Ecofriendly Agapanthus – Myth or Reality?
Agapanthus ‘Sarah’.
The typical large-growing form of Agapanthus, A. praecox subsp. orientalis, has become a weed that is increasingly worrying some regional authorities and environmental organisations.
As a result, on 1 July 2008 it was included in the Auckland Regional Pest Management Strategy as a Surveillance Pest Plant. This sparked debate from the public, and media attention was rekindled by recent suggestions to include it as a National Pest Plant Accord species.
Agapanthus is widely grown and the numerous cultivars available are popular for amenity plantings and home gardens, providing year-round foliage, hardiness and low maintenance, with the bonus of a long flowering period and showy flowers. To meet the demand for Agapanthus selections that the public can still buy and grow, the New Zealand nursery industry has released a range of low-growing (‘dwarf’) cultivars that they promote under various terms, including ‘Auckland safe’, ‘eco-friendly’, ‘environment safe’, ‘low-fertility’ and ‘sterile’.
The key questions here are which, if any, cultivars are truly of low fertility? And are the dwarf cultivars able to hybridise with typical large-growing Agapanthus praecox subsp. orientalis? In response to concerns over Agapanthus in its region, Auckland Regional Council (now Auckland Council) contracted Landcare Research botanists Kerry Ford and Murray Dawson to help answer these questions. Over two years, Kerry and Murray conducted detailed fertility assessments of Agapanthus. Their research investigated the sterility and low-fertility claims made of two dwarf cultivars, Agapanthus ‘Finn’ and A.‘Sarah’, and their ability to hybridise with the common tall-growing A. praecox subsp. orientalis. A fertile dwarf cultivar, A. ‘Streamline’, was included for comparison.
A wide range of methods were used to test both male and female fertility: observations of floral morphology and pollen viability, artificial crossing experiments (self, sib and outcrosses), observations of pollen tube growth, seed counts and germination, and flow cytometry. “These fertility assessments revealed that none of the plants assessed were fully sterile and all were capable of producing seedlings,” said Murray.
Agapanthus ‘Finn’ had the lowest overall fertility. It was self-sterile and yielded <10% seed in any outcross with it. Pollen viability was also low at 40%. These results may set a good benchmark for low fertility in Agapanthus. Agapanthus ‘Sarah’ was self-sterile and had the lowest female fertility (about 6% seed set) when outcrossed. However, male (pollen) fertility was relatively high at 85%. As expected, the typical tall-growing Agapanthus praecox subsp. orientalis had the highest fertility across the various assessments (74% seed set when sib-crossed; >95% pollen viability). Note that when self-pollinated this wild-type Agapanthus had low self-fertility (9.5% seed set). This highlights that self-sterility claims made of some cultivars are rather meaningless. Also as expected, A. ‘Streamline’ was fertile, but also had moderate self-fertility (with 40% seed set).
So where do these results leave the industry? Kerry and Murray recommend further fertility testing of existing selections claimed to be of low fertility (e.g. Agapanthus ‘Agapetite’, A. ‘Baby Pete’, A. ‘Double Diamond’, A. ‘Goldstrike’, A. ‘Pavlova’, A. ‘Peter Pan’, A. ‘ Senna’, A. ‘Thunderstorm’ and A. ‘Tinkerbell’). Female sterility – the inability to set seed – is the important criterion to screen for. “There is huge potential for deliberate breeding programmes to create fully sterile cultivars, which we are exploring further,” said Murray.
With further research it may well be possible to find ways of using and enjoying this popular garden plant without harming the environment. That way everyone wins.
This project was funded by the Auckland Regional Council and a full report is available online. An Envirolink small advice grant, in association with Tasman District Council (TSDC85), has allowed this information to be made more widely available.