Phormium cookianum
Phormium cookianum, more commonly known as mountain flax or wharariki, is endemic to New Zealand and is a very useful plant for any native planting project. Despite its common name, it is found throughout New Zealand from coastal slopes to alpine treelines.
Phormium cookianum is smaller than the New Zealand flax Phormium tenax/harakeke, rarely reaching more than 1.6m, and its leaves and seed pods droop and twist more. The flowers are a source of nectar for the native birds, especially tui and bellbird/korimako.
Trimming: Trim Phormium cookianum as needed by removing old, damaged, or split leaves at the base with clean, sharp secateurs. Avoid cutting across the leaves, as this can leave untidy blunt ends and spoil the natural arching form.
Deadheading: Deadheading is not usually required, but spent flower stems can be cut back to the base once flowering has finished to tidy the plant.
Fertilising: Feed in spring with a balanced slow-release fertiliser, or apply compost around the base to support strong foliage growth. Avoid overfeeding, as this can encourage soft, floppy growth.
Watering: Water regularly during the establishment phase. Once established, Phormium cookianum is reasonably drought tolerant, but it will benefit from deep watering during extreme dry periods, especially in exposed or free-draining sites.
Pest control: Watch for mealybug, scale, and snails, which can hide between the leaf bases. Signs include sticky residue, sooty mould, cottony white clusters, or chewed foliage. Treat with horticultural oil or insecticidal soap, and remove pests by hand where practical.
Disease management: Look out for leaf spot, crown rot, and root rot, particularly in wet or poorly drained conditions. Signs include spotted or browning leaves, soft growth at the crown, wilting, or poor vigour. Remove affected foliage, improve drainage and airflow, avoid overwatering, and use an appropriate fungicide if disease persists.
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