flower bed back to a diverse state that isn’t overrun with only black and yellow blooms.

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Dr Heidi Parkes

By Dr Heidi Parkes

Senior Information Extension Officer QLD Dept of Agriculture & Fisheries.