Plant of the Week – May 18th 2026 – Early Hair Grass (Aira praecox) – Botany in Scotland

Last week I wandered on Blackford Hill (Edinburgh) trying to find the Spring Cinquefoil (Potentilla verna). I couldn’t find it, but I was struck by the profusion of the tiny grass, Aira praecox, known as the Early Hair Grass. It was just coming into flower, on the shallowest of soils. It was a sunny day, and the silvery inflorescences attracted my attention. Often the grass grows upright – in fact the name Aira comes from the Greek word airo, meaning “to raise”. At Blackford Hill it is mostly prostrate, having been trampled on with heavy footwear.

Aira praecox at Blackford Hill, Edinburgh, mid-March 2026. Image: John Grace

We are fortunate to have several hills within our Scottish parklands, where exposure and geology combine to create habitats that sometimes resemble Europe’s alpine regions. The hills are used by dog-walkers and runners; mostly such folk are oblivious to the role of these hills as a refuge for wild, and occasionally rare, plants. But Aira praecox is not rare, it is widespread everywhere in the British Isles except for parts of southern England and central Ireland. It tends to grow in infertile dry habitats. It is a native plant, germinating on vegetation gaps in the autumn. Grime et al (1988), in a survey based in the Sheffield region, show it to be an extreme stress-tolerator, found on soils with the broadest range of pH, including wasteland and spoil heaps, but never in woodlands or wetlands.  I was surprised to find considerable disagreement in the literature about its preferences for soil pH. Continental authors have said it is a ‘calcifuge’, meaning it should be confined to acid soils, but here we sometimes see it at the edge of pavements, in the angle between wall and pavement where the thin accumulation of soil is usually rich in carbonates from paving material (not acid at all). More about this later.

Aira praecox, Lamlash. Image: Chris Jeffree

As the name praecox suggests it is early: in late April or early May the internodes of the flowering stalks begin to elongate rapidly, causing the inflorescences to emerge from their sheaths. Most flowering and seed production is in late May, and so the plant avoids competition with the more ‘thuggish’ tall and spreading species.

Its original (pe-industrial) habitats presumably included grasslands and heathlands, many of which have been ‘improved’ for agriculture, forest plantations or development projects. Aira praecox has however become thoroughly urbanised and is found in towns and cities. In our Urban Flora project, we recorded it in churchyards, car parks, on roads, paths, buildings, railway-banks and at marine sites. Outside conurbations, there remain many ‘natural’ sites – Cope and Gray (2009) mention cliff-tops, rock-outcrops, dunes, quarries and eroded soil in the uplands. All these habitats have shallow soil with gravel and some of them are likely to be quite infertile and heavily-grazed.

Aira praecox, Irvine. In this case the plant is growing in relatively fertile soil with other species, and its shoots are erect. Image: Chris Jeffree

This is a very small grass – no more than 25 cm and usually much less, often just 5 cm. The leaf-blades are rolled inwards, presumably to reduce transpiration and thus survive periods without rain. Being so small, it is easily overlooked. Cope and Gray (2009) cite the earlier book on grasses by Graves (1822) who wrote “too diminutive to be an object of attention to any but the Botanist”.

Indeed, several British ecologists of the 20^th Century did take a keen interest in Aira praecox, and their work appears in prestigious publications such as the Journal of Ecology. For example, Newman (1963) studied Aira praecox on heathland soils in Thetford, demonstrating how the plant survives the dry spring-time conditions from April to June during the time of flower and seed development. Pemadasa and Lovell (1974) worked on the control of flowering in sand dune species on Anglesey, where A. praecox was one of five winter-annuals. They found that all of them required low temperatures (less than 10 ^oC) for ‘floral induction’ and a combination of higher temperatures (10-20 ^oC) and long days for rapid flowering. There are several such papers, describing carefully planned experimentation, but these days (the 21^st Century) such studies are not generally funded, despite their importance as the basis for predictive models on how plants may respond to global heating and drying.

Detail of flowering shoots, Ardrossan. Image: Chris Jeffree

My hunt for more information (using Google Scholar) led me to research from the Czech Republic, where Aira praecox is close to its eastern limit. It took me a while to grasp the terminology of the paper by Cerny et al (2007) where a ‘classical’ approach¹ is used to name plant communities. In the Czech Republic this plant occurs on dry grasslands and sandy soils, habitats which are threatened by human activities.  But ‘anthropogenic’ habitats include newly opened sand-pits, quarries, military training areas, camping sites and forest roads, all heavily disturbed and offering open ground suitable for colonisation. This confirms Grime’s view that this species is a ‘pioneer’. The Czech research says it is a calcifuge (avoiding soils with high pH, literally fleeing from calcium) but I beg to differ – we find it grows at the edges of pavements (usually calcium rich and high pH)².

Distribution of Aira praecox in Britain and Ireland, from BSBI/maps

Another difference is that the Czech authors say the species is intolerant of drought, but the British research says otherwise. I turned to one of my favourite grass books, Agnes Arber’s The Gramineae, published in 1934. She shows a cross-section of the leaf of Aira praecox. The inrolled leaves have their stomata in the groove, a typical feature of drought-tolerant species and heathland plants like Heather Calluna vulgaris.

Cross-section sketch of leaf blade, copied from Arber (1934) showing stomata (‘stomates’) sunk in grooves.

I wonder, is it possible that under the influence of urbanisation, the species has undergone significant evolution, and that different subspecies should now be recognised? According to Cope and Gray the plant ‘is probably self-fertile and largely inbreeding’ but it seems we lack knowledge of its breeding system. Perhaps we have local races (‘microspecies’). More work is required. This sounds like a topic for an interesting PhD studentship.

Aira praecox has spread throughout the world, presumably via seeds sticking to boots and clothing. I was however surprised to see how it is considered ‘invasive’ in parts of North America, and especially in woods of Garry Oak³, where it develops before the tree canopy has cast its shade. Here’s what I found:

The global spread is evident from the map shown below. I was interested to see how the grass has managed to gain a foothold on many of the small oceanic islands.

Aira praecox global distribution, from GBIF. It is native only to Europe. Note the oceanic islands.

Notes

1.known as the method of the Zürich-Montpellier school, in which plant communities are given names such Airetum praecocis as in this quote from the Abstract of the paper “Within the Airetum praecocis, two subassociations (typicum and plantaginetosum lanceolatae) are distinguished, differing by their successional age”.

2.Cerny et al (2007) say that Aira praecox is a typical calcifuge plant growing mainly on slightly to strongly acidic soil, but Grime et al. (1988) report its range to extend to pH 7-8 (alkaline).

3.Garry Oak (Quercus garryana) was ‘discovered’ by Scottish Botanist David Douglas and named after the Nicholas Garry, the deputy governor of the Hudson’s Bay Company from 1822 to 1835.

Image credit. The image at the heading of this article is by Malcolm Storey, rights holder, permitted by Creative Commons licence CC-BY-NC

References Consulted

Arber A (1934) The Gramineae. Cramer.

Cerny TC et al (2007) Vegetation with Aira praecox in the Czech Republic compared to its variability in Western Europe. Phytocoenologia 37, 115-134.

Cope and Gray (2009) Grasses of the British Isles. BSBI.

Grime JP, Hodgson JG and Hunt R (1988) Comparative Plant Ecology. Unwin.

Newman EI (1967) Response of Aira praecox to weather conditions: I. Response to drought in spring. The Journal of Ecology, 539-556.

Pemadasa MA and Lovell PH (1974) Factors controlling the flowering time of some dune annuals. The Journal of Ecology, 869-880.

©John Grace