Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
Every August when I see purple flowers blooming, usually around local ponds, I think to myself that those “asters” are blooming early this year. Then I look more closely and realize that they are not asters. They are New York ironweed, a native plant that blooms in August, a month ahead of the asters.
What is New York Ironweed?
New York ironweed (Vernonia noveboracensis) is a perennial plant that is native to the Eastern part of the United States. In its native habitat, it is usually found growing along streambanks because it prefers moist areas. It grows in both full sun and partial shade. The plants are quite tall, 4 to 6 feet in height and can spread 3 to 4 feet. Bloomtime is August. Up close, the flowers look like they are having a bad hair day!
There are quite a few varieties of ironweed. The most popular are the New York ironweed and the giant ironweed (V. gigantea) which can grow up to 7 feet. Most people grow the New York ironweed in their gardens.
Why is it Called Ironweed?
It does seem strange that a tall green plant with purple flowers would be called “ironweed”, but if you revisit the plants in the fall after the flowers have finished and the seed heads have formed, it will become obvious. The beautiful purple flowers will have been replaced by rusty yellow seed heads resembling rusty iron. Also, the stems of the plants are very tough like iron. Both attributes are what give the plants their “ironweed” name.
How to Grow New York Ironweed
Ironweed may grow in moist areas in the wild, but it will also grow perfectly well in your garden. The tall plants make a lovely back of the border display. The ones you see for sale in nurseries and online are domesticated varieties. It’s usually not a good idea to plant wild plants in your garden. They tend to grow and spread aggressively, crowding out your tender, domesticated plants.
If 4 to 6 feet is too tall for your garden, in the late spring, you can cut back the stems. They will grow back but shorter so that the plants won’t overwhelm a small garden.
Ironweed adds interest to your garden because the flowers attract butterflies such as the American Lady, Sachem, Crossline Skipper and Fiery Skipper. If you let your plants go to seed, the birds will thank you. They love snacking on the seed heads.
The plants readily reseed themselves so be sure to remove the spent flowers in the fall if you are not feeding the birds. If you allow the flowers to go to seed, you will find many new seedlings in your garden the following spring.
How to Divide New York Ironweed
New York Ironweed should be divided every 3 to 4 years to keep the plants healthy. Carefully dig up your plants. Using a sharp knife, carefully cut away any dead parts of the crown and roots, usually in the center and then cut the remaining crown and roots into pieces that you can replant. Each piece should have both foliage and roots. Be sure to space the plants at least 12 inches apart.
How to Grow New Ironweed From Cuttings
Another way to obtain ironweed plants is through cuttings. Take your cuttings in the late spring when the plant is actively growing. Cut 5 to 6 inches off an actively growing stem. Remove the leaves from the bottom 2 to 3 inches and dip that end in rooting hormone. Rooting hormone encourages the formation of roots. Place the cutting up to the level of the leaves in sterile soil. Keep the soil moist while your plant is growing its new roots. Be patient, it may take a few weeks. You will know that your cutting has roots when it starts to grow new leaves. Plants without roots are unable to grow leaves.
How to Grow New York Ironweed From Seed Outdoors
The best way to grow ironweed from seed is to sow the seeds in your garden in the fall. Plant the seeds no deeper than ⅛ inch deep. Barely cover them with soil. The seeds will germinate in the spring. Another way to start your seeds outdoors is to plant them, again ⅛ inch deep in pots that you leave outdoors so that the seeds can experience winter. They will germinate in the spring. Ironweed seeds need to experience a period of cold to germinate. When the seedlings are at least 2 inches tall, you can transplant them into your garden. Remember, they will grow into very large plants, so space your seedlings 12 inches apart.
How to Grow New York Ironweed From Seed Indoors
You can start your seeds indoors and fool them into thinking that they have experienced winter. Most people do this by planting the seeds in flats, covering the flats with plastic to prevent them from drying out and then refrigerating the flats for a number of weeks. In the case of ironweed, it should be for 3 months. Some people go to extremes with their ironweed seeds and alternate refrigerating and freezing their seeds to really mimic winter conditions. To do this, they put their flats in the refrigerator for a day and then the freezer for a day. Then back to the refrigerator for another day followed by a day in the freezer. They keep this up for a week before removing the flats to a warm room (70⁰F) to mimic spring weather. Whichever cold stratification method you use, you can transplant your seedlings into your garden when they are at least 2 inches tall and all danger of frost has passed. Use the same 12 inch spacing as above.
Questions & Answers
Question: In regard to growing New York Ironweed, I notice that you mentioned that 3 months cold stratification is needed but if you alternate the temperature of the seeds from the freezer to the fridge every day is that for 3 months or only 1 week?
Answer: Yes, you are correct. Gardeners who use the alternating freezer/fridge stratification method do it for one week. I prefer cold stratifying in my refrigerator for 3 months because I think that most closely mimics the length of time seeds would normally experience winter weather.
Question: Ironweed is my absolute favorite plant for its resilience and beautiful colors. Would it be possible to completely uproot one and relocate it?
Answer: Please do not ever remove a wild plant from its natural environment. It may even be illegal in your area. Removing wild plants from their native areas disrupts the entire ecosystem of that habitat, and if the plant is endangered, you may be contributing to its extinction. In the case of ironweed, the wild version is extremely invasive and will take over your garden. Always purchase plants from a reputable nursery. The ironweed that they sell is a domesticated version that is not as invasive. Another good option is if you know someone who is growing the domesticated version in their garden, you can ask for a division when they divide their plants. Or, you can ask for seeds in the fall or cuttings in the spring and grow your own.
Question: Can I tranplant 2 year old plants at the end of the season?
Answer: Yes, New York ironweed is a perennial and fall is the best time to plant or transplant perennials. You can do it any time up to when the ground freezes.
© 2018 Caren White
Caren White (author) on August 31, 2018:
Claudia, what a welcome surprise that was!
Claudia Porter on August 31, 2018:
I love New York Ironweed. The purple is such a bright spot in late summer fields and gardens. We are lucky because we had a couple come up on their own and they come back every year.