What does it look like?
Willow-leaved hakea is a fast-growing, upright shrub that can grow up to 5m tall. The flat and elliptical leaves are widest in the middle and can grow up to 12cm long. New growth is rose coloured. During the spring it has pale yellow to white flowers which appear in small dense clusters among the leaves.
Prickly hakea, as the name suggests is a very prickly shrub or tree up to 5m high with numerous branches starting at the base. Young twigs are covered in short, fine hairs, older stems are smooth. The leaves are dark green to grey-green, smooth, and needle-shaped. It has small, cream flowers from June to September, and wooden fruit capsules which are purplish-brown with paler markings.
Downy hakea is a spreading shrub, hairy in most of its parts. The shoots are round, shaggy and hairy, and leaves are simple, round (30-80 x 8-1.5mm) hairy at first and some hairs remain rigid and spiny. Flowers are solitary and few in bunches. Flower stems are hairy 3-5mm long, and it has white flowers from June-August. Fruit are 3.7-4.3 x 3~3.5cm knobbly and shortly beaked. Seeds are 30-33 x 10-14mm with black wings extending down both margins.
Fork-leaved hakea is a large, rather erect shrub or tree 1 - 4m tall with variable leaves. Leaves can be either simple (a single leaf blade) or compound (with several leaflets) 3 - 13cm long and 1 - 1.6mm wide. It has sweetly scented white flowers in clusters, during April - August. The woody fruits are egg-shaped and shiny brown, tapering to a small beak, and seeds are small, winged and black.
Hakea are slightly tolerant of shade and frost, highly tolerant to drought and intolerant to poor drainage. Due to this, Hakea species can successfully grow on thin, poor soils, including gumlands, scrub, open hillsides and sandy soils, and can form dense populations. It is often found in dunes and dune lake areas, on roadsides, and in gumlands. Hakea are early succession species and may be replaced by natives or other species if no further disturbance events occur (that is, fires) in invaded areas.
All species are scattered throughout Northland particularly in gumland areas. Willow leafed hakea is at the south end of Ninety Mile Beach, and it has long been naturalised on the gumlands of North Auckland. Downy hakea is most common in the Te Paki area, with some scattered populations around Dargaville and Poutō peninsula. Prickly hakea is common to abundant in the North, including at Kai iwi lakes and Lake Te Kahika. Fork-leaved hakea is the least common of the species, but is still present in the far north and Whangarei areas.
Why is it a problem?
All hakeas are adapted to fire and low soil nutrients assisting them to become aggressive competitors in sandy and other low nutrient soils. In South Africa, prickly hakea is highly invasive, downy hakea and fork-leaved hakea are moderately invasive and willow leafed hakea is not invasive.. These species have also established and become invasive in other countries such as Portugal.
A seed bank is maintained in the canopy. Winged seeds, two per fruit, are released on death of adult plant. Prickly hakea produces a much larger seed bank than the other species. Fork-leaved hakea has a long juvenile period (6 years) and lower seed production than prickly hakea, so is not usually as invasive.
Vectors of spread: gravity and wind dispersed seeds that are released after fire, but some are also released continuously. These plants do not establish below their own canopy. Controlled by Metsulfuron-methyl at gorse rates.
Hakea can be shaded out if sites of dense native vegetation are freed up/cleared, allowing native species to overtop hakea. this is best done in winter to maximise regeneration and minimise fire risk.
- Pull out small plants (all year round): mulch
- Cut and squirt (autumn): make 1 cut every 100mm around the trunk and fill each cut with 2g metsulfuron-methyl 600g/kg.
- Cut trunk and paint stump (autumn-winter): cut trunk near to the ground, and swab freshly cut stump with metsulfuron-methyl 600g/kg (5g/L).
- Overall spray (autumn): metsulfuron-methyl 600g/kg (5g/10L)+penetrant; or glyphosate (150ml/15L).