Edison’s Favorite Rudbeckia
August 14, 2023
By Karen Maxwell, Horticultural Specialist
Did you know that Thomas Edison kept a “Best in My Index” list of plants during his rubber research? In January of 1929, he noted: Rudbeckia laciniata “Very stiff viscous rubber, good plant.” Indeed, the rubber content was listed as 3.6%, higher than some of the goldenrods he tested. The Rudbeckia genus has twenty species including popular perennials, such as the Black-eyed Susan which multiplies by underground rhizomes and Brown-eyed Susan which is a biennial and reproduces by self-seeding. At the Estates, we grow Black-eyed Susan Vines in pots around the property; however, these plants are not Rudbeckia, they are Thunbergia alata.
Edison rarely, if ever, used common names for plants. Rudbeckia laciniata is known by North American wildflower enthusiasts as Cutleaf Coneflower. The Cutleaf Coneflower looks very much like its cousin, the Coneflower, or Echinacea purpurea, another American native wildflower. Both are perennials in the Asteraceae Family, but that is about all they have in common. Let’s take a quick look at the coneflowers or Echinacea. Historically, Echinacea was grown for its medicinal value. Today, they have been hybridized to include more than 100 varietals, many of which are sterile or have a dense, but beautiful double flowerhead which renders the nectar unavailable to pollinators, as seen in E. purpurea ‘Double Scoop.’
Practically speaking, most of our readership (Zones 9-10) should only consider growing Echinacea as an annual or short-lived perennial. Should you wish to give it a go, I would recommend sticking with the Echinacea purpurea and not a hybrid, to provide the wildflower benefit to pollinators if this is your gardening goal. The inclusion of wildflowers in the garden is encouraged to benefit our pollinators, both insect and avian, and attracting songbirds is most rewarding. If Thomas Edison were growing Rudbeckia laciniata in any quantity, it surely would have been to the delight of his wife, Mina who loved birds. Personally, I cannot brag about any success in attracting butterflies and birds to my black-eyed Susan or brown-eyed Susan (Rudbeckia hirta and Rudbeckia triloba respectively).
Rudbeckia laciniata or Cutleaf Coneflower, produces great seedheads of thistle-sized seed, loved by finches – goldfinches and house finches alike, which are both grown for their health properties but have always been enjoyed as a staple in a cut flower garden, and this one is an underutilized stunner. The plant has a single row of yellow petals surrounding a greenish to yellow nubby center and the variety ‘Hortensia’ or ‘Goldquelle’ looks like a double yellow pom pom. Grow R. laciniata in rich, well-drained soil – these native plants would naturally be found in wetland areas in partial shade.
While R. laciniata looks similar to the multitude of hybridized echinacea, here is how to distinguish them: The name “Cutleaf Coneflower” is indicative of the foliage having cuts or deep lobes, usually 3-5, where echinacea leaves are never lobed. The center of echinacea flowers is typically dark colored and quite bristly to the touch with R. laciniata being soft and yellowish, earning it another common name: Green-headed Coneflower.
In the garden, R. laciniata can grow in clumps up to three feet tall and can be dead headed regularly to prolong flowering. Provide support such as a fence or trellis for these heavy bloomers, especially during rainy season. As the plant diminishes, keep the seed heads to attract a charm (group of goldfinches) to your garden.
Realizing that August is a brutal month in Southwest Florida for gardening, here’s a couple of quick house-keeping tips to do early in the morning: Check your palms for any signs of significant yellowing. (Yellowing throughout the fronds, not just on one or two aging fronds). An even disbursement of yellow speckling along the fronds indicates a potassium deficiency – and this is especially true for non-native palms. The heavy rains of summer can easily leach potassium from the soil, thereby starving the palms of this necessary nutrient. Apply an 8-0-12 fertilizer per instructions based on the size of your palm(s) this month, while they are still in active growth mode.
For those of us who like to keep our poinsettias in the ground, now is the time to give them a good pruning to force branching and blooming for the holidays. Use clean sheers, wiped down with alcohol, to prune your stems back to only three or four leaves, or down to about eight inches from the soil top. Continue feeding monthly until they bloom with a balanced fertilizer (20-10-20). If it is granular, be sure to water well before and after application. Keep your plants protected from any artificial evening light so blooming will not be inhibited. Remember that if you purchased nursery grown poinsettias, they will rarely look so lush again – those plants were regularly fed with a liquid fertilizer and pesticides while they grew in captivity. Your aim is to re-grow a natural looking, beautiful poinsettia.
To learn more about gardening in Florida, sign up for the gardening classes and talks, which will be on the Edison Ford website soon.