Phragmites

It’s an emergent aquatic plant, meaning it’s rooted in wet ground or shallow water but grows up above the surface. Mature plants can reach up to about three metres tall. It has stiff, bamboo‑like stems with long, wide, flat leaves that taper to a pointed tip. In summer and autumn, phragmites produces large, fluffy flower heads that are often purplish in colour. The seeds are covered in fine, silky hairs. Over winter, the dense growth dies back, leaving thick stands of dry stems.

Phragmites prefers the edges of still or slow‑moving water, including freshwater and brackish wetlands, drainage ditches and ponds. It’s less salt‑tolerant than spartina but can cope with fluctuating water levels and will sometimes grow away from the water’s edge.

This plant thrives in nutrient‑rich conditions and tolerates a wide range of water quality, from moderately nutrient‑loaded to highly enriched waters. Added nutrients can encourage faster growth and denser stands.

Phragmites has a long history of being highly invasive overseas and is considered one of the worst potential aquatic weeds in New Zealand.

It forms dense stands that build up thick layers of dead plant material. This blocks light from reaching the ground, suppressing native plants and reducing biodiversity. Phragmites is also allelopathic, meaning it releases chemicals that inhibit the growth of other plant species nearby.

Each winter, the tall, dense growth dies back, leaving large amounts of dry material behind. This creates a fire risk. Dense stands can also block drains and waterways, contributing to flooding, invading low‑lying pasture, and restricting access for fishing, boating and recreation.

Phragmites spreads locally through creeping underground rhizomes, and new plants can establish from small rhizome fragments. Overseas, spread along roading corridors has been a major pathway for landscape‑scale invasion. While phragmites is not known to set seed in New Zealand, its ability to spread vegetatively makes it extremely difficult to contain once established.