Black-eyed Susans (Rudbeckia hirta) belong to the Asteraceae family of daisies, renowned for their bright yellow flowers with dark centers that earned them the moniker ‘black-eye.’ Their beautiful flowers attract pollinators like bees and butterflies, resulting in the production of seeds easily blown away by the wind leading to a sometimes problematic spread of territory.
Here are some steps to keep black-eyed Susans from spreading:
- Grow seeds or seedlings close together.
- Grow black-eyed Susans under direct sunlight.
- Remove rhizomes before suckers sprout.
- Prune dead flowers to prevent seeding.
Black-eyed Susans are attractive flowers but are notorious spreaders. Gardeners with limited space often find this trait troublesome. Read on to learn how to manage black-eyed Susans properly to prevent them from spreading.
1. Grow Seeds or Seedlings Close Together
As wildflowers, black-eyed Susans naturally re-seed to ensure the longevity and proliferation of the species. Out in the open, natural elements like pollinators and wind also contribute to how far and wide the plant can spread.
These flowers make an excellent addition to your garden, provided you know how to manage and control their spread. Left unattended, black-eyed Susans can easily dominate your entire yard in one season.
Although it may not seem like a problem if you love their yellow-golden hues, you might want to limit their spread if you plan to place other plants with contrasting colors in your garden. After all, black-eyed Susans look exceptionally well with other plants with darker leaves and flowers like violets.
When growing black-eyed Susans from scratch, usually, you should sow the seeds or plant the seedlings at least 18 inches (46 cm) apart to give them enough space to receive sunlight and spread.
However, since you want to limit the spread of your black-eyed Susans, you may want to plant them closer together. Growing them 10-12 inches (25-30 cm) apart on your garden bed should be enough for the plants to grow happily and healthily while staying compact.
The limited space will also help the plants grow upwards instead of outwards. Moreover, the limited space will prevent the plants from self-sowing the seeds at large distances after the flowering season.
It will also be beneficial to keep the plants against structures that would protect them from the wind. It can reduce the likelihood of the seeds being sown far away from the plot and prevent the plants from falling over.
The seeds of black-eyed Susans typically germinate 5-10 days after sowing. If you don’t need any more seedlings or if they are growing outside their designated area, you can pull them out. They grow relatively quickly at temperatures between 70-75 °F (21.1-23.9 °C), and you’ll find yourself with way more plants than you imagined.
On the other hand, if you want to ensure a healthy new batch of black-eyed Susans, you can collect the seeds or seedlings and transplant them into pots indoors to protect them from the cold.
You can then move them back outdoors when the temperatures get warm enough. When black-eyed Susans sow seeds in the fall, they typically germinate in spring.
2. Grow Black-Eyed Susans Under Direct Sunlight
While black-eyed Susans are best grown on the ground under direct sunlight outdoors, they can also thrive in pots in partially shaded areas like the balcony or the side of a house. They are pretty versatile plants, making them popular among avid gardeners.
Although it’s perfectly fine to grow your black-eyed Susans in partial shade, their affinity to sunlight will compel them to grow toward the light. As a result, they will lean or spread out to get more sunlight.
One way to keep them from spreading is to grow them under direct sunlight alongside other plants. This way, they will grow taller and become less likely to spread.
Another essential benefit to growing black-eyed Susans outdoors is to keep your pets safe. Ingestion of large amounts of these plants can cause discomfort in your cats or dogs, ranging from vomiting to diarrhea. Keeping your flower garden inaccessible to your pets can benefit your plants and your pets.
3. Remove Rhizomes Before Suckers Sprout
There are several varieties of black-eyed Susans, depending on their lifespan. Some are annuals, while others are perennials. If you have a perennial type, it’s more likely to spread.
In addition to re-seeding, perennial black-eyed Susans are aggressive spreaders through their underground stems or rhizomes. The mechanism helps the plant survive and grow new suckers throughout the seasons.
While it helps ensure you enjoy new growths for the following seasons, controlling the plant’s spread can be challenging. The rhizomes can grow in various directions underground, resulting in suckers sprouting everywhere.
However, before cutting off the rhizomes and getting rid of the suckers, you need to consider a few things:
- Perennial black-eyed Susans do not produce as many blooms in the following years. This lack of consecutive blooms is why gardeners often treat them as annuals or biennials. If you want to have plenty of flowers next summer, you may want to grow new and younger plants.
- Rhizome cuttings are ideal for propagation. You can use the cuttings to grow a new batch of black-eyed Susans indoors or in a greenhouse during the cold season. When all frost has passed, you can move the plants to your garden by spring.
With the things discussed above in mind, you may start deciding what to do with the rhizomes. Whether you plan to transplant or throw them away, you must cut them properly.
Some gardeners locate and cut the rhizomes in-ground using a spade or a trowel and pull them out. Others use the suckers to pinpoint the location of the rhizomes. Using sharp tools like a trowel is generally okay to cut the rhizomes, and your black-eyed Susans can recover soon enough.
However, you also risk inflicting secondary infection on your plants through the injury. Although there isn’t enough research about microbial infections in Rudbeckia, some bacteria, viruses, or fungi may still infect your plant through the wound site. Such disease may also spread to nearby plants, which are more vulnerable to infections.
To avoid such a risk, you can dig around your plant and carefully pull it out of the ground. Using sterile garden shears, you can cut the rhizomes around 1-2 inches (2.5-5 cm) away from the bulk of the root.
If you want to prevent the rhizomes from regrowing and causing the same problem, you can plant the black-eyed Susan into a clay pot that you can bury in the ground. It will serve as a physical barrier to block the rhizomes’ growth and production of suckers.
4. Prune Dead Flowers to Prevent Seeding
If you have limited space in your garden, you may find deadheading the spent flowers on your black-eyed Susans extremely beneficial. This self-propagating plant can quickly spread in all directions in your garden in one season.
When the flowers die, the plants self-sow their seeds into the ground. The seeds will then take up to 10 days to germinate. However, when temperatures drop below 50 °F (10 °C), the seeds typically wait until the temperatures get warm enough to grow.
Black-eyed Susans bloom from June to October, depending on your region. This long-blooming period produces numerous seeds during the season, leading to unwanted seedlings everywhere.
Pruning is one of the best ways to prevent black-eyed Susans from dominating your garden during summer. Remember to use sterile gardening shears when pruning your plants. It will help avoid the risk of infection to your plant.
If you want to prevent or limit the spread of your black-eyed Susans, prune spent or dead flowers regularly. You can schedule pruning once a week or two weeks during the flower season from June to August. It will encourage the growth of new and fresh blooms.
You can then collect the seeds to dry for a future batch of seedlings and adequately dispose of the dead flowers.
When the blooming season is almost over and the plant has spent most of the flowers, you can prune the outer shoots down 30-50% of their total length. On the other hand, prune the central stem only 1-2 inches (2.5-5 cm).
How well your black-eyed Susans can endure the cold will determine whether or not they’d bounce back in spring. Otherwise, you can pull out the dead plants and start sowing the seeds in March.
Black-eyed Susans make for a beautiful addition to your flower garden. However, they can be challenging to control as they tend to spread too much, taking over much space intended for other plants.
You can manage them with the proper knowledge, preparation, and patience. The best method is to prune spent flowers during flower season and trim down your plant in the fall. It may seem like tedious work, but it’s all worth it when you see them in full bloom come next summer!