Chances are, you know this plant and you possibly also have it in your garden. It's just about everywhere, still sold in garden centers, but is also a major threat to native plants...Agapanthus.
Several regional councils list agapanthus as a pest plant including Tamaki Makaurau, Northland, Waikato, Gisborne and Bay of Plenty. Other councils say they haven't listed it as it would be too costly to remove it.
So why is it such a problem? Agapanthus is a prolific seeder; it is very good a producing many seeds that spread well and germinates densely. It also spreads through rhizomes under the ground. Agapanthus is tolerant of hot and cold temperature, as well as various conditions including salty and poor soils. Spread of densely growing weeds such as Agapanthus out compete and crowd out native species, causing massive biodiversity loss.
What can you do? One easy step is to remove the flower heads before they see to prevent further spread of the plant - as the old gardening adage goes 'one year's seed is seven years weeding'!
Digging out is hard work, but reasonably effective, however, there is a lot of root that needs to be removed in order to stop it coming back. Spraying is also effective, but will also require repeat treatments.
Which ever removal method you choose, planting other species in its place will help to reduce more weeds growing in the space. Rengarenga lily (arthropodium cirraum) is a very hardy native that does well in part shade.
Harakeke (Phormium tenax) is also a great option as it does well in costal spaces and in poorer soils and provides food and habitats for a wide range of animals.
Another alternative is Lily turf (Liriope muscari), an introduced species from Asia, but is not known to be weedy.
Whilst there are new varieties being developed that are either likely sterile (Agapanthus ‘Thunderstorm’ and ‘Agapetite') or low fertility varieties, there are so many native and non weedy alternatives that it is easy to avoid agapanthus altogether.
Happy weeding!