Northern Ireland's Priority Species

Hierochloe odorata – holy grass

Hierochloe odorata

Hierochloe odorata (L.) P. Beauv.
Family: Poaceae

This is one of Ireland's rarest plants. Its only known site in the whole island is at Selshan on the County Antrim shore of Lough Neagh, where it was first found as recently as 1946. Outside Ireland, holy grass occurs across central and northern Europe, northern Asia and North America. In Great Britain, only known from a number of sites in Scotland, where it was first discovered in about the mid-1830s near Thurso.

In brief

  • Only known from one site by Lough Neagh at Selshan, County Antrim

  • Originally found growing on the edge of the lough, but survived in damp pasture after the lake was lowered

  • Best seen when flowering in April

  • A priority species because Northern Ireland holds the entire Irish population which has declined

  • Threatened by drainage of the site and possibly disturbance by grazing animals

  • Not seen in the wild for the last 20 years.

Species description
This is a member of the grass family, growing up to a maximum of about 50cm in height. It can be distinguished from other grasses with which it grows by the neat rounded shiny spikelets, each of which contains three tiny flowers. The spikelets are produced at the ends of the branches of a loose one-sided panicle. The plant has a strong smell of hay when crushed and was traditionally used as a strewing herb or incense in parts of Scandinavia and North America. This may account for its association in Orkney with the sites of Norse churches.

Life cycle
The flowers of holy grass are of two sorts: male and hermaphrodite produced in a ratio of 2:1. The hermaphrodite flowers are wind-pollinated (as in all grasses) and potentially can produce seed which can germinate to form new plants germination in cultivated plants reportedly takes place within about two weeks. However, nothing is known of the actual extent to which the Irish plants produce seed, or if produced, how successful is germination. It is possible that the population at Selshan has been maintained largely by vegetative spread. Plants from Selshan which were taken into garden cultivation some years ago spread aggressively by rhizomes.

Similar species
When not in flower, holy grass is virtually impossible to differentiate from the other grasses amongst which it grows. In flower, it is quite unlike any other accompanying species, although it has some slight resemblance to quaking grass (Briza media) a species not found at the Selshan site.

How to see this species
The plant is restricted to two small areas either side of the Selshan Drain, Lough Neagh. Permission from the landowners will be required to access the sites. The sites lie within the Lough Neagh ASSI and the area where holy grass has been seen over the past twenty years is a damp, occasionally flooded, pasture. Visits should be made in April or May to catch the plant in flower.

Current status
Holy grass is known only from an area around the mouth of the Selshan Drain which lies within the Lough Neagh ASSI. The site was visited in the 1980s by several observers who confirmed the continued presence of the plant, up to 1985. After a twenty year gap, searches in 2005 were unsuccessful and the current status is consequently uncertain.

Why is this species a priority in Northern Ireland?
Northern Ireland contains the only population of this species in Ireland.

Threats/Causes of decline
The site is vulnerable to damage by over-grazing, physical disturbance, mowing or other agricultural activity.

Conservation of this species

Current action

  • The main area of the site is protected as an ASSI

  • It is being monitored by field botanists.

Proposed objectives/actions

  • Confirm the plant's continued presence at the sole site

  • Ensure that the population is maintained.

What you can do
It is important that the continued presence of this species at its sole Irish site be confirmed. It is also possible that other, unknown, sites are present in the Lough Neagh area. Any sightings should be sent to The Botanical Society of the British Isles, c/o Botany Department, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU or to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, Tel: 028 9039 5256, [at]

Further information

Flora of Northern Ireland Hierochloe odorata page

Caithness Community Web Site: history and photos of holy grass

Caithness Community Web Site: history of the discovery of holy grass in Scotland

Hackney, P. (1992). Stewart and Corry's Flora of the North-east of Ireland. 3rd edn. Inst. of Irish Studies, Belfast.

Preston C.D., Pearman, D.A. and Dines, T.D. (2002). New Atlas of the British & Irish Flora. Oxford University Press.

Text written by:
Paul Hackney