Cardinal flower is normally planted from potted nursery starts in the spring, or by seeds sown in fall. It is a fairly fast-growing plant that usually flowers in its first year. Individual plants are short-lived, but cardinal flower perpetuates itself constantly by self-seeding and sending out offshoots that quickly colonize an area. As is true of other species in the Lobelia genus, cardinal flower is seriously toxic to humans and to pets. The plant contains several toxic alkaloid compounds, including lobelamine and loeline, which can cause a variety of symptoms, ranging from digestive upset to convulsions and even death. Cardinal flower is uniquely free of common pests and diseases, but the basal foliage is sometimes targeted by snails and slugs.
Light
In colder areas, cardinal flowers appreciate full sunshine. In hotter climates, it will do best with afternoon shade to provide shelter from the intense heat.
Soil
The cardinal flower loves rich, moist-to-wet soil that often causes other plants to collapse with rot, but it struggles in dry, barren soils. To help retain soil moisture, try adding a layer of mulch around your plants. Amending soil with heavy amounts of compost can also improve soil moisture levels.
Water
This plant appreciates plenty of water. Cardinal flower can even tolerate prolonged seasonal flooding. Be sure to maintain a consistent watering schedule that keeps the soil evenly moist. Heavy twice-a-week watering may be necessary during hot months if no rain is falling.
Temperature and Humidity
Cardinal flower can routinely handle the wide range of temperatures across USDA zones 3 to 9. It is known to survive down to minus 34 degrees Fahrenheit, and some zone 2 gardeners have grown it successfully. The named cultivars and hybrids, however, may be somewhat less cold-hardy than the native species plant. Since cardinal flowers love moisture, higher humidity levels are ideal; these plants aren’t well suited for arid climates.
Fertilizer
Cardinal flowers generally do not require fertilizer throughout the year. Adding compost and organic material in the late winter or early spring will provide the necessary nutrients for the growing season ahead. This one-time application is generally sufficient for healthy growth.
Types of Cardinal Flower
In addition to the native species, Lobelia cardinalis, there are several cultivars that have been developed to expand the range of flower colors:
‘Queen Victoria’ features the plant’s trademark vivid red flowers on burgundy stems.‘Black Truffle’ also has crimson flowers, but very dark purple foliage that is nearly black.‘Rosea’ features pink flowers.‘Alba’ Is topped with white flowers.‘Angel Song’ is filled with cream- and salmon-colored flowers.
There are also a couple of hybrids to consider:
Lobelia x speciosa ‘Vadrariensis’ has dark violet flowers. It grows to 4 feet tall.Lobelia x speciosa ‘Star Ship Deep Rose’ is a 24-inch-tall plant with deep pink flowers.
Pruning
You may want to remove spent flower spikes to keep your plant looking clean and to encourage further blooming. Just keep in mind that this may not allow the plant to self-seed, which could impact next year’s colony. If you find your plant getting a bit unruly during its growing season, feel free to trim it back to help maintain a bushier, less leggy look.
Propagating Cardinal Flowers
Cardinal flowers can be propagated by seed, division, or by transplanting young plants that develop around the mature plant. Here’s how to propagate by division: If you would like to remove young volunteer plants that have formed around your mature plant by self-seeding, simply dig them up in the fall and place them wherever you like.
How to Grow Cardinal Flowers From Seed
To propagate by seed, you have a couple of options. These plants easily self-seed, so you can simply leave the seed pods on the plant and allow them to fall naturally. Another option is to collect the seeds, sowing them around the plant when they are ripe If you would like to collect seeds to start indoors, here’s how:
Potting and Repotting Cardinal Flower
Cardinal flower is not a common choice for planting in pots and other containers, but it can be done. Use a large, well-draining pot filled with ordinary commercial potting mix. These moisture-loving plants will require frequent watering when grown in containers. These are not plants that can be moved indoors to grow as houseplants for the winter. Instead, move the potted plants to a sheltered location out of the wind for the winter months.
Overwintering
In most regions, cardinal flower requires no special preparation or protection. Do not cut stalks in the fall–wait until spring. In regions where their hardiness is borderline (zone 3), a layer of mulch over the root crowns will moderate freeze-thaw cycles over the winter and ensure the plants return the following spring. These plants can survive extremely low winter temps but frequent thaw-refreeze cycles may cause them to perish.
Common Pests and Plant Diseases
The cardinal flower is a very hardy plant troubled by very few pests or diseases. The most common pests are snails and slugs, so take protective measures against them if they show up. Fungal infections such as rust and leaf spots may arise if the plants are crowded and if airflow around them isn’t good. Fungicides can usually treat the problem, but you can also simply cut the plants down to the ground and allow them to grow back fungus-free. Make sure to keep neighboring plants cut back so that the cardinal flowers have room to breathe.
Common Problems With Cardinal Flower
Like many native wildflower species, cardinal flowers are pretty fuss-free plants. The common complaints are fairly easy to address:
Yellowing Leaves
If you find that the leaves of cardinal flowers are turning yellow, it might indicate nutrient-deficient soil. A rich compost applied around the base of the plants can help.
Toppling Flower Stalks
When cardinal flower grows in shady conditions, the stalks may become overly leggy as they reach for the sun. In some cases, you may need to use stakes or hoops to support the flower stalks against the wind.
Clumps Become Sparse
Cardinal flower is a fairly short-lived plant that dies back after flowering, though a colony will continue to sustain itself by the offshoots that are created. But an older clump may spread out and become sparse. The solution is to dig up the plant, divide the crown, and replant the pieces with closer spacing.
