Spinifex sericeus is that small ‘tumbleweed’ commonly seen on North Island beaches during summer. It is a native sand-dune grass commonly seen on the seaward face of dunes through most of the North Island and the upper part of the South Island. It produces strong, long runners that creep across the dunes both above ground and under.
The Dune Restoration Trust website says spinifex is also called silvery sand grass or kowhangatara. “Where stands of spinifex are vigorous, runners will trail over recent erosion scarps caused by storms and high seas … [and] encourage the build-up of wind-distributed sand along the scarp and eventually a return to a low-angle dune face…”
The plant thrives in strong winds, salt spray, full sun, shifting sands and drought. Spinifex sericeus is also found in Australia.
Spinifex plants are either male or female, the latter’s flowers being those tumbleweed seed-heads that are released in late summer. When the wind blows, they roll along the beach until they lodge against an obstacle, trap sand in their spines to bury the seeds which then have a chance to germinate. The Terrain website notes that seeds can also be carried by the tides and still germinate when they come to rest.
The always-informative Oratia Native Plant Nursery website says: “The individual sexes [of spinifex] can often cover huge areas of a beach, and it is not unusual to find all of one sex dominating an entire beach. The male flowers are erect spikelets producing masses of wind-blown pollen, while the female plants … form the distinctive spiny tumbleweed seen wheeling along the beach later in the season. Each spine contains a single seed at its base, but many are infertile, having not been pollinated. The miracle is how the female manages to collect the wind-blown pollen when the individual sexes are often at opposite ends of the beach, or even on separate beaches.”
Coastlands Plant Nursery at Whakatane, which is the national Dune Revegetation Centre, notes differences between spinifex plants from the east coast and the west coast even when grown in identical conditions in the nursery.
The entry for spinifex in The Cultivation of New Zealand Native Grasses by Lawrie Metcalf (Random House, 2008) mentions that spinifex lost much of its habitat to the introduced marram grass (Ammophila arenaria). However, it has transpired that marram grass created steep dunes more prone to wind erosion. Spinifex and its partner pingao (Desmoschoenus spiralis, another native) bind dunes in a lower profile.