The handsome cardinal flower, or Lobelia cardinalis "Splendens", is a brilliantly colorful perennial in USDA Planting Zones 4 through 11. This hardy marginal bog plant performs best and is at its showiest and most impressive when used to complement wetland fern irises. Cardinal flowers perform well when planted directly into the boggy landscape, or scattered throughout as potted specimens. When Old Man Winter threatens, proper winterization will pull your beauties through to glorify many growing seasons yet to come.
Prune off dying cardinal flower plant material as it appears throughout the fall. Remove the debris to prevent it from building up.
Apply about 1/2 inch of organic mulch such as well rotted manure or compost to marginal bog cardinal flowers late in the fall. This will nourish the plants throughout the winter.
Cut marginal cardinal flowers back to 2 to 4 inches tall if the ground doesn't normally freeze in your area.
Mulch marginal cardinals lightly with 1/2 inch of straw prior to the first freeze. This is necessary only in very cold areas where the ground heaves and causes root damage during brutal winters. Otherwise, the plants will fare better without mulch or litter around their bases. This provides them with maximum air circulation.
Submerge as much of the plants as possible below the ice line, where they can survive cold winters as long as their crowns don't freeze. Cardinal flowers will winter best outdoors with about 2 inches of water over their crowns. In places where more than the top 1 or 2 inches of the pond surface freeze, you'll need to bring your cardinal flowers indoors.
Bring potted cardinal flowers indoors and drain the excess water from the pots. Store them in a refrigerator or other very cool spot where the crowns won't freeze until spring. Water enough to keep the soil soggy to prevent them from drying out.
Remove any straw mulch from marginal cardinal flowers in the spring when new growth begins. Fertilize both marginals and potted cardinals with a slow-release product for blooming plants. This will help to revive the plants and jump start them into the growing season. Apply 1/2 inch of organic mulch to marginals.