Giant lobelias are relatives of the common garden lobelia, which grow in the cool, misty mountains of central Africa. They are beautiful plants, with dense, symmetrical rosettes of deep green to purple leaves, reminiscent of agaves. These lobelias produce huge spikes of tiny flowers, which tower up to 18 feet above the basal rosette. Most of these rare plants are threatened in their natural habitat by grazing animals, land clearing for agriculture and illegal plant collectors. Luckily, these stunning beauties can be successfully grown in cultivation in moist, cool, frost free climates. A few species, including Lobelia gibberoa, L. deckenii and L. aberdarcia, are available from specialty nurseries.
Select a sheltered location in bright shade, with moist, well drained soil.
Use the shovel to dig a hole twice as wide and six inches deeper than the pot.
Mix in with one part fine organic compost to two parts backfill. Avoid using animal manures. Mix in 1/4 cup of dolomite lime powder with the backfill. Place six inches of amended backfill in the bottom of the hole.
Remove the giant lobelia from its pot and carefully place it in the hole on top of the amended backfill. Adjust the backfill level so that when the hole is filled in, the plant will be growing at the same level as it was in the pot.
Fill the hole with backfill, firming gently to remove any air pockets. Create a three inch well around the outermost edge of the planting hole to prevent irrigation runoff.
Spread a three inch layer of organic mulch, such as shredded bark, on the surface of the planting hole, avoiding the area two inches on all sides of the basal rosette.
Water the planting hole so that the outer well fills up with water once, and then allow it to drain. Repeat once more to thoroughly water the plant.
Irrigate your giant lobelia once a week to maintain evenly moist soil. Do not over water these plants, but never allow the soil to become dry. Mist the plant occasionally with a fine spray of water early in the morning.
Feed giant lobelias once a month during the growing season with balanced liquid plant food diluted to half the recommended dose.