Crocosmia, often recognized by its former name Montbretia, is a popular perennial valued for its late-summer spikes of vibrant, fiery orange, red, and yellow flowers. Its sword-like foliage and arching flower stems provide texture in the garden, making it a favorite for gardeners seeking vertical interest. While generally considered a resilient and easy-to-grow plant, Crocosmia can present specific cultivation challenges that lead to a decline in its appearance or flowering performance. Understanding the common issues, which range from environmental stress to pest and disease pressure, is the first step toward maintaining a healthy plant.
Failure to Bloom and Vigor Issues
A common frustration is a Crocosmia clump that produces abundant foliage but few or no flowers. This problem is frequently linked to non-living, or abiotic, environmental factors. The most frequent cause of non-flowering is overcrowding, which occurs as corms multiply rapidly beneath the soil. This forms a dense, congested mat that starves the plants of nutrients and water, inhibiting the corms from storing enough energy to initiate flower production.
Inadequate light is a factor, as Crocosmia requires full sun for its best blooming potential. A lack of direct sunlight results in weak growth and reduced flower production. Planting the corms deeper than the recommended 3 to 4 inches can also discourage flowering. Furthermore, an overabundance of nitrogen fertilizer promotes lush foliage growth at the expense of flower bud development.
To correct these issues, congested clumps should be lifted and divided every three to five years. Plants should also be relocated to a position receiving at least six hours of direct sunlight.
Common Pests That Attack Crocosmia
Crocosmia is generally robust, but certain pests can damage the foliage and new growth. Spider mites can become problematic, especially during hot, dry weather. These tiny sap-sucking pests feed on the leaves, causing a fine, yellowish stippling that eventually turns the foliage brown and ragged.
Infestations are identified by fine webbing, particularly around the leaf tips or stems. A strong spray of water can dislodge the mites, while insecticidal soaps or neem oil can treat persistent outbreaks. Another common garden pest is the slug and snail, which are particularly attracted to the tender, emerging shoots in the spring. These mollusks chew irregular holes in the developing leaves, which can stunt the plant’s growth.
Fungal and Rot Diseases
Crocosmia is susceptible to pathogens that thrive in high humidity and poor air circulation. Crocosmia rust is a fungal infection that appears as small, raised, brownish pustules on the leaves. This rust is more prevalent when plants are overcrowded or situated in shadier, humid spots within the garden.
Corm rot is a major threat linked to poor soil conditions, often caused by the fungi Fusarium or Rhizoctonia. It develops when the plant is grown in poorly drained or waterlogged soil, allowing pathogens to attack the corm. Symptoms include stunted growth, yellowing foliage, and failure to emerge in the spring, with affected corms becoming soft. Management involves improving soil drainage by incorporating grit or organic matter and promptly removing infected material to prevent the spread of spores.
Managing Overgrowth and Invasive Spread
The vigorous growth habit of Crocosmia can become a problem if left unchecked. Certain species, such as Crocosmia × crocosmiiflora, are known for their aggressive spreading. This proliferation leads to dense, overgrown clumps that compete heavily for resources, diminishing flower production.
To contain the spread, dense clumps must be managed through division. Gardeners should lift the dormant corms every three to five years, typically in the early spring, and separate the individual corms before replanting the healthiest ones. For aggressive varieties, planting the corms in a container sunk into the ground can effectively restrict their spread.